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Yerkes, Robert Mearns

Yerkes, Robert Mearns





Robert Mearns Yerkes was a leading figure in comparative psychology, a branch of psychology that studies animal behavior and often makes comparisons from species to species. The ultimate goal is to find general principles that may sometimes shed light on human behavior. Yerkes published several books on the subject. Among them was The Great Apes: A Study of Anthropoid Life, an influential book he coauthored with his wife, Ada Watterson Yerkes. He also started the first U.S. scientific journal devoted solely to the study of animal behavior. In 1929, Yerkes founded the Yale Laboratories of Primate Biology, the first laboratory for nonhuman primate research in the United States. The laboratory was later renamed the Yerkes National Primate Research Center.

Early in his career as an animal researcher, Yerkes also worked with John Dodson to develop the Yerkes-Dodson law. This law originally related the strength of a stimulus to the speed of avoidance learning in mice. It has since been used, however, to explain the effect of arousal on human performance. The basic idea is that there is an optimal level of arousal for the best performance on any task, and this level depends upon the task's difficulty.

In addition, Yerkes played a major role in the rise of human intelligence testing. During World War I, he headed a committee that developed the first group intelligence tests, which were used to assess Army recruits. These tests, known as the Army Alpha and Beta tests, captured the public's interest. After the war, when Yerkes and others promoted the use of group tests in the general population, they found a ready market. Their tests became the forerunners of standardized tests such as the SAT.

The Alpha and Beta tests had been given to some 1,750,000 Army recruits during the war. Afterward, this huge trove of data was studied intensely. Yerkes's own analyses led him to controversial conclusions about apparent racial and ethnic differences in intelligence. They also raised alarms about a supposed decline in the nation's brainpower. These conclusions have since been refuted on several grounds; at the time, however, they fueled much social debate and helped lead to laws limiting immigration.


In an autobiographical essay written in middle age, Yerkes recalled that he had been a "moody, strong-willed, unsuggestible child, difficult to control." Throughout Yerkes's career, a stubborn streak in his personality sometimes led to conflicts with other scientists. Yet it also gave him the tenacity to hold onto his dream of building a primate research center, where both his name and his scientific legacy live on to this day.

Early life

Yerkes was born on May 26, 1876, in Breadysville, Pennsylvania. He was the oldest child of Silas Marshall Yerkes and Susanna Addis Carrell Yerkes. Growing up on a farm, he developed a lasting interest in the domesticated and wild animals that were all around him: cows, horses, mules, sheep, hogs, chickens, turkeys, ducks, pigeons, rabbits, dogs, cats, rats, mice, snakes. The bond Yerkes probably felt such a strong bond with these creatures in part because of his lack of human playmates. His sister, born four years after Yerkes, had died at the age of three from scarlet fever. His other sister and two brothers were even younger, so Yerkes spent much of his time playing alone.

Yerkes, stricken with scarlet fever at the same time as his sister, only narrowly escaped her fate. The family's doctor was an older cousin, whose caretaking of Yerkes during the crisis made a deep impression on him. As he later wrote in an autobiographical essay, "Ever since, in my daydreams, I have imagined myself as physician, surgeon, or, in other guise, alleviator of human suffering." Yerkes set his sights on becoming a doctor.

In the essay, Yerkes described his mother as "a woman of rare sweetness of disposition and unusual ability" as well as the most important influence in his early life. His father was a different matter, however. Yerkes later recalled that he and his father "had little in common intellectually, and more often than not we disagreed in practical matters." This tension was just heightened by the fact that the father wanted his sons to stay on the farm, while Yerkes had big dreams of a medical education.

Nevertheless, Yerkes's formal education got off to a slow start. When he first began attending the local country school at the age of eight, he was unable to read well and too shy to make friends easily. Yerkes soon adapted, however, and even found that he enjoyed the lessons. He particularly liked "arithmetic and algebra, because I found them stimulating, interesting, game-like . . . and physiology and hygiene, because their objectives, information, and principles impressed me as particularly important."

Yerkes attended the ungraded local school for seven years. At age 15, he and a cousin were sent to the West Chester State Normal School, a school for training teachers. This was Yerkes's first experience with living away from home and his introduction to higher education. He planned to study at the normal school, then transfer to the Jefferson Medical College of Philadelphia. After only a few months, however, Yerkes found himself back home. The family was having trouble paying for his education. There were heavy debts on the farm and three younger children to feed and clothe.

College years

At this point, a kindly uncle came to his rescue. The uncle was a homeopathic physician in Collegeville, Pennsylvania, home of Ursinus College. He offered Yerkes a chance to earn his way through college by doing chores around the uncle's house and stable. Yerkes jumped at the opportunity. In 1892, he entered Ursinus Academy, a preparatory school where he studied ancient languages. A year later, he was admitted to the college program.

At Ursinus College, Yerkes majored in chemistry and biology. He also took pre-med classes in human anatomy and physiology. In addition, he performed the chores at his uncle's house. Despite the busy schedule, however, Yerkes later remembered this as a happy time. He also never forgot the generosity of his uncle, describing him as "a wise, broad-minded, generous gentleman, a beloved physician, and a staunch, dependable friend."

In 1897, Yerkes graduated from Ursinus College. His plan all along had been to go straight to medical school after graduation. Once again, however, fate intervened. Yerkes was unexpectedly offered a loan of $1,000 to do graduate work in psychology, biology, and philosophy at Harvard University. At age 21, he made a decision that shaped his whole future when he chose to attend graduate school at Harvard instead of medical school in Philadelphia. As he recalled,

Readily I convinced myself that I was young to enter medical school and might better devote at least a year to special work in Harvard before completing my medical training. It was my earnest desire to work with pre-eminently able investigators and teachers.

That fall, Yerkes entered Harvard, although not as a graduate student. Instead, he first had to take some undergraduate classes and prove his fitness for graduate study. In 1898, he was awarded the A.B. degree and granted graduate status. Yerkes by this point already knew that he was keenly interested in both psychology and zoology. He decided to combine these interests by studying the new field of comparative psychology. Encouraged by his teachers, he set his sights on a psychology degree rather than a medical one.

At least two professors, who became Yerkes's colleagues and friends, had a lasting influence on him during his student days. One was Hugo Münsterberg, a leader in applied psychology, which looks for practical uses for psychology in settings such as business, industry, health care, education, and government. Applied psychology later became an interest for Yerkes, too, when he set out to solve practical problems with intelligence tests. Another professor was biologist Charles Davenport, a leader in the eugenics movement, which held that the human race could be improved through selective breeding. Like Davenport, Yerkes later became an outspoken supporter of eugenics.


  • The Dancing Mouse: A Study in Animal Behavior. New York: Macmillan, 1907.
  • With J.W. Bridges and R.S. Hardwick. A Point Scale for Measuring Mental Ability. Baltimore: Warwick and York, 1915.
  • "The Mental Life of Monkeys and Apes: A Study of Ideational Behavior." Behavioral Monographs 3 (1916) 1–145.
  • "Provision for the Study of Monkeys and Apes." Science 43 (1916) 231–4.
  • (Editor). "Psychological Examining in the United States Army." Memoirs of the National Academy of Sciences 15 (1921) 1–890.
  • Almost Human. New York: Century, 1925.
  • With B.W. Learned. Chimpanzee Intelligence and Its Vocal Expressions. Baltimore: Williams and Wilkins, 1925.
  • "The Mind of a Gorilla." Genetic Psychology Monographs 2 (1927) 1–193, 375–551.
  • With Ada W. Yerkes. The Great Apes: A Study of Anthropoid Life. New Haven, CT: Yale University Press, 1929.
  • Chimpanzees: A Laboratory Colony. New Haven, CT: Yale University Press, 1943.

Harvard University

In 1902, Yerkes received his Ph.D. in psychology, along with an offer to stay on at Harvard as an instructor. There was just one catch: The job, which involved both teaching comparative psychology and doing research, did not pay well. Yerkes had borrowed heavily to finance his education, so taking the job would be a hardship. When asked by Münsterberg whether he could afford to accept the position, Yerkes later recalled that his response was, "No, but I shall, nevertheless." Yerkes taught at Harvard for the next 15 years, first as an instructor, and then as an assistant professor.

While at Harvard, Yerkes married botanist Ada Watterson. Their two children, Roberta and David, were born during this period as well. Years later, Ada teamed up with her husband to write The Great Apes, which proved to be one of Yerkes's most important books. In an autobiographical essay penned around the same time as the book, Yerkes noted that his marriage to Ada had "perfectly blended our lives and incalculably increased our professional and social usefulness."

Yerkes's early animal research addressed such topics as sensory function, instinctive behavior, learning, and problem solving. In 1907, Yerkes published a classic book titled The Dancing Mouse: A Study in Animal Behavior, in which he explored the genetics and behavior of mutant house mice. The next year, Yerkes and Dodson coauthored a paper that presented what became known as the Yerkes-Dodson law. In 1911, Yerkes founded the Journal of Animal Behavior, the first U.S. journal devoted specifically to animal behavior research.

That same year, Yerkes bought a farm in Franklin, New Hampshire, which he planned to use as both a summer home and a location for studying primates. During 1914–15, he also studied primate behavior on an estate in Montecito, California. In 1916, he published an article in the journal Science, in which he made the case for establishing a laboratory especially for this type of research.

Yet, despite these successes, he faced several challenges. Research in comparative psychology was seen as a low priority at Harvard. Yerkes was advised to switch to educational psychology if he wanted to get ahead. Although he later claimed that he had disregarded the advice, he did begin to venture into areas outside animal behavior. Among other things, he wrote a psychology textbook and coauthored a book about self-psychology.

In 1913, Yerkes started working half-time as a psychologist in the Psychopathic Department at Boston State Hospital. While at the hospital, Yerkes became aware of the urgent need for practical tests that could be used to assess the mental abilities of patients. Here was a problem that combined aspects of both educational and applied psychology.

The Binet-Simon scale, the first useful test of intelligence, had recently been imported to the United States from France. In its original form, the test assessed intelligence in terms of age levels. Other researchers had suggested that the results could be turned into a score called an intelligence quotient (IQ), which involved dividing mental age by chronological age. Yerkes and his colleagues at the hospital devised their own method for converting the test into a point scale. Using this method, the test could now be scored simply by tabulating the number of points earned on a wide range of items. Their method removed the need to link results to the subject's age.

By 1917, Yerkes had built a reputation solid enough to get himself elected as president of the American Psychological Association (APA). Still, Harvard declined to promote him to a position as full professor. When the University of Minnesota asked him to head up the psychology department there, Yerkes accepted. As soon as Yerkes made plans to move to Minnesota, however, he got sidetracked by America's entry into World War I.

World War I

When the United States entered the war, Yerkes was 40 years old. He was eager both to serve his country and to advance his career. Beyond that, however, he also wanted to show the nation just how valuable the young science of psychology could be. As APA president, he convinced the association's council to form 12 committees that would explore possible military applications for psychology. Yerkes named himself head of the committee that was charged with studying possible applications of intelligence testing.

To form his committee, Yerkes called on all the top U.S. intelligence testers of the day. They included Henry Goddard, who had introduced the Binet-Simon scale to the United States, and Lewis Terman, who had just developed an Americanized version of the scale called the Stanford-Binet. From the outset, Yerkes had big ambitions. He aimed to greatly expand intelligence test methods within a very short period of time.

The first version of the Binet-Simon scale had been published by French psychologist Alfred Binet and his associate, Théodore Simon, in 1905. Binet's original goal for the scale was relatively modest and very practical: He wanted to identify mentally retarded schoolchildren who might benefit from special education programs. The test was designed specifically for children, and it was meant to be given on an individual basis.

Any test Yerkes devised for the military would have to differ from this model in several key ways. First, it would have to be designed for adults rather than children. Second, given the huge number of recruits, it would need to be given in a group rather than individually. Third, Yerkes was not satisfied with the idea of a test that would merely weed out mentally unfit recruits. Instead, he wanted to develop a test that could also identify those recruits with superior ability who might make good officers.

Yerkes managed to convince the U.S. Army to give his idea a try. His committee quickly put together two prototype tests: one for recruits who could read English, and another for those who could not. Results from a trial on 80,000 men were promising enough that the Army authorized testing of all new recruits by the beginning of 1918. Yerkes told the newspapers that psychology was now in a position to help win the war.

The tests were promptly revised and renamed Army Alpha, for literate recruits, and Army Beta, for illiterate ones. Soon, the tests were being given at a rate of 200,000 per month. By the war's end in November 1918, about 1,750,000 men had taken one of the tests—an incredible logistical feat. Many corners had been cut to accomplish this feat, however, rendering the data of questionable value. In addition, the trial period had been too brief to draw any firm conclusions about the usefulness of the test. Army commanders themselves were divided in their opinions. Nevertheless, Yerkes had succeeded in placing group intelligence testing on the map.

National Research Council

Yerkes had been elected to membership in the National Research Council in 1917. After the war, he had to choose between working with the council in Washington, DC, or belatedly assuming his post at the University of Minnesota. Yerkes chose the National Research Council, in part because he wanted to oversee publication of a lengthy report about the wartime testing program. Beyond that, however, he hoped that taking this job would help him to find financial support for a long-time dream: to establish a laboratory for studying nonhuman primates. No such lab existed in the United States at the time, yet Yerkes was determined to see his dream become a reality.

First, though, he would have to attend to several other projects for the council. Yerkes founded and chaired the Committee on Scientific Problems of Human Migration. At the same time, he chaired the Committee for Research in Problems of Sex. In 1921, Yerkes also served as editor for a massive report titled "Psychological Examining in the United States Army," which detailed findings from the Army Alpha and Beta tests.

Yerkes never lost sight of his goal of doing primate research, however. In 1923, he began raising two apes in his home. Chim was later recognized as a bonobo, which resembles a chimpanzee but is more slender, while Panzee was a common chimpanzee. Yerkes described his research on the pair in a book titled Chimpanzee Intelligence and Its Vocal Expressions. The following year, Yerkes spent the summer in Havana, Cuba, where he was able to observe a large primate colony. This work led to yet another book, titled Almost Human.

Yale Laboratories of Primate Biology

In 1924, Yerkes returned to the academic world. He joined the faculty at Yale University as a professor of psychobiology, the study of mental functions and behavior in relation to other biological processes. In 1925, he received funding for four years of primate research in New Haven, Connecticut, the home of Yale. While this was certainly a step in the right direction, Yerkes continued to push for a primate research center located in a warmer clime. Finally, in 1929, the Rockefeller Foundation provided the funds he needed to set up the Yale Laboratories of Primate Biology in Orange Park, Florida, not far from Jacksonville.

Yerkes had been waiting for such an opportunity all of his professional life. Building and running the facility would prove to be huge undertakings, however. These duties kept Yerkes so busy, in fact, that he had relatively little time to do research of his own. According to Donald Dewsbury, a comparative psychologist who has studied the history of the field, Yerkes's most fundamental accomplishment during this time may have been "the demonstration that chimpanzees could be kept successfully, bred, and studied in captivity. Much later progress was possible only because Yerkes invested heavily in housekeeping and developing methods of keeping and caring for chimpanzees."

Yerkes served as director of the laboratory until 1941. Although he was often preoccupied with administrative tasks, other researchers made good use of the facility. Nearly 200 articles from research conducted at the laboratory were published while Yerkes was director there. The most notable studies covered topics such as the role of the brain's frontal lobe, by Carlyle Jacobsen; learning, by Kenneth Spence; morphine addiction, by Shirley Spragg; and mating behavior in chimpanzees, by Yerkes and James Elder.

Yerkes's time at the laboratory was not without controversy, however. As 1939, the renewal date for the Rockefeller Foundation grant approached, the foundation asked several leading scientists to review the laboratory's progress. The reviewers issued a report that criticized many of the laboratory's policies and practices. First, the foundation had wanted to make sure that researchers from around the country had ready access to the facility. Yerkes preferred to rely on permanent staff, however; many of the reviewers saw this as a sign that he wanted to control the research done there. Second, the scientists took issue with Yerkes's insistence on using the facility to study chimpanzees, which are classified as apes. Chimpanzees are closer relatives of humans than monkeys, but they are also rather difficult to study in captivity. The reviewers felt that monkeys would have been less expensive to raise and equally appropriate for many research purposes. Third, they cited Yerkes's style of observational research, which was out of vogue at the time. In addition, some reviewers were offended by Yerkes's personal style, especially what they interpreted as his need to dominate others.

Eventually, the Rockefeller Foundation did renew its funding, but with a decreasing budget each year. Just when it looked as though the laboratory's days might be numbered, the dean of the Yale Medical School worked out a deal to save the facility. Part of the deal, however, was that Yerkes would retire. Yerkes, who was in his sixties, realized that he had no choice but to comply. Upon his retirement in 1941, Yale renamed the facility the Yerkes Laboratory of Primate Biology in his honor.

After Yerkes's death, Yale officials decided that the long distance between the university and the Florida laboratory did not allow for the best use of the facility. Emory University, located in Atlanta, took over ownership of the lab. In 1965, the facility was moved to the Emory campus. In 2002, the facility was renamed once again as the Yerkes National Primate Research Center. Today, it is one of eight national primate research centers funded, in part, by the National Center for Research Resources of the National Institutes of Health.

Life after Yale

In 1944, Yerkes also retired from his position as professor at Yale University. The next several years were spent working on an autobiographical book, which he called "The Scientific Way." Unfortunately, the 425-page manuscript was rejected by several publishers, including Yale University Press, where his daughter Roberta was an editor. The book was never published.

Despite this final setback, Yerkes was able to look back over a long and highly productive career. He had received numerous honors, including honorary degrees from Ursinus College and Wesleyan University and the Gold Medal of the New York Zoological Society. He had also been elected to the National Academy of Sciences, and he had served as president of the American Society of Naturalists. Yerkes died of a heart attack on February 3, 1956, at the age of 79.


In a 1996 article, Dewsbury wrote that Yerkes was "arguably the most important comparative psychologist and psychobiologist of the [twentieth] century." Yerkes is, first and foremost, remembered for his success in establishing the study of nonhuman primates as a field of scientific research. He also made other important contributions to psychology, however, two of which are the Yerkes-Dodson law and major advances in intelligence testing.

Yerkes-Dodson law

Main points The Yerkes-Dodson law, as originally stated, relates the strength of a stimulus to the speed of avoidance learning. In their research with mice, Yerkes and Dodson used three levels of task difficulty: easy, medium, and hard. They also used three levels of stimulus strength: weak, intermediate, or strong. They found that, if the task was easy, it was learned most quickly when the stimulus was strong. If the task was difficult, however, it was learned most readily when the stimulus was weak.

Specifically, the mice in Yerkes and Dodson's experiment were placed in specially designed boxes. Soon, the mice became cornered; in order to escape, they had to choose between entering either a black passageway or a white one. If they chose the white passageway, they were always allowed to pass through, and they would return to a roomier nest box. If they attempted to enter the black passageway, however, they would always receive an unpleasant electric shock. In this case, then, the stimulus varied with the strength of the shock, and the task difficulty varied depending on how much the two passageways differed from one another in brightness. Each mouse was tested a number of times. The researchers then measured how quickly the mice learned to pick the white passageway every time.

If it was easy to tell the difference between the two passageways, the mice learned to avoid the shock most quickly when the shock was strong. If it was hard to tell the difference, however, they learned fastest when the shock was weak. Yerkes and Dodson concluded that "the relation of the strength of electrical stimulus to rapidity of learning or habit-formation depends upon the difficultness of the habit, or, in the case of our experiments, upon the conditions of visual discrimination."

Explanation Yerkes and Dodson's findings languished in relative obscurity for several decades. In the 1950s, however, psychologists introduced the concept of general arousal. They noted that the relationship between arousal and performance tends to take the shape of an upside-down U. A certain amount of arousal is thought to produce the best performance; too much or too little arousal, on the other hand, is detrimental.

Proponents held that the optimal level of arousal varies depending on the difficulty of the task at hand. The level tends to be relatively high for easy tasks and low for difficult ones. Some researchers pointed to Yerkes and Dodson's research as an early demonstration of this principle. It is certainly possible to see getting an electric shock as something that might lead to general arousal. Yerkes and Dodson, however, never used the term arousal in their writings; psychologists applied it to their findings many years later.

Researchers have since tried to study arousal theory in a wide variety of ways. "Arousal" has been correlated with the level of electric shock, threats, incentives, and even as the amount of caffeine in someone's system. Not surprisingly, these diverse studies have yielded mixed results. Some have found the predicted effect on performance, but others have not. Many psychologists now believe that the concept of general arousal is overly broad, since it fails to distinguish between such states as stress, anxiety, fear, motivation, and attention. This limitation does not necessarily reflect poorly on the Yerkes-Dodson law, however. The law can still be taken the way Yerkes and Dodson intended: simply as a description of the relationship between the strength of a stimulus and the speed of avoidance learning.

Examples Over the years, researchers have tried to use arousal theory to predict people's performance on many kinds of tasks. One example is eyewitness memory. Using the modern restatement of the Yerkes-Dodson law, many researchers have predicted that an increase in emotional arousal from low to moderate levels should improve memory. If arousal increases even more, however, going from moderate to high, memory should start to decline again. This prediction raises an interesting question: Does arousal theory mean that eyewitness testimony is unreliable in situations where emotions run high, such as when someone is the victim of a violent crime or involved in a terrifying car crash?

It might seem logical that high emotion would interfere with memory, but research has shown that this is not necessarily the case. In fact, studies have found that people often remember the details of very emotionally-charged events quite well. While there may indeed be some differences in how emotional events are remembered, the relationship between emotion and memory seems to be more complex than previously thought. It appears to depend on a host of factors, including the nature of the event, the type of details being recalled, the amount of time that has passed, and whether or not there are any cues to jog the person's memory. Overall, there is little reason to believe that emotional arousal automatically impairs the ability to store a memory.

Intelligence testing

Main points Most of Yerkes's research was done in animals. A detour from this path, however, led to a lasting achievement: the development of the first tests of mental ability designed to be given to large groups of people. This accomplishment paved the way for the mass intelligence testing that is still a very common practice in American schools.

America's entry into World War I was a turning point, not only for the nation, but also for psychology. In The Mismeasure of Man, American paleontologist and author Stephen Jay Gould noted that Yerkes

was a superb organizer, and an eloquent promotor of his profession. Yet psychology still wallowed in its reputation as a "soft" science, if a science at all. Some colleges did not acknowledge its existence; others ranked it among the humanities and placed psychologists in departments of philosophy.

Yerkes wanted to show that psychology could be not only practical, but also as rigorous as chemistry or physics. The new field of intelligence testing seemed to be a fast track to both of these goals.

Yerkes had some background in the field. He and his colleagues had just devised a method for converting the Binet-Simon test into a point scale. He also benefited from the assistance of several leading experts on intelligence testing. Nevertheless, the task he took on was Herculean. In very short order, he and his committee devised two new tests for use with Army recruits. Unlike previous intelligence tests, these assessments would be given to large groups of men at one time, rather than to each person individually.

The Alpha test was designed for men who could read and write. It included arithmetic problems, word pairs to be rated as synonyms or antonyms, number sequences to be completed, scrambled sentences to be unscrambled, analogies, and multiple-choice questions that drew on general knowledge or "common sense." The Beta test, in contrast, was intended for men who could not read and write English. This group included not only recruits with learning problems, but also recent immigrants and those with a limited education. Men who took the Beta test were asked to trace the path through mazes, find the missing element in pictures, imagine how pictured shapes might be fitted together, and substitute symbols for numbers in a code. Each test took less than an hour to complete and could be given to a large group all at once.

Test scores were reported using letter grades from A to E, including pluses and minuses. The grade of A was said to indicate "a high officer type when backed up by other necessary qualities." B indicated "splendid sergeant material," and C indicated a "good private type." On the other hand, men who scored D, while usually fair soldiers, were thought to be unsuited for tasks requiring much skill, planning, or alertness. Those whose scored E were deemed unfit for regular Army service.

The tests had been cobbled together very quickly, with little time for refining the tasks and procedures. The rushed development may well have reduced the tests' accuracy and reliability. Aware of this problem, Yerkes and his colleagues said that men who received low scores on the Alpha test should be retested on the Beta. Those who got low scores on the Beta should be retested using an individual intelligence test. When this policy was followed, test scores tended to rise on each retesting. Given the realities of wartime mobilization, however, the policy was ignored more often than not. Most men were hurried through a single testing session, the results of which stayed with them for the rest of their military careers.

It is unclear just how much the Army actually relied on Yerkes's tests. There seems to have been some skepticism about the results. There also was some resentment of the testing program, which took up valuable space and time. According to Gould, "Yerkes's corps encountered hostility in some camps; in others, they suffered a penalty in many ways more painful: they were treated politely, given appropriate facilities, and then ignored." In addition, a second Army testing program headed by psychologist Walter Dill Scott divided the available resources. Some observers claimed that Scott's program was more useful.

Explanation In the decades since the war, Yerkes's testing methods have continued to draw criticism. For one thing, several items on his tests seemed to be biased toward members of the majority American culture. Men taking the Alpha test were expected to know that Overland cars were made in Toledo, Crisco was a food product, and Christy Mathewson was a famous baseball player. Those taking the Beta test were asked to notice the missing details in pictures of a tennis court, a person bowling, and a phonograph. Clearly, anyone from outside mainstream, middle-class America might have been at an unfair disadvantage on such items.

Another problem was the difference in the way directions were given for the Alpha and Beta tests. Men taking the Alpha test were given a clear verbal explanation about the purpose of the testing. Relatively complete written directions were also printed on the Alpha test forms. In contrast, men taking the Beta test were told nothing about test's purpose. The directions for various tasks were barked out in very brief commands. The intent was to overcome the language barrier for recent immigrants. The real effect, however, was probably that many recruits felt totally bewildered.

The instruction manual for the tests was quite detailed. As a result, it is possible to very closely reconstruct the suggested test procedures. The clipped commands and gestures used when giving the Beta test sometimes seem to border on slapstick. For example, Gould has cited these instructions given for a picture completion task: "'This is test 6 here. Look. A lot of pictures.' After everyone has found the place, 'Now watch.' Examiner points to hand and says to demonstrator, 'Fix it.' Demonstrator does nothing, but looks puzzled. Examiner points to the picture of the hand, and then to the place where the finger is missing and says to the demonstrator, 'Fix it; fix it.' Demonstrator then draws in finger. Examiner says, 'That's right . . .'" After a few more demonstrations, the examiner says, "'All right. Go ahead. Hurry up!' During the course of this test the orderlies walk around the room and locate individuals who are doing nothing, point to their pages and say, 'Fix it. Fix them.'" After three minutes, the examiner announces that time is up.

While the Beta test did not require reading, it did require the ability to use a pencil and a knowledge of numbers. For people with no formal education, these requirements were often a big hurdle. In addition, different Army camps seemed to use different criteria for deciding which men would take the Alpha test and which would take the Beta. Finally, although the Army was supposed to provide an adequate building at each camp for the testing, this was not always realistic. Often, the tests wound up being given in cramped rooms in which the men sitting in the back had difficulty seeing or hearing the examiner.

It seems likely that many recruits did not perform their best under such stressful conditions. To demonstrate this point, Gould tried giving the Beta test to a modern group of 53 students at Harvard University. He stuck to Yerkes's procedure as closely as possible. These students had a couple of advantages over the World War I recruits, however: They knew what was happening, and they did not have the pressure of real-life consequences riding on their results. Nevertheless, more than 10% of students from one of the world's leading universities scored just a C, meaning they would have been seen as mentally fit for no higher a rank than private.

For actual World War I soldiers, scores on both the Alpha and Beta tests did tend to agree overall with officers' ratings of their men's intelligence. There was also a lower but still moderate association between test scores and actual military performance. What is true for a whole group of men, however, is not necessarily so for any particular individual. The tests probably underrated the intelligence of many men who simply did not understand what was expected of them. In general, the tests seem to have been most accurate for literate, native-born Americans.

The end of the war brought Yerkes's testing program to an abrupt halt. Yerkes claimed that his test had helped to win the war, but not everyone agreed. Some critics argued that there were too many flaws in the tests for the results to be meaningful. But while the tests may not have won the war, they certainly helped Yerkes score a victory. If nothing else, he had introduced the idea of intelligence testing on a large scale.

Examples After the war, Yerkes began to sort through the mountain of data that had been collected. He soon reached some questionable conclusions that led to much public debate. The first dealt with the average mental age for Army recruits; Yerkes claimed it was shockingly low: just over age 13. This mental age was thought to be barely above the cutoff for mild mental retardation. The supposed decline in national intelligence became a rallying cry for eugenicists. They argued that the nation's declining intelligence was due to unfettered breeding by the poor, the feebleminded, non-whites, and immigrants. It became all too easy to twist such "science" into bigotry.

Of course, Yerkes himself might have helped to set the record straight. He could have pointed out that problems with the hastily thrown-together tests could have led to faulty data. In fact, there were clear signs that this was true. For example, an unusually large number of men scored zero on parts of the Army intelligence tests, indicating that they simply did not understand the instructions.

Instead, Yerkes adopted the eugenicist view. Gould quotes Yerkes as saying that an average mental age of 13 only confirmed "that the average man can manage his affairs with only a moderate degree of prudence, can earn only a very modest living, and is vastly better off when following directions than when trying to plan for himself." Furthermore, Yerkes believed that this low level of intelligence was due to genetics and therefore unchangeable. Trying to help the average man improve his lot in life was just a wasted effort. As Yerkes put it, "much of our effort to change conditions is unintelligent because we have not understood the nature of the average man."

Yerkes reached two other controversial conclusions. He noted that blacks and recent immigrants tended to score lower on the Army intelligence tests than native-born whites did. Once again, it might have been sensible to conclude that the tests were biased toward members of the majority culture. In fact, there were strong hints that group differences in test scores reflected differences in life experiences. For example, there were many more black recruits than white ones who had not attended school. Looking back, it seems logical that social conditions—such as racial discrimination and poor conditions in black schools—may have led to fewer educational opportunities for blacks. Less education, in turn, probably hurt black recruits' performance on the tests.

Yet Yerkes chose to put a eugenicist spin on the numbers. He believed that less schooling among blacks simply meant that they were not as inclined toward education as whites. Critics such as Gould have since noted that Yerkes ignored his own data on this point. There were regional differences in schooling, with evidence of wider educational opportunities for blacks in the northern states than in the southern ones. In turn, black recruits from some northern states tended to score higher on the Army tests than either southern blacks or southern whites.

Some of Yerkes's conclusions about the test scores of recent immigrants now seem equally dubious. When Yerkes broke down the test results by country of origin, he found that recruits whose ancestors came from the "Nordic" countries of northern Europe tended to score higher than those of "Slavic" or "Latin" ancestry. This finding suited the views of racial supremacists quite well, since they believed in the superiority of Nordic peoples. But Yerkes glossed over one key fact: Most immigrants from the Slavic and Latin countries of eastern and southern Europe had arrived in the United States only recently. Many, therefore, did not speak English well. In contrast, the main wave of immigration from northern Europe had passed years before. Most recruits from those countries were already fluent in English, an obvious advantage when taking the tests.

Yerkes also found that the average test scores for foreign-born recruits rose the longer they had lived in the United States. This result held true regardless of their country of origin. In hindsight, this seems to be a clear sign that test scores were tied to people's knowledge of the English language and familiarity with American culture. In other words, it was strong evidence for an environmental influence on intelligence test scores. Yerkes himself acknowledged the possibility when he wrote, "At best we can but leave for future decision the question as to whether the differences [in scores] represent a real difference in intelligence or an artifact of the method of examination."

Yet many eugenicists chose to see the same data as evidence for a genetic difference in intelligence among groups of people. They used this viewpoint to argue for restrictions on immigration. Gould is one of many later critics who have decried the eugenicists' misuse of scientific data. In The Mismeasure of Man, he wrote:

The army mental tests could have provided an impetus for social reform . . . Again and again, the data pointed to strong correlations between test scores and the environment. Again and again, those who wrote and administered the tests invented tortuous, ad hoc explanations to preserve their hereditarian prejudices.

Comparative psychology

Main points Yerkes's work on the Army intelligence tests had considerable impact on society at large. In the context of Yerkes's career, however, this work was really a sidelight. His main area of interest was always comparative psychology. In this area, Yerkes made his most important contributions to science. Today, he is remembered mainly for his research on nonhuman primates. Early in his career, though, he also studied learning in mice, turtles, green crabs, frogs, and crawfish.


Konrad Lorenz

Konrad Lorenz (1903–89), an Austrian naturalist, was one of the founders of ethology. This field, which has sometimes been described as the biological study of behavior, sprang up in Europe in the first half of the twentieth century. Its emphasis was on observing animals in natural surroundings, although Lorenz worked largely with captive animals.

Lorenz's father was a physician who wanted his son to follow in his footsteps. While young Lorenz obediently earned a medical degree, he soon realized that his true love was animals. He returned to school, and in 1933, he received a Ph.D. in zoology from the University of Vienna.

Soon thereafter, Lorenz began the work for which he is best known. At his family's home in Altenberg, Austria, he spent summers studying the behavior of greylag geese. Lorenz observed that the geese lived a family existence that was in many ways similar to human family life. Lorenz also identified the process of imprinting, in which a young animal that is exposed to a foster "mother" in place of its real mother during a critical period in development will become attached to the substitute. Lorenz raised goslings that, removed from their real mother, accepted him as their mother figure. The scientist was often seen walking down a path or rowing a boat with a line of goslings following behind. Lorenz also found that mallard ducklings would imprint on him, but only if he squatted down and quacked.

Lorenz theorized that animals have fixed-action patterns, genetically programmed behavior patterns that remain dormant until a specific stimulus triggers them. In birds, fish, and insects, such critical behaviors as courtship, nesting, and caring for the young are, to a large extent, fixed-action patterns. In mammals, and especially in humans, behavior is more modifiable and dependent on learning. Nevertheless, Lorenz believed that fixed-action patterns still play a role. In 1973, Lorenz received the Nobel Prize in Physiology or Medicine. He shared the prize with two other founders of ethology: Nikolaas Tinbergen, a Dutch-born zoologist who helped develop the theory of fixed-action patterns; and Karl von Frisch, an Austrian zoologist who studied the communication system of bees.

Comparative psychology is a branch of psychology that studies animal behavior and frequently makes comparisons among species. Beyond that, however, there have never been clear-cut boundary lines between comparative psychology and other kinds of animal research. In common practice, the study of learning, motivation, and memory in animals—especially rats, monkeys, and pigeons—is now often categorized as experimental psychology. The study of physiological aspects of behavior in humans and other animals is often categorized as physiological psychology. Other kinds of psychological research on animal behavior have traditionally fallen under the heading of comparative psychology.

In the 1930s and 40s, ethology sprang up as another new discipline. This closely related field also studied animal behavior, but from a more biologically oriented point of view. Ethologists tended to be trained in zoology and based in Europe (see accompanying sidebar). Their research—focused mainly on birds, fish, and insects—was conducted in the field, where they could observe animals in their natural environment. Ethologists were typically focused on studying instinctive behavior and the evolution of behavioral patterns. Comparative psychologists, on the other hand, tended to be trained in psychology and based in North America. Their research—focused mainly on mammals—was done in laboratories or research centers, where the psychologists could control experimental variables and gather data for statistical analysis. Comparative psychologists were often interested in developing general theories of behavior and learning.

Yerkes played a big role in shaping the prevailing view of what comparative psychology should be. He always believed strongly in the importance of doing experiments under controlled laboratory conditions. He also believed however, that it was critical to understand an animal's natural habits and instincts. Therefore, when ethology began to emerge as a new field, Yerkes welcomed it enthusiastically.

For Yerkes, the study of animal behavior was only a means to the end of understanding human psychology. Over the years, he became fond of some of the chimpanzees he studied. Yet for him, animals were always just stand-ins for humans. He believed that any knowledge gained from studying animals should be used to serve humanity through better education and breeding—a policy he referred to as "human engineering."

Explanation Early in his career, Yerkes helped break new ground in comparative and experimental psychology. For example, his book The Dancing Mouse: A Study in Animal Behavior was one of the earliest studies of behavioral genetics. Yerkes also studied sensory function, learning, and problem solving in several species. In addition, he collaborated with John B. Watson, the father of behaviorism, to develop new methods for the study of color vision. Later, he studied sexual, social, and maternal behavior in primates. Beyond that, Yerkes was an extraordinary organizer, administrator, and promoter of large-scale research projects. In a book titled Comparative Psychology in the Twentieth Century, Dewsbury concluded that "no one made a more substantial or more sustained contribution to comparative psychology than Robert Mearns Yerkes."

Yet Yerkes also had significant failings as a scientist. His later research with chimpanzees, in particular, was often colored by his personal biases—especially his research on family groups and gender roles. For example, Yerkes interpreted the social groups formed by captive chimpanzees at the Yale research facility as evidence of unchanging family bonds. Later researchers studying chimps in the wild, however, such as noted English zoologist Jane Goodall, have found a different pattern. Under natural conditions, chimpanzee clusters tend to break apart and then regroup on a regular basis. This fact was not known until after Yerkes's time, however. Working without benefit of such knowledge, Yerkes seems to have imposed his own sense of family values onto what he observed among the chimpanzees.

Yerkes's opinions about gender roles also seem to have affected his research. In experiments with chimpanzees, Yerkes concluded that males were normally dominant over their female partners. Males granted special privilege to females during estrus, however, the phase of the chimpanzee menstrual cycle when females are most receptive to mating. Thus male dominance was seen as natural, and any special privilege that a female might gain was bestowed on her by the male only in return for sex.

Examples The studies that Yerkes conducted to find this supposed effect have not held up well to scientific scrutiny. In a so-called food chute test, Yerkes put male-female chimpanzee pairs into a cage. He then dropped pieces of banana into the cage through a chute. The experimenter recorded which chimpanzee got the food and observed how the animals interacted. Yerkes reported that the males usually got the food. When the females were in estrus, however, the males allowed the females to have the food. As Yerkes explained in his book Chimpanzees: A Laboratory Colony:

The behavioral picture is clear-cut. A male who previously has completely controlled the situation and taken the food time after time as if it were a matter of course yields without protest, although possibly somewhat reluctantly, to the female when, at the beginning of genital swelling and willingness to mate, she claims the food. Thereafter as long as she is sexually receptive and also acceptable to her mate, she may if she so desires continue to control the food-getting situation without competition or conflict. But the very day detumescence [the subsiding of genital swelling] begins, the behavior of the males changes ...

Despite Yerkes's claim, however, the results do not seem to be clear-cut at all. Critics have pointed out several weaknesses in the design and analysis of the food-chute study. For one thing, Yerkes chose to use menstrual cycles rather than pairs of chimpanzees as his unit of statistical analysis. From a statistical point of view, this choice was problematic. From a theoretical point of view, it showed a tendency to see the animals as interchangeable parts rather than distinct individuals.

An even bigger problem may have been the way Yerkes tended to ignore any results that did not confirm his ideas. Although Yerkes used several chimpanzee pairs in his study, he focused mainly on the results from one pair, Jack and Josie, who showed the expected pattern in their relationship. Yerkes found reasons to discount the findings from other pairs that showed the relationship less definitely and completely.

Such selective use of data—using those results that support the researcher's hypothesis and throwing out the rest—goes against the basic principles of science. In a 1998 article, Dewsbury noted that some modern primate researchers have come to refer to the practice jokingly as the "Yerkes transformation." Yet there is no evidence that Yerkes himself thought he was doing anything wrong. To the contrary, he presented a full account of his results as well as his reasoning for ignoring most of them. Given the similar problems with Yerkes's intelligence test work, it seems possible that he simply failed to keep pace with new developments in statistical analysis. It is also possible that his personal biases were so strong that he deluded himself about the appropriateness of his methods.

Later scientists still have not settled some of the issues raised by Yerkes. For example, Goodall observed that females were indeed groomed more often and were more successful at begging food during the estrus phase. Other scientists, however, have found no difference in male-female food sharing during estrus compared to other times. Dewsbury concluded that "recent data do not resolve either the issue of dominance reversals [during estrus] or the replicability of the effect described by Yerkes."


The informal observation of animal behavior is as old as humanity. Animals have been the subjects of systematic study since at least the time of the ancient Greeks. During the early years of the twentieth century, however, the study of animal behavior became more experimental in nature. Rather than just observing animals in the field, psychologists were now researching their behavior in laboratories, where conditions could be precisely controlled. It was a heady time for comparative and experimental psychology. Yerkes was one of the pioneers who helped map out this new direction.

Birth of a new discipline

Yerkes was a college student during the 1890s, the same decade in which comparative psychology first emerged as a separate discipline. In 1894, German philosopher-psychologist Wilhelm Wundt published his Lectures on Human and Animal Psychology, which helped establish animal research as a respectable field of study. That same year, British psychologist C. Lloyd Morgan published An Introduction to Comparative Psychology, which helped to set the agenda for animal studies to come. Among other topics, Morgan discussed habit formation and instinctive behavior.

In the late 1890s, at Clark University, Linus Kline and William S. Small began work that led to the first psychological studies of rats navigating mazes. Around the same time, at Harvard University, Edward L. Thorndike produced his classic thesis, "Animal Intelligence: An Experimental Study of the Associative Processes in Animals." Most of Thorndike's thesis dealt with learning in dogs, cats, and chicks. During this decade, then, the groundwork was laid for the kind of research on animal learning that would become so important in future decades.

By 1899, courses in comparative psychology were being taught at the University of Chicago and Clark University. Both Clark and Harvard University also had laboratories devoted to the field. The stage had been set for a period of rapid growth and progress, and Yerkes was poised to help lead the charge into the twentieth century.

The first years of the new century were a golden age in comparative psychology. In an article in American Psychologist, Dewsbury outlined three core issues that were explored during this period: the evolution of instinctive behaviors, the relationship between behavior and development, and the nature of intelligence and other higher mental processes. These issues are still at the heart of comparative psychology today. In fact, according to Dewsbury, "one might argue that all of 20th-century comparative psychology is but a footnote to this period and a series of attempts to resolve issues that were brought into focus at this time."

By the early 1900s, a whole generation of comparative psychologists was being trained at universities around the country. Early on, comparative psychology appeared to be headed for a central role in the still-young science of psychology. The leading psychologists of the day were eager to show that psychology was every bit as scientific as chemistry or physics. Comparative psychology, with its close ties to biology, seemed to be custom-made for this purpose. Soon, however, the tide began to turn. Psychologists started to focus on proving their usefulness by finding practical applications for their work. As Yerkes himself found during his tenure at Harvard, applied psychology became the surest path to job advancement. Comparative psychology suddenly seemed much less appealing to ambitious young psychologists.

Comparative psychology survived the crisis. For several decades, it served as a training ground for psychologists who went on to work in other fields. By the 1930s, comparative psychology had re-established itself as a separate discipline, although it never recovered the brief prominence it had enjoyed at the turn of the century. Today, it remains a relatively small specialty; it does overlap considerably with other fields, however, such as experimental psychology, physiological psychology, neuroscience, and ethology.

Key issues in comparative psychology

In the field's infancy, comparative psychologists needed to choose their ultimate goal. Should they study animal behavior for its own sake? Or should the overriding goal always be to shed light on human behavior? Early comparative psychologists quickly took sides on this issue, and some even kept one foot in both camps. For example, in the late 1890s, Kline designed a laboratory course at Clark University where the students studied animal instincts and habits, regardless of any relevance to humans. At the same time, however, Kline studied other animal behaviors from a decidedly human point of view. For example, he studied "the migratory impulse vs. love of home" in both humans and nonhuman animals.

Yerkes came down on the side of using animal research to reach insights into human psychology. This approach has sometimes come under attack, however. Some critics have argued that it blinds scientists to the true nature of other species, encouraging them to see animals merely as convenient stand-ins for humans. Others have argued that it may obscure the true essence of humanity, since it makes it harder to see which behaviors humans really do share with other animals and which are uniquely or primarily human.

Another critical issue in the early days of comparative psychology was deciding which species to study. Early comparative psychologists tended to study a wide range of animals. For example, Kline's course covered amebae, earthworms, slugs, fish, chicks, rats, and cats. By the 1920s, however, laboratory rats had become by far the most popular subjects. Fairly or not, comparative psychology earned a reputation as rat psychology. This reputation has proved hard to shake, despite the efforts of psychologists, including Yerkes, who extended their research to other species.

Yet another issue that needed to be settled was whether comparative psychology would be conducted in a laboratory or in the field. Laboratory studies had the advantage of offering greater control, although it was hard to say how the artificial setting might affect the results. Field studies offered a glimpse at more natural behavior, but the uncontrolled circumstances made it hard to sort out causes and effects. Laboratory research has largely won out in psychology. The laboratory studies of comparative psychologists have been complemented over the years, however, by the field observations of zoologists.

Evolution of instinctive behaviors

At the dawn of the twentieth century, the new field of comparative psychology was being heavily influenced by British naturalist Charles Darwin's theory of evolution. In 1872, Darwin had published a book titled The Expression of the Emotions in Man and Animals that was a forerunner of later writings on comparative psychology. In this book, Darwin suggested that many expressions of emotion are genetically based rather than learned. As such, they are the product of evolution, and their roots can be traced back to humankind's shared past with other animals. Therefore, neither emotions nor their expression are uniquely human. Darwin believed that other animals may experience some of the same emotions as humans. These animals also may display those emotions in ways that resemble the facial expressions and gestures of people.

Today, scientists are divided on whether Darwin was right about this matter. Some prefer to consider the expressions of animals strictly as communication signals. Others have no qualms about attributing emotions to animals. Even among the latter group of scientists, however, there is no clear agreement about which emotions animals feel, and whether the animals experience those feelings in the same way as humans do.

Whatever the final verdict on emotions in animals, Darwin's ideas raised a crucial question: Should humans be seen as just one species among many? Or should they be seen as unique and distinct from other animals? Many comparative psychologists, who wanted to draw parallels from animal to human behavior, took the first position. Wundt, for instance, wrote that "the mental life of animals shows itself to be throughout, in all its elements and in the general laws governing the combination of the elements, the same as the mental life of man." Others, however, cautioned against anthropomorphism—ascribing human thoughts and feelings to nonhuman animals.

Darwin also wrote about the process of natural selection, in which the fittest members of a species tended to survive longer and produce more offspring than other members. In the post-Darwin era, scientists began to look for genetically programmed behavior patterns that might aid survival. Such patterns, which develop without the need for learning, are known as instinctive behaviors. For example, Morgan studied newborn chicks and ducklings hatched in an incubator. He found that pecking, walking, scratching, preening, stretching up and clapping the wings, scattering and crouching when alarmed, and making a danger sound were all inborn behavior patterns. Chicks and ducklings arrived in the world already programmed to display such behaviors, although they sometimes needed practice to reach a high level of skill.

The evolution of behavior remains a core concept in comparative psychology to this day. While most psychologists now accept that some behaviors are instinctive, they also stress that the way these behaviors are expressed can be affected by development and experiences.

Animal learning as lab science

Morgan also studied the way animals learned brand-new behavior patterns that, like instinctive behaviors, became automatic and unconscious. He called this process habit formation. Morgan was mainly interested in answering two questions: how a new behavior pattern was learned in the first place and how it became automatic afterward. He broached a number of topics that later became central to comparative psychology. These topics included the role, if any, of consciousness in guiding animal behavior, the effect of imitation on learning, and the process of learning by trial and error.

Inspired by Morgan, turn-of-the-century scientists such as Thorndike, Small, and Kline began transferring the study of animal learning into the laboratory. In his groundbreaking thesis on animal intelligence, Thorndike defined intelligence in terms of an animal's ability to form new mental associations. He also described ingenious devices for studying animal learning and showed how they could be used in controlled research. For example, he constructed puzzle boxes for cats and then studied their behavior as they attempted to escape.

Earlier psychologists, writing about human thought and learning, had also focused on associations. For them, however, this meant the association of ideas or mental processes with one another. In contrast, Thorndike viewed associations as links between the situation in which an organism found itself and the organism's impulse to act. This view was an important step toward the behaviorists' concept of learning as the association between a stimulus and a response.

Around the same time, Russian physiologist Ivan Pavlov was studying the digestive systems of dogs. He noticed that dogs would salivate when they saw their keeper, apparently in anticipation of being fed. This observation led him to perform a classic series of experiments. Pavlov showed that, by repeated association, a previously neutral stimulus (such as a bell) could be substituted for a natural stimulus (such as food) to produce a natural response (such as salivation). This process became known as classical conditioning. Pavlov's work was first published in his native Russian. In 1909, Yerkes and Sergius Morgulis coauthored a paper that brought Pavlov's ideas to an English-speaking audience.

Thorndike's and Pavlov's theories laid the groundwork for the behaviorist revolution, led by Watson starting in 1913. Watson wanted to remove consciousness from the realm of psychology. Instead, he believed that psychology should focus strictly on behavior. Watson held that most behavior was the direct result of stimuli in the environment. In a nutshell, behavior that led to positive consequences was rewarded and continued, while behavior that led to negative responses was eliminated.

Later in his career, Watson began to focus more on the implications of behaviorism for humans. In the early years, however, he was concerned mainly with animal behavior, including behavior that was instinctive as well as learned. With Yerkes, Watson developed new methods for the study of color vision in animals. With psychologist Karl Lashley, he collaborated on studying terns, a type of sea bird, found on a cluster of islands off the coast of Florida.

As behaviorism took a more extreme turn in the coming decades, it branched off from comparative psychology. Eventually, the radical behaviorism of B.F. Skinner became so popular that behaviorist-based experimental psychology eclipsed its comparative cousin. It is worth noting, though, that the two disciplines grew up side by side in the first decades of the twentieth century. In fact, when the Journal of Animal Behavior was founded in 1911, Yerkes became the journal's editor, and Watson became the editor of an associated monograph series. Watson himself championed Yerkes for the editor's position, saying "there is no one else to do it who has the courage, the orderliness, and the persistence."


Yerkes won many accolades over the years. He also received his share of criticism, however. The most serious charge was that he let his personal beliefs and feelings override his objectivity. In work on group intelligence testing during World War I, Yerkes was criticized for the racial and ethnic bias that seemed to distort his analysis. In research on primate behavior, he was taken to task for the gender stereotypes that seemed to sway his findings.

Group differences in intelligence

Yerkes reached three controversial conclusions based on data gathered with the Army intelligence tests. First, he claimed that the average mental age in the United States was a mere 13 years. Second, he said there were genetically based racial differences in intelligence, with whites outperforming blacks. Third, he said there were also genetically based ethnic differences in intelligence within the white population, with individuals whose ancestors came from northern Europe surpassing those from southern or eastern Europe. In an article quoted by Dewsbury, Yerkes wrote:

If we may safely judge by the army measurements of intelligence, races are quite as significantly different as individuals . . . Almost as great as the intellectual difference between Negro and white in the army are the differences between white racial groups.

Yerkes published his massive report on these findings in 1921. Two years later, Carl Brigham, a young psychologist who had been one of Yerkes's assistants, published his own book on the subject. Titled A Study of American Intelligence, Brigham's book repeated many of the same claims made by Yerkes. Brigham also noted that immigration from southern and eastern Europe had been increasing in recent years. Based on the eugenicist views that he and Yerkes shared, and that were common at the time, Brigham warned that growing numbers of presumably inferior immigrants would further taint the gene pool in the United States. He urged that immigration restrictions be imposed before it was too late.

The next year, Congress passed the Immigration Act of 1924, which limited the number of immigrants who could enter the country. Strict quotas were set for each national group. Since the quotas were based on the makeup of the U.S. population in 1890, before the recent wave of immigration from southern and eastern Europe, the quotas for those areas were quite low. Public sentiment against immigration was strong enough that the bill probably would have passed in any case. Nevertheless, the support of respected psychologists such as Yerkes and Brigham certainly bolstered the cause.

Even at the time, however, their conclusions did not go completely unchallenged. Walter Lippman, a columnist for New Republic magazine, wrote a series of articles in which he ridiculed Yerkes's claim that the average intelligence of recruits was on par with that of a typical 13-year-old. At the same time, several reviews published in psychology journals commented on Brigham's tendency to neglect or dismiss data that did not agree with his interpretations. They also noted statistical oddities that called into question the validity of the data.

The strongest challenge, however, came from psychologists who embraced the views of Franz Boas, the leading American anthropologist of the time. Boas argued that many racial and ethnic characteristics were passed down from generation to generation not by heredity, but by culture, through shared values, language, and childrearing customs. One of the first researchers to apply this culture concept to group differences in intelligence test scores was Otto Klineberg, a graduate student in psychology who happened to study anthropology with Boas.

In 1926, Klineberg began working on his dissertation. While giving intelligence test items to Yakima Native American children in the state of Washington, Klineberg noticed that the children were almost completely unaware of time. Even when urged to hurry, they still took their time, but they also made relatively few mistakes. Here was a clear example of a cultural, rather than genetic, difference that would put the Yakima children at a disadvantage on any timed intelligence test. Yet it was unrelated to any real difference in mental ability. Instead, it was rooted in cultural values that equated speed with carelessness.

This experience attuned Klineberg to cultural factors affecting intelligence test scores. Soon, he followed up on his dissertation with studies of the psychological characteristics of African Americans and Native Americans. His 1935 book Negro Intelligence and Selective Migration argued that it was superior cultural and environmental advantages that caused northern blacks to score higher on intelligence tests than their southern black counterparts. He found that, when black students moved from racially segregated schools in the South, which usually were poorly funded, to integrated schools in the North, their intelligence test scores tended to improve. In fact, their scores rose to the level of northern-born blacks once they were in the integrated schools.

By the 1930s, most psychologists had conceded that culture and environment played a major role in causing group differences in intelligence test scores. Brigham even admitted that he had overstated the case for genetic differences. He acknowledged that the tests of the day assessed not only pure intelligence, but also knowledge of language and culture. In Brigham's own words (as quoted by Gould): "Comparative studies of various national and racial groups may not be made with existing tests. . .One of the most pretentious of these comparative racial studies—the writer's own—was without foundation."

Gender roles

Yerkes also had firmly traditional views on gender roles. In his unpublished autobiographical book, quoted by Dewsbury, Yerkes wrote that "women are more deeply concerned with the perpetuation of the species than are men; more wrapped up in the problems and chores, privileges and satisfactions of housekeeping." He also believed that, because of innate differences between women and men, "from birth educational practices should be adapted to sex as well as to individual characteristics."

Yerkes's views seem to have influenced the way he ran the Yale Laboratories of Primate Biology. There were no female scientists or students at the facility during his entire time as its director. In the 1930s, a female graduate student named Eleanor Gibson approached Yerkes about working with his chimpanzee colony. Yerkes told her that he did not allow women in his laboratory. This response was undoubtedly his loss, since Gibson went on to do important research on perceptual learning and development.

Critics have charged that Yerkes's ideas about gender roles also may have affected the way he interpreted the results of his chimpanzee studies. In studies of chimpanzee pairs where food was dropped into the cage through a chute, Yerkes claimed that males ordinarily controlled access to the food due to dominance. Females only got to take control when the males granted them that privilege in return for sex. In Chimpanzees: A Laboratory Colony, Yerkes wrote that "the patterns of dominance and privilege differ notably. In the former, action tends to be prompt, clear-cut, decisive, commanding or demanding, while in the latter it more often is delayed, tentative, questioning, or suggestive of inhibition." Yerkes saw the males as naturally more active, and the females as more passive.

In Yerkes's anthropomorphic descriptions of the animals' behavior, the females often sound rather silly, while the males come across as patronizing. Consider this description of Jack and Josie, in which both chimpanzees are trying to gain access to the food chute:

Presently he came to the chute ready for the experiment. She came also and attempted to take control, but he gently shouldered her to one side . . . Jack did not seem irritated by Josie's assumption of right to take control; instead, by playful, gentle, and good-natured tactics he managed to dominate and have his own way.

Several feminist authors have taken Yerkes to task for his choice of language. They have also objected to the implied message that his findings reveal a broader truth about primate (including human) relationships; namely, that males are naturally dominant and females naturally submissive. For example, in her 1948 book Adam's Rib, Ruth Herschberger wrote satirically:

On March 15, 1939, as Josie stood resolutely beside the food chute, she little realized that she had become representative of all womanhood, a model upon which personnel directors and police captains could in the future base their decisions and argue their case. Nor did her cage-mate, Jack, as he elbowed her gently aside, realize that he was from that moment the incarnation of the dominant male, an inspiration to all humans who sought "friendly masculine ascendancy" over their womenfolk.


Yerkes helped establish comparative psychology as a scientific discipline in its own right. Beyond that, the Army intelligence tests that his committee developed during World War I became the model for future group tests of mental ability. In addition, his studies of chimpanzee behavior laid the groundwork for modern research at nonhuman primate laboratories.

Group tests of mental ability

Research The Army Alpha and Beta tests opened the floodgates to a host of other group tests of mental ability. Today, most U.S. students are assessed with at least one of these tests at some point while in school. The SAT is just one familiar example. This sort of test is typically given to a whole class of students or group of individuals at once. The test often consists of numerous multiple-choice questions, which are answered on a special answer sheet that can be scored by a machine.

One of the big advantages to such tests is that they are standardized. This means that the tests themselves have undergone extensive testing before ever being used in an actual classroom or other real-life situation. During the development phase, a test is given to a representative sample of individuals under clearly spelled-out conditions, and the results are scored and interpreted according to set criteria. The goal is to establish a standardized method of giving, scoring, and interpreting the test in the future. This helps ensure that as much as possible of the variance in scores will be caused by true differences in ability, and not by differences in the testing procedure.

Norms are also provided to help with the interpretation process. These are the test results gathered from a particular group of test takers during the development phase. The norms can then be used as benchmarks for interpreting individual test scores in the future. Depending on the test, different kinds of norms may be provided. For example, age norms and grade norms indicate the average scores of a group of test takers who are of a certain age or in a particular grade.

The other major advantage to group tests is their efficiency. It might take hundreds of hours for a skilled examiner to administer 100 individual intelligence tests. In contrast, it might take just a few hours to give a group test to an entire roomful of people. Clearly, group testing is much less expensive and time-consuming. In fact, without the development of group tests, intelligence testing would never have become the large-scale industry that it is today.

Group tests also have some disadvantages, however. The group setting makes it impossible to take into account individual factors—such as being sleepy, sick, uncooperative, or anxious—that might affect a person's score, but that have nothing to do with his or her intelligence. The group format also does not allow an examiner to note why a particular answer was chosen or question was skipped. It simply scores how many correct answers were chosen. No distinction is made between questions that were missed because the person simply did not know the answer and those that were missed because the person could not read them, did not understand them, or simply was taking his or her time in an effort to avoid careless mistakes.

Another drawback relates to the multiple-choice format that these tests favor. Multiple-choice questions may call for the use of different psychological strategies than the open-ended questions often found on individual tests. For one thing, multiple-choice questions, which are based on the assumption that there is one right answer, may penalize creative thinkers, who often see the same problem from many different angles. Nevertheless, research has shown that scores on the best group tests are generally highly correlated with those on individual tests. In other words, a person who gets a certain score on a group intelligence test is likely to also get a similar score on an individual intelligence test.

The mass testing of mental abilities remains controversial. Yet many organizations have concluded that the pros outweigh the cons. Group tests assessing various mental abilities have become a fixture in American society. These are just a few common examples:

  • Multidimensional Aptitude Battery. This is a test of general thinking ability, designed to be given to groups of adolescents or adults. It is an adaptation of the Wechsler Adult Intelligence Scale–Revised (WAIS–R), the most widely used individual test of adult intelligence.
  • Cognitive abilities tests. These are two distinct group tests designed to assess the general mental ability of schoolchildren.
  • SAT. This is a test of general scholastic ability that is used to help colleges make decisions about which students to admit.
  • Graduate Record Examinations (GRE). These scholastic ability tests are used to make graduate school admission and placement decisions.
  • Armed Services Vocational Aptitude Battery. This is an example of a test that attempts to measure several specific aptitudes. It is used to screen military recruits and help place them in appropriate jobs.
  • General Aptitude Test Battery. This test also assesses several aptitudes. It was developed by the U.S. Department of Labor and is currently used by the U.S. Employment Service to help guide job placements.

Certainly, a huge amount of data has been amassed over the years on the validity and reliability of various group intelligence tests. In addition, great strides have been made in the way such tests are standardized and normalized. Nevertheless, the underlying philosophy and basic procedures for most group tests still bear a strong family resemblance to their ancestor: Yerkes's World War I Army tests.

The SAT While all group intelligence tests owe a debt to Yerkes, one test has a more direct link to him. The original version of the modern SAT was developed by Brigham, Yerkes's junior colleague in the Army testing program. Soon after the war, Brigham began adapting the Army Alpha test for use in screening college applicants. In the 1920s, Brigham first tried out his new test on freshman at Princeton University and applicants to the Cooper Union, an all-scholarship college in New York City.

Several years earlier, in 1900, the College Entrance Examination Board had been founded. The board was set up by the presidents of a dozen leading universities, who sought to simplify the application process for the benefit of both prospective students and admissions officers. In order to do that, the board wanted to devise a common entrance exam that could be used by all the universities. That way, an applicant would have to take only one entrance exam, rather than a separate test for each school to which he or she applied. At first, the exam consisted of essay tests in specific subject areas. When the board heard about Brigham's research, however, they put him in charge of a committee, which was asked to develop a test that could be used by a broad range of colleges as an objective measure of academic potential. The test also needed to streamline the admissions process and level the playing field for students from a wide variety of backgrounds.

In 1926, Brigham's test, which later came to be known as the SAT, was given to high-school students for the first time. Then, in 1933, officials of Harvard University set out to find a way of evaluating candidates for a new scholarship program. The program was intended to help academically gifted young men who had not graduated from the elite Eastern boarding college preparatory schools that supplied most of Harvard's students. The officials settled on Brigham's test, because they thought it measured pure intelligence rather than the quality of a student's high-school education. By the late 1930s, the SAT was being used as a scholarship test by all of the prestigious Ivy League schools.

Use of the SAT soon spread beyond its Ivy League roots, and the test remains very widely used today. In fact, in 2003, a record 1.4 million high school seniors took it. Yet, in spite of—or perhaps because of—the SAT's popularity, the test has been a lightning rod for controversy over the decades. Critics have charged that the test systemically underestimates the academic ability of females, applicants over age 25, and those whose first language is not English. In addition, some studies have shown that the SAT does not predict college performance—such as freshman grades, undergraduate class rank, college graduation rates, or attainment of a graduate degree—as well for black students as it does for white ones.

In general, studies have shown that high school grades are better predictors of college grades than SAT scores are. The SAT still does a fair job of predicting how well a college freshman will perform, however. When SAT scores and high school grades are both used, their combined predictive ability is slightly better than that of grades alone. One problem with using grades alone is that they are less comparable, since they may reflect not only a student's ability, but also the difficulty of the courses the student has taken and the standards of the school. On the other hand, SAT scores alone can not reveal anything about a student's motivation or work habits. Therefore, most psychologists currently recommend that, if SAT scores are used at all, they should be combined with grades, portfolios, or other evidence of academic potential.

As an interesting aside, it is worth noting that a version of the SAT introduced in 2005 includes a new essay-writing section. In part, then, the test has come full circle. Yerkes and his followers introduced the idea that large groups of people could be tested and compared quickly using objective methods. Many people still believe that group tests can be quite useful as an efficient screening tool. Even advocates of this approach recognize that it has its limits, however. To fully assess any individual's capabilities, it is necessary to look at other dimensions besides a test score.

Case studies Several colleges and universities have studied the validity of the SAT. The aim of such studies is to measure the predictive power of SAT scores for that particular college's student body. The College Board (the current name for the College Entrance Examination Board) encourages such research through its Validity Study Service. The service itself has come under fire in recent years however. Critics claim that it encourages the use of flawed research methods that overstate the SAT's benefits.

Nevertheless, validity research at individual institutions has generally found that the SAT has relatively weak predictive ability. The National Center for Fair and Open Testing (nicknamed FairTest), an organization that opposes the misuse of standardized testing, has documented some of the less encouraging results. For example, researchers at the University of Pennsylvania looked at high-school class rank, scores on the SAT I (the main test), and scores on the SAT II (optional subject area tests). They compared all these factors to students' cumulative grade point average (GPA) in college. The researchers found that the SAT I was the poorest predictor of all, explaining a mere 4% of the variation in college grades. The best predictor was high-school class rank, but it still explained just 9% of changes in cumulative GPA. Even when SAT scores and class rank were combined, they accounted for only 11% of the variation, failing to explain almost 90% of the variation in college grades.

Relevance to modern readers High-stakes tests are tests that are used to make major decisions about a student, such as promotion to the next grade, graduation from high school, or admission to college. The use of standardized tests for such purposes started with Brigham's SAT, and it has grown to massive proportions over the decades. The No Child Left Behind Act of 2001 promised to spur even more growth in the mass testing movement. The act required that each state give standardized tests of language arts and mathematics to all third- and eighth-graders by 2005. In 2007, standardized tests of science were to be added to the mix.

Some psychologists and educators oppose such mass testing on principle. Others, however, believe the tests could be beneficial, but only if they are well designed and fairly used. According to the APA's Code of Fair Testing Practices in Education, professionals are obliged

to provide and use tests that are fair to all test takers regardless of age, gender, disability, race, ethnicity, national origin, religion, sexual orientation, linguistic background, and other personal characteristics . . . Fairness implies that every test taker has the opportunity to prepare for the test and is informed about the general nature and content of the test, as appropriate to the purpose of the test. Fairness also extends to the accurate reporting of individual and group test results.

Such issues are obviously very important to students, whose lives may be negatively affected by unfair or misleading test scores. Another concern for students is the possibility that high-stakes group testing may produce "teaching to the test." In other words, schools and teachers may start focusing on the narrow range of skills assessed by the test in order to raise scores, while neglecting other equally important areas of study. When this happens, it is students who are the ultimate losers, since they may miss out on the benefits of a well-rounded education.


1876: Born on May 26 in Breadysville, Pennsylvania.

1897: Graduates from Ursinus College in Pennsylvania.

1898: Earns an A.B. degree from Harvard University.

1902: Receives a Ph.D. in psychology from Harvard.

1902–1917: Teaches comparative psychology at Harvard.

1908: Publishes the Yerkes-Dodson law, developed with John Dodson, which related the strength of a stimulus to the speed of avoidance learning.

1911: Founds the Journal of Animal Behavior, the first U.S. scientific journal devoted solely to animal behavior research.

1913–1917: Works half-time as a psychologist in the Psychopathic Department at Boston State Hospital.

1915: Introduces a point scale for measuring intelligence, developed with J.W. Bridges.

1917: Elected president of the American Psychological Association. Became a member of the National Research Council.

1917–1918: Chairs a committee that developed the Army Alpha and Beta intelligence tests during World War I.

1919–1924: Works for the National Research Council.

1923–1924: Raises a bonobo and a chimpanzee in his home.

1924–1944: Holds a post as professor of psychobiology at Yale University.

1929: Publishes The Great Apes: A Study of Anthropoid Life, coauthored with his wife, Ada Watterson Yerkes.

1929–1941: Founds and directs the Yale Laboratories of Primate Biology, the first laboratory for nonhuman primate research in the United States.

1956: Dies on February 3.

The flip side of this issue is the question of whether students who take test preparation classes can substantially raise their test scores. Experts differ in their estimation of how much such classes really help. FairTest claims, however, that a good coaching program can raise an individual student's scores by 100 points or more. Since many such programs offered by private companies are quite expensive, FairTest believes that this adds an element of income bias to the SAT. It also implies, however, that use of a test preparation service—whether it is a pricey class or a free tutoring program—may give students an edge on the test.

Primate research

Research While group intelligence testing was an important sidelight in Yerkes's career, animal research was really his central focus. In particular, Yerkes broke new ground by establishing the first U.S. laboratory exclusively for the study of nonhuman primates. This accomplishment opened the door to future psychological and medical research in apes and monkeys.

Today, the Yerkes laboratory is one of eight National Primate Research Centers funded by the National Institutes of Health. The goal of these centers is to establish nonhuman primate models of human health and disease for biomedical research. All of the current centers are affiliated with academic institutions. They are devoted to primate research related to major human diseases, such as AIDS, cancer, Alzheimer's disease, Parkinson's disease, and cardiovascular disease. At the Yerkes National Primate Research Center, current research interests include aging, AIDS, drug addiction, malaria, Parkinson's disease, transplantation, and vision disorders. They also include primate evolution and social behavior—two subjects that have long been at the core of comparative psychology.

The evolutionary studies currently being done at the Yerkes center make use of the latest tools and techniques to compare humans with chimpanzees. Some studies use modern technologies that would astonish Yerkes, including sophisticated brain imaging techniques and computer-based tests of learning and memory. The center also makes use of genomic mapping to identify differences in DNA among primate species. It turns out that human nuclear DNA is 98.4% identical to that of chimpanzees.

Some studies of social behavior use advanced neuroscientific techniques to study behavior at the molecular and cellular level. The goal is to learn more about the nerve cell mechanisms underlying social behaviors. Conversely, the researchers also hope to understand how social experience may affect the anatomy and physiology of the developing brain.

As valuable as these techniques are, however, they will never replace careful observations of animals under more natural circumstances. Observational studies are still a cornerstone of comparative research. Yerkes's scientists are also conducting studies of the complex social structure of ape society. It appears that apes and humans have a lot in common when it comes to their social psychology, including their tendency toward both aggression and social cohesiveness. Chimpanzees even have their own version of "you scratch my back, and I'll scratch yours." Scientists have noted that one chimpanzee will sometimes do a favor for another in exchange for favors received, a process known by researchers as reciprocal exchange.

Case studies Among the most fascinating published case reports are studies of apes born at the Yerkes Field Station. For example, American psychologist Sue Savage-Rumbaugh has studied language ability in bonobos. She is best known for her work with Kanzi, the first ape to learn language in the same manner as human children.

Kanzi was born in 1980. He was raised by a female bonobo named Matata. The pair came to the Georgia State University Language Research Center when Kanzi was six months old. For the next two years, researchers worked every day with Matata, trying to teach her to communicate with lexigrams, symbols that were composed of shapes and lines presented on a keyboard. Kanzi was always with his mother, but he did not seem particularly interested in the lessons. Meanwhile, Matata was making very slow progress, and the researchers were becoming discouraged.

When Kanzi was two-and-a-half years old, the Yerkes center requested that he be weaned from his mother so that Matata could be returned to the field station for a brief visit. While his mother was gone, Kanzi unexpectedly began using the lexigrams on her keyboard. Although he had never before shown any sign of interest in the lexigrams, he had apparently learned to connect them with both the objects in his world and the spoken English words that the symbols represented. This was the first time that an ape had ever been shown to match specific words spoken by a trainer to corresponding symbols. The fact that Kanzi had accomplished this without specific training was quite remarkable.

Kanzi's keyboard originally had just 10 lexigrams. Gradually, however, the researchers began adding new symbols and using them in conversations with Kanzi. Gestures, pictures, videotapes, and activities were all used to help the animal learn to associate the right words with the lexigrams. The researchers claim that Kanzi eventually learned to understand 500 words and use more than 200 lexigrams on his keyboard. These claims are backed up by studies done under controlled conditions.

Yerkes would undoubtedly have been very gratified by such innovative primate research. His own contribution has by no means been forgotten. In a tribute to the


Harlow's monkey infant experiment

Harry Harlow (1905–81) was an American comparative psychologist who made his mark by studying mother love in monkeys. In 1930, Harlow joined the faculty at the University of Wisconsin, where he established the Psychology Primate Laboratory. At the time, it was widely believed that humans and other social animals lived in organized groups mainly for the purpose of regular sexual contact. Harlow had a different idea: that mother love and social ties might also be important.

In 1957, Harlow began working with rhesus monkeys, which are more mature at birth than human infants, but which nonetheless are similar in development. In a series of landmark studies, Harlow separated young rhesus monkeys from their natural mothers, giving them instead two artificial substitutes: one made of wire, and the other made of cloth. Even when the wire "mother" was outfitted with a bottle for feeding, the infant monkeys showed a clear preference for cuddling with the softer cloth "mother," especially when they were scared. In related studies, Harlow showed that monkeys who were deprived of maternal contact and comfort as infants grew up to be poor mothers themselves.

Harlow also showed that young monkeys who were raised with real mothers and young peers naturally learned to play and get along with other monkeys. Those that were raised with real mothers but no young playmates were often fearful or inappropriately aggressive, while those raised without either real mothers or peers were socially inept and often unsuccessful at mating as adults. Taken as a whole, Harlow concluded that his studies showed that society was not based on sex alone. He also found that mother love by itself was not enough to help a youngster grow up to be socially competent. Instead, normal parenting and mating behavior as adults depended on both healthy maternal and peer contacts early in life.

center that bears his name, Savage-Rumbaugh and her colleagues have dubbed the lexigram language "Yerkish."

Relevance to modern readers Nonhuman primates are humans' closest relatives in the animal world. As a result, they share many characteristics with humans, including complex communication systems, long-lasting social relationships, and the use of tools. Studying the psychology of nonhuman primates can teach people about their own psychological nature (see accompanying sidebar).

Humans and nonhuman primates also share a similar physiology. By studying the brains of monkeys and apes, researchers have gained insight into how the human brain works. In addition, primate research has been crucial to understanding biological processes, such as reproduction, and medical conditions, such as AIDS and addiction. Over the years, primates have also been used in Nobel Prize-winning research; some of these studies have resulted in a yellow fever vaccine (1951), a polio vaccine (1954), and key discoveries about visual processing in the brain (1981).

One continuing concern is the ethical treatment of animals used for research purposes. Several safeguards have been put in place to help prevent abuses, however. Four federal government agencies—the U.S. Department of Agriculture, the U.S. Public Health Service, the National Research Council, and the Food and Drug Administration—regulate different aspects of animal research. In addition, the APA has issued its own Guidelines for Ethical Conduct in the Care and Use of Animals. According to these guidelines, "psychologists should conduct their teaching and research in a manner consonant with relevant laws and regulations. In addition, ethical concerns mandate that psychologists should consider the costs and benefits of procedures involving animals before proceeding with the research."

When these high standards are met, primate research can be of great benefit to both psychology and society at large. No single individual has had a greater impact on primate research in the United States than Yerkes. It is fitting that his namesake laboratory continues to carry on the work that was dearest to his heart. In an autobiographical essay written around the same time that his primate lab opened, Yerkes wrote:

It is as if I am now on the threshold of a great undertaking which from the first was dimly envisaged and later planned for with increasing definiteness and assurance . . . It promises the fulfillment of my persistent dream for the progress of comparative psychology and the enhancement of its values to mankind through the wise utilization of anthropoid apes and other primates as subjects of experimental inquiry.



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American Psychological Association Joint Committee on Testing Practices. Code of Fair Testing Practices in Education. 2004 [cited May 18, 2004].

Burkhardt, Richard W. Jr. "The Journal of Animal Behavior and the Early History of Animal Behavior Studies in America." Journal of Comparative Psychology 101 (1987) 223–30.

Christianson, Sven-Åke. "Emotional Stress and Eyewitness Memory: A Critical Review." Psychological Bulletin 112 (1992) 284–309.

Dewsbury, Donald A. "Comparative Psychology and Ethology: A Reassessment." American Psychologist 47 (1992) 208–15.

Dewsbury, Donald A. Comparative Psychology in the Twentieth Century. Stroudsburg, PA: Hutchinson Ross Publishing Company, 1984.

Dewsbury, Donald A. "Issues in Comparative Psychology at the Dawn of the 20th Century." American Psychologist 55 (2000) 750–53.

Dewsbury, Donald A. "Robert M. Yerkes: A Psychobiologist With a Plan." In Gregory A. Kimble, C. Alan Boneau, and Michael Wertheimer, eds. Portraits of Pioneers in Psychology, Volume 2. Washington, DC: American Psychological Association, 1996.

Dewsbury, Donald A. "Robert Yerkes, Sex Research, and the Problem of Data Simplification." History of Psychology 1 (1998) 116–29.

Fancher, Raymond E. The Intelligence Men: Makers of the IQ Controversy. New York: W. W. Norton, 1985.

Gould, Stephen Jay. The Mismeasure of Man. Rev. ed. New York: W. W. Norton, 1996.

Herschberger, Ruth. Adam's Rib. New York: Harper and Row, 1948.

Plucker, Jonathan. Robert Mearns Yerkes. Indiana University. 2003 [cited May 18, 2004].

Winton, Ward M. "Do Introductory Textbooks Present the Yerkes-Dodson Law Correctly?" American Psychologist 42 (1987) 202–3.

Yerkes, Robert Mearns. "Autobiography of Robert Mearns Yerkes." In Carl Murchison, ed. History of Psychology in Autobiography, Volume 2. Worcester, MA: Clark University Press, 1930, 381–407.

Yerkes, Robert Mearns. Chimpanzees: A Laboratory Colony. New Haven, CT: Yale University Press, 1943.

Yerkes, Robert Mearns and John D. Dodson. "The Relation of Strength of Stimulus to Rapidity of Habit-Formation." Journal of Comparative Neurology and Psychology 18 (1908) 459–82.

Yerkes, Robert Mearns and Sergius Morgulis. "The Method of Pavlov in Animal Psychology." Psychological Bulletin 6 (1909) 257–73.

Further readings

American Society of Primatologists. 2004 [cited May 18, 2004].

CARE: APA Board of Scientific Affairs, Committee on Animal Research and Ethics. American Psychological Association. 2004 [cited May 18, 2004].

College Board. 2004 [cited May 18, 2004].

Georgia State University Language Research Center. 2004 [cited May 18, 2004].

Green, Christopher D., ed. Classics in the History of Psychology. York University [cited May 18, 2004].

Goodall, Jane. Through a Window: My Thirty Years With the Chimpanzees of Gombe. Reissue. Boston: Houghton Mifflin, 2000.

Jane Goodall Institute. 2004 [cited May 18, 2004].

Murphy, Kevin R., and Charles O. Davidshofer. Psychological Testing: Principles and Applications, 5th ed. Upper Saddle River, NJ: Prentice-Hall, 2000.

National Center for Fair and Open Testing. 2004 [cited May 18, 2004].

National Primate Research Centers. National Institutes of Health. 2004 [cited May 18, 2004].

Primate Info Net. Wisconsin Primate Research Center, University of Wisconsin-Madison. 2004 [cited May 18, 2004].

Savage-Rumbaugh, Sue and Roger Lewin. Kanzi: The Ape at the Brink of the Human Mind. New York: John Wiley and Sons, 1994.

Secrets of the SAT. PBS Frontline. 1999 [cited May 18, 2004].

Testing and Assessment. American Psychological Association. 2004 [cited May 18, 2004].

Yerkes National Primate Research Center. Emory University. 2002 [cited May 18, 2004].

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