I. INFANT DEVELOPMENTHarriet L. Rheingold
II. THE EFFECTS OF EARLY EXPERIENCEEverett W. Bovard
Infancy is the period of time that begins with the organism’s emergence into the world as a separate being and ends with the organism’s achieving a measure of independence. Because the organism is immature and environmental stimulation is new, infancy is the simplest and clearest arena in which the controversy of nature and nurture has been waged. This controversy is one main issue in the study of the origins of behavior. Another main issue is the effect of early experience upon later behavior. A study of the origins of behavior, however, can transcend these issues: the processes by which an infant organism comes to learn the nature of the world into which it is born and to adapt its behavior to the physical and social properties of that world merit scrutiny in their own right. That the infant changes so rapidly in structure and function, in physical growth and behavioral achievement, does not render the study impossible. Rather, these characteristics of the immature organism contribute to the challenge: to discover even in infancy the principles governing behavior.
The facts and theories concerning the origins of behavior in infancy can be organized in many ways. Change with age constitutes one way. The development of different classes of behavior—sensory, motor, social, and language—constitutes another. A historical account of methods employed in the study of infancy, from the anecdotal to the experimental, constitutes a third. These are traditional ways and may be found in textbooks of child and developmental psychology. The contributions of different theoretical schools—for example, psychoanalytic theory, behavior and learning theory, and the theories of Piaget—to developmental psychology would also provide an interesting system of organization.
The present account uses still a different system of organization. Here, the facts and theories concerning the origins of behavior in infancy will be organized under four main principles: (1) the infant is responsive to stimulation; (2) the infant is an active organism; (3) the infant’s behavior is modifiable; and (4) the infant modifies the environment, particularly the social environment.
This organization possesses the advantage of carrying the account to the advancing margin of developmental psychology and of starting with the intact organism, whole, living, and already behaving. It also bypasses old controversies. Furthermore, since the principles apply to the behavior of older as well as younger organisms, they will serve to establish continuity between the infant, child, and adult. And, since they apply to the behavior of all living organisms, they will serve to establish continuity between man and animal.
The principles challenge the notion, still extant, that the infant is insensitive to environmental stimulation, that his behavior is random and uncoordinated, that he is passive rather than active, that sleep and ingestion are his most important activities, that his behavior patterns are innate and fixed, and that he has no power to affect his environment. Yet, it would be inaccurate to assert, as do some in protest to the older notions, that the infant is competent. Although he is responsive, active, and profits by experience, although he becomes increasingly competent in handling his environment, he is nevertheless still deficient in skills, limited in experience, and wanting in language.
The period of time covering infancy requires a word of definition. Birth marks its beginning, to be sure, but what of its end? The end differs with the species; in general, the more intelligent the species, the longer its period of infancy. Still, the end of infancy is a matter of opinion. To set the’ end for the human infant by the calendar does not allow for varying rates of development. To set the end by developmental stage is more reasonable, but there is no agreement on the criteria. Here, for the human infant the criterion of speech is proposed, that is, speech adequate to obtain simple wants. It is an arbitrary criterion, to be sure, but it does correspond to the beginning of what Pavlov termed the “second signal system.” For the infant “reality is signalized almost exclusively by stimulations … which come directly to the special cells of the visual, auditory, or other receptors of the organism” ( 1955, p. 262). The infant is a nonverbal organism; he vocalizes, but he does not speak; he communicates but not with words.
Behavior, then, is the topic of this account of infancy. The physiological and neurological substrates of behavior are largely ignored, although their relevance is plain. The behavior of the human infant, furthermore, is the topic of discussion. Behavior in infancy regardless of species is another topic; a comparative psychology of infancy lies beyond the scope of this article. And, even then, behavior in the first few months of life will receive more attention than that in later months.
The infant is responsive
The first principle is that the human infant is responsive to a wide range of external stimulation. At birth, or shortly after, every sense of the neonate is functioning, although it must be conceded that the neonate’s sensitivity is not commensurate with that of later life (Peiper  1963, p. 92). Furthermore, from birth the infant possesses almost all the sensory systems of the adult human. In this respect he differs from such mammals as, for example, the rat, dog, and cat, whose young are born blind and deaf. As a consequence, the human infant from the beginning lives in a broad environment.
The nature of the environmental stimuli to which an organism responds tells us not only which sensory systems are functioning but also which experiences can affect its behavior. Stimuli are called upon to serve several functions in any theory of behavior: they elicit behavior, they reinforce behavior, and they serve as discriminative stimuli for behavior (i.e., they signal whether the behavior will be reinforced). Thus, if we wish to study the effects of early experience on later behavior, we must know the stimuli to which the infant organism is responsive.
Although motor responses, such as eye movements or changes in bodily activity, have been most frequently used as indicator responses, physiological responses, such as changes in respiration and heartbeat, and neurological responses, such as the electroretinogram and the electroencephalogram, have also been employed. Conditioned-response studies of the classical type can also supply information on the infant’s sensitivity to particular stimuli and his ability to discriminate between two similar stimuli. Instrumental, in distinction to classical, conditioning could supply similar information by the manipulation of reinforcing and discriminative stimuli, although it has not yet been so employed. The methods of demonstrating sensitivity, therefore, are varied, but the indicator response is always a change in behavior or function in relation to a stimulus.
In the following partial account of the infant’s sensitivities, attention will be paid to method as well as to results. Munn (1955), Peiper (1949), and Riesen (1960) offer more complete accounts and also wrestle with the difficulties encountered in measuring the sensitivities of so response-limited an organism as the human infant.
The skin senses
Although there is no clear evidence of pain sensitivity at birth, most infants react to cutaneous irritation by the end of the first week of life. Sensitivity to pain increases rapidly, however, and by the end of the first year the infant localizes the site of stimulation and deliberately withdraws a stimulated extremity. The face is the most sensitive area. Pinpricks are the common experimental stimuli, and the measures are crying, changes in general activity, widening of the eyes, and turning of the face, limb, or body away from the stimulus, often accompanied by a visual search for the spot being irritated.
The infant is also responsive to tactile stimulation, especially about the mouth and face. He turns his head toward a touch near his mouth; this has been called the rooting or search reflex and is especially prominent when he is hungry. The infant is also responsive to the stimulation provided by a nipple within his mouth; to such stimulation he responds by sucking.
The infant is sensitive to heat and cold, to the lowering or elevating of ambient temperature, and to warm and cold stimuli in the mouth, on the skin, or close to the skin. Locally circumscribed temperature stimulation evokes turning of the stimulated body part toward or away from the stimulus, as well as such general responses as restlessness, grimacing, and crying [seeSkin senses and kinesthesis].
The newborn can clearly distinguish sweet from salty, bitter, and sour substances, but, according to Peiper (1949), there is no evidence that he can differentiate between salty, bitter, and sour substances. Measures used by different investigators are sucking and changes in fontanel pulse, respiration, body movement, and facial expression. Reported results in this area, as well as in many others, are often sketchy and sometimes contradictory. Several factors contribute to such a state of affairs: the condition of the infant at the time of testing; whether it is awake or asleep, quiet or disturbed; the number of trials; the order of stimuli and interactions based on the order; the variability of the response; and the extended latency period characteristic of the immature organism. On the basis of clinical observation, Peiper feels that the taste sense of the infant is far more delicate than the experimental evidence indicates.
The determination of the infant’s sensitivity to smell has presented the methodological problem of ensuring that only olfactory receptors and not those of taste and pain are being stimulated. The indicator responses often include facial and bodily signs of aversion or pleasure. Studies carried out in the early 1960s (e.g., Engen et al. 1963) employing changes in leg motion, general bodily activity, respiration, and heart rate showed that newborn infants responded to the smell of acetic acid, asafetida, phenylethyl alcohol, and anise oil and that thresholds of sensitivity appeared to decrease during the first four days of life. These, along with older studies (see Munn 1955), demonstrate the infant’s sensitivity to olfactory stimuli [seeTaste and smell].
The eyes of the infant are relatively large for his size, compared with the eye of the adult, and are anatomically functional at birth. From the beginning, the pupils react to light. The infant’s eyes also fixate on light and track it briefly, although not always with conjugate movements. He responds to bright light with the eye-neck reflex (throwing the head back), with a decrease in bodily activity, and with changes in respiration, showing thereby that he discriminates different intensities of illumination.
Movement. Visual pursuit of moving objects is not fully developed at birth, but by 10 to 15 days of age infants do follow moving objects briefly after some lag (Kistiakovskaia 1959). At 3 to 4 weeks of age, pursuit is smooth, and the head, as well as the eyes, turn. At this age, infants will look from one object in the environment to another, and at 2 to 3 months of age they will follow the transit of a dangling ring moved in arcs from right to left and from head to foot.
Patterns, objects, depth. From the first month of life the infant can see equally well at distances of 5, 10, and 20 inches; he can also see black and white stripes as narrow as 1/8 inch at a 10 inch distance, acuity corresponding to a visual angle of a little less than one degree. The infant, from the first days of life, also discriminates patterns, looking longer at simple black and white drawings than at plain colored surfaces. In the first few months of life, furthermore, the infant looks longer at complex patterns than at simple ones and longer at a solid sphere than a flat circle (Fantz 1961). These data were obtained by the stimulus preference method: a longer duration of regard for one of two simultaneously presented visual patterns was assumed to indicate a preference and hence the ability to discriminate. A variation of the method, contrasting regard of familiar and novel stimuli, yielded similar results for patterns (Saayman et al. 1964) and for objects of different shapes (Venger 1962). The discrimination method in which the subject is rewarded for selecting one of two (or more) stimuli would probably constitute a more convincing test of the ability to differentiate; it is, however, a more difficult and time-consuming method, requiring training over many trials. Ling (1941) employed this method in training infants six months of age to discriminate a circle from other forms by making the circle sweet to the taste and movable. Thus, all the evidence suggests that from a very early age the infant prefers patterned to nonpatterned visual stimuli and that shortly thereafter he discriminates the form and shape of objects.
By the time the infant can locomote on his own, he perceives a drop-off (the visual cliff) and tends to avoid it (Walk & Gibson 1961); his tendency to avoid the drop-off increases as the depth increases, further evidence that the infant is sensitive to visual differences in surface texture.
Color. There is considerable evidence to suggest that infants also perceive color. At three and four months of age they look longer at colored than at gray stimuli (Spears 1964). Older children reach more often for colored than gray objects and select red and yellow more often than green and blue. Munn (1955) discusses the methodological problems in equating the brightness of different hues for the infant eye; this reservation aside, it seems likely that the infant perceives color [seePerception, articles onPerceptual developmentanddepth perception; Vision].
There is no doubt that the infant is sensitive to sound. From birth he responds to sound, especially to loud noises, by blinking and starting, by changes in respiration and general bodily activity, or, occasionally, by turning head and eyes to the source of sound. He responds to sounds of lower intensity, too; an absolute threshold between 38 and 48 decibels of a tone of 1,000 cycles per second has been found by measuring cardiac acceleration (Bartoshuk 1964). During the first month of life, according to normative studies (Bayley 1933), the infant responds to the click of a telegraph snapper by blinking, frowning, starting, increasing or decreasing activity, or crying; during the second month he responds to the speaking voice by head turning, vocalizing, or a ceasing of activity; and in the fourth month he will turn his head toward a sound made at his right or left side [seeHearing].
This account of the infant’s sensitivities to stimulation summarizes what is presently known as the result of controlled investigations. It is obvious that it would be unwise to declare that the neonate lacks this or that sensitivity, for the statement is always dependent upon the method. Careful probing with improved techniques almost always yields evidence of keener sensitivity than had been suspected.
In general, sensitivity to certain stimuli means not only that these stimuli can elicit behavior but also that they can reinforce behavior and serve as discriminative cues. It is these additional functions of stimuli which are important for the third principle, that the infant’s behavior is modifiable; the evidence suggests that stimuli in all the sense modalities can affect the infant’s behavior.
The infant is an active organism
The chief characteristic of a young organism, in distinction to an older one, is its almost constant physical activity when awake, often in the absence of observable stimulation. This characteristic makes its appearance very early in the life of the human infant. His awake periods lengthen rapidly, and during them he may be observed to look, to move his body, to touch, reach, and grasp; even the newborn turns his head and eyes, raises and lowers his arms, opens and closes his hands, flexes and stretches his legs, makes sucking movements, vocalizes, and cries—and this is only a partial inventory of his activities. More important still, this activity leads him into contact with his environment and thereby increases the amount of stimulation to which he is exposed [seeStimulation drives].
Of his many activities, attention will be paid here to only a few: visual exploration, postural and locomotor behavior, manipulating and reaching toward objects, vocalizing and smiling, and distress responses. For each it will be shown how contact with the environment is effected and how his own stimulation is thereby increased. The possibility of modification of behavior by feedback from the contact, the third principle in this account, will be anticipated.
By the end of the first month, the infant looks at the world around him and is quiet as he looks. He looks at near and far objects, at the play of light, at bright and shiny things, at everything that moves within his scope. Patterned objects hold his gaze. Chief among these are people; they offer not only patterned visual stimuli that move, but also complex auditory and tactile stimuli. The infant moves his eyes and his head as he visually explores one part of his environment after another. Now he looks intently at one constellation of stimuli, moves on to another, and returns to the first. As he grows older, he may become quiet for a moment as he looks, only in the next moment to become physically active, to kick and vocalize; what he sees apparently excites him. During his waking hours he is constantly observing, attending, investigating. Before the infant can move himself to any object, before he can reach out to anything, his eyes have made contact with it, he has explored it visually, and he has come to know its dimensions. Because he is so physically limited, vision plays an important role in his life.
Posture and locomotion
The general physical activity of the infant soon takes specific form. He raises his head, arches his back, rolls over, pivots, sits up, crawls, creeps, and then walks, a progression of achievements easily listed but arduously won. To be sure, he learns by trial and error, with no tuition, obeying no command, and unaware of the goal striven for. What is important here, however, is not so much the achievement as the persistence and energy with which the achievement is attained. Failures do not deter him, neither do hurts and bruises. The result of this tireless and sustained activity is physical contact with the world seen and, not least, a measure of independence from his caretakers.
Manipulation and reaching
In the meantime, his hands have not been idle. At first they closed on what they chanced to touch: breast, bottle, clothes, the bars of his crib, his own hands, or a toy. Now he holds, releases, scratches, fingers, but still does not look at what his hands manipulate. At 3 months of age his hands begin to make waving movements on the appearance of an interesting object; at 4 months the hands definitely approach the object (Gesell & Thompson 1934). During these months the infant is also clasping his own hands and observing them with interest. Before long the infant brings objects to his mouth, transfers them from hand to hand, waves and shakes them, hits and bangs them. In reaching toward an object, his glance shifts from his own hand to the object (Piaget 1936). At 41/2 to 5 months of age, the hand opens in anticipation of contact, and grasping has been achieved (White et al. 1964). Finally, the infant brings two objects together, drops a pellet into a bottle, hits a spoon against a cup, places one block on top of another. Except for his first holdings and fingerings, he looks at what he touches, the eye guiding the hand.
Each object grasped supplies information about its physical nature; at the same time, each supplies additional stimulation. By the coordinated use of eye and hand he builds up his acquaintance with the world. As with visual exploration and locomotion, his grasping and manipulating of objects occur without tuition or encouragement. The object is the stimulus, but only in part; the repeated striving for it and the endless manipulations and inventive combinations stem from him alone.
The infant, when in a state of comfort, vocalizes from the first day of life. At first he utters soft, low sounds, but day by day they become louder, more vigorous, and more frequent. He coos, sustaining the vowel sounds, and now and then introduces a consonant. Separate syllables are heard, and several are strung together. The syllables become linked into two-part chains, the second repeating the first; by six months of age he is saying the sounds, but not yet the words, “mama” and “dada.” By now he has already made all the sounds he will need for later speech. Before a year of age he has at least three words in his vocabulary, including “mama” and “dada.” At the same time he “speaks” his other sounds with the inflection of the language he hears (see McCarthy 1954 for a review of the development of language).
The stimuli for his vocalizing are not always patent. He vocalizes in response to the sight of people and things and to their sounds. As early as three months of age, he not only initiates a “conversation” but engages in one. Often, though, he appears to vocalize to himself alone, to make sounds for their own sake. He plays with sound, experimenting with new combinations, now in low, now in loud tones. The infant appears in fact to enjoy vocalizing; thus, these sounds have often been called pleasure sounds to differentiate them from distress sounds.
The investigator of the infant’s behavior would like to discover the dimensions of auditory and visual stimuli which evoke his vocalizations (although it seems likely that they will not account for all his vocalizations) and to discover the role of the response which people make to his vocalizing (although he often vocalizes in the absence of such a response). These interesting topics notwithstanding, the vocalizing of the infant by itself is a convincing illustration of his active nature. He experiments with sound; he practices like a virtuoso; he explores the effect of his vocalizations upon himself and others. By any count, vocal behavior is prominent.
The infant’s smiles, like his vocalizations, are attractive to the beholder and are usually greeted with a smile in return. The first smile, especially, is eagerly awaited and signals for the parent the beginning of the infant’s human quality: social responsiveness is now mutual. The infant smiles often, first coaxed by a varied display of stimuli from the adult, including talking, looking, smiling, and light stroking of the chin or cheek; later, at three months of age, he smiles at just the appearance of the adult. In fact, the adult need now only present himself, with sober face and without speech, to evoke in the infant the full smiling response, a sequence of behaviors composed of, in order, intent regard (associated with decreased activity), brightening of the face, smiling, bursts of activity, and vocalizing. Smiles are almost always accompanied by vocalizations; vocalizations, in contrast, may occur in the absence of smiles.
Although the sight of another human being is the stimulus for the infant’s smile, so also is the sight of some inanimate objects and even geometric patterns (Preyer 1881; Piaget 1936). When the responses to people and rattles were contrasted, it was found that the infants smiled and vocalized to both people and rattles, but more often to people (Rheingold 1961).
The infant produces yet another class of vocalizations, those made when he is in a state of discomfort. They may be called distress vocalizations and include not only crying but also the precursors of crying: protests and fusses. Crying, to take the most prominent of the distress vocalizations, has been inhibited in the newborn by playing a tape recording of a normal heartbeat (Salk 1962) and by giving the infant an empty nipple to suck (Kessen & Leutzendorff 1963). Although crying characterizes infancy, it may be so common a behavior that, these studies aside, it has generally tended to escape notice as a topic for scientific investigation. It is, furthermore, unpleasant to the hearer, and for this, or other culturally derived reasons, we hold the conviction that the infant should not cry. Whatever the reasons, we tend to overlook the significance of crying as a means of vocal communication between infant and caretaker. Crying, however, does effect contact with the social environment: it brings to the infant’s side people who not only minister to his primary biologic needs but also supply stimulation. Often the infant’s cries are terminated by just the appearance or voice of a person or by being picked up. Only imperceptiveness to the intelligence of the infant could permit us to label such behavior “being spoiled.” Distress vocalizations may be viewed, then, as a mode of dealing with the environment.
The main purpose of this section has been to show by means of a few representative response classes that the infant is an active organism. He not only responds to external stimuli, but he often seems to be active in the absence of definable external stimulation. These activities carry him into contact with the environment. At the same time, they also increase the amount of stimulation to which he is exposed. By means of them, as we shall see next, he tests, weighs, assesses, examines, and discovers the nature of the world in which he lives.
The infant’s behavior is modifiable
The third principle is that the infant’s behavior is modifiable by experience; he is capable of learning. From the beginning, he is learning the nature of things and of people, their look and feel, the response they give to his overtures (which may be called the contingencies of his environment), and the regularity with which they respond (which may be called the schedules)
Controversy has arisen in the past over the extent to which behavior in the immature organism is fixed and stimulus-bound and the extent to which it is flexible and modifiable: maturation was set in opposition to learning, nature to nurture, heredity to environment. The lively issues of the past are well set forth by McGraw (1946). Although the issues possess theoretical importance, the processes of maturation and learning are no longer set in opposition. It is understood now that any behavior, no matter how simple or how early, is already the result of an interaction of genetic material and environmental conditions. To claim that the infant’s behavior is modifiable claims nothing about the origin of the behavior, whether innate or learned, for it is always possible to begin the study of learning with any response or behavior the organism already possesses. The advantage of this premise is that it permits the formulation of questions more accessible to experiment. Thus, we now ask what behaviors can be modified by what kinds and amounts of experiences occurring at what ages in the life of the organism. In short, we seek to analyze the processes of learning.
Experimental studies of learning
Although the evidence for the modifiability of the infant’s behavior is everywhere to be seen by even the casual observer, the process of learning can best be studied under controlled conditions. It is to such studies rather than to anecdote that we turn our attention. By now, many principles governing the learning of older humans and animal subjects are known. Do the same principles govern the learning of the immature organism? What qualifications need we make to extend these principles to the human infant and to the infants of other species?
The human infant is certainly not the easiest subject for experimental study. His physical characteristics change rapidly; he is alert only briefly; he has few responses suitable for objective measurement. What responses he has cannot be called forth by verbal explanation, as with older human subjects, or by deprivation, as with infrahuman subjects. For extraneous stimulation to be controlled, the infant must be brought to the laboratory for study, a procedure which not only makes demands on his mother but, because of the strangeness of the laboratory, often suppresses the behavior of interest. Conducting research in the familiar environment, the home, reduces some of these difficulties but introduces others. Each difficulty poses a problem to be solved, and, in various ways, investigators have solved some of the problems and have succeeded in demonstrating that the infant’s behavior is modifiable.
Excellent reviews of laboratory studies of infant learning are presented by Munn (1954) and Lip-sitt (1963). Only a few studies will be presented here. Some are so recent that they have not yet found their way into reviews. Others possess especial interest because they illustrate how early in life behavior may be modified, how various are the behaviors and the reinforcers, or how different are the experimental procedures.
Sucking. That the nature of the first feeding of an infant at the breast may affect subsequent feedings was reported by Gunther (1961). If the nipple did not stimulate the back of the infant’s mouth, the infant did not suck; instead he showed an “apathy” which reappeared at subsequent feedings. Or, if at the first nursing the infant’s upper lip or the mother’s breast covered his nostrils, obstructing respiration, the infant cried and “fought” with his fists. On the second or third experience of the same conditions, he cried whenever he was turned toward the mother. Thus, the consequences of the infant’s behavior modified his subsequent behavior, an example of instrumental conditioning (although Gunther viewed the nipple as a releasing stimulus and cast her observations into an ethologi-cal framework).
Sucking in infants three and four days of age was experimentally modified by Lipsitt and Kaye (1964). In the classical conditioning procedure of pairing an auditory stimulus with insertion of a rubber nipple into the baby’s mouth, the infant came to suck to the sound alone after only 20 pairings. It should be noted that the infant sucked an empty nipple; there is no question here of milk as a reinforcer. Milk as a reinforcer, however, was used in a study (Stanley et al. 1963) of neonatal puppies that with only slight modification could be adapted to the human infant. Sucking was increased by pairing insertion of a nipple in the mouth with milk given from an eye dropper and decreased by pairing insertion of the nipple with a solution of quinine and water. Classical conditioning with elements of instrumental conditioning was therefore demonstrated. Incidentally, studies of this nature, as noted earlier, also supply information on the sensitivity of the young organism. Here, the results demonstrate not only learning but also the gustatory sensitivity of the newborn pup: it can discriminate between milk and quinine.
Head turning. Other responses besides sucking can be modified. The infant’s turning of his head was brought under experimental control by Papousek (1961). As early as the second week of life, after ten trials a day during the first week of life, infants turned their heads to the left at the sound of a bell. By the beginning of the second month, the conditioned response was well established. At three months of age, the infant learned to turn his head to the right at the sound of a buzzer and to the left at the sound of a bell; response differentiation was effected. In the fourth month, infants learned to reverse the differentiation, now turning their heads to the left at the sound of the buzzer and to the right at the sound of the bell. The learning may be classified as classical conditioning, because the auditory stimulus was paired with milk; it may also be classified as instrumental conditioning, because head turning was followed by food. One sees here, in the modification of a simple and elementary response, the first steps in the development of adaptive behavior.
Orienting behaviors. Visual stimulation can also modify the infant’s behavior. On an auditory signal, infants learned to turn the head and eyes toward a light appearing at the side of their cribs (Karlova 1959). Six-month-old infants not only looked but also reached for the light and smiled at it. Karlova labeled these responses “unconditioned orienting reflexes.” Later on, infants learned to look for the light after only a few pairings with an auditory signal. The response was retained for several days and then was quickly extinguished when the light did not follow the auditory signal. These results are interesting on two counts: learning, even in the first year of life, can occur very quickly, and stimuli other than food can modify the infant’s behavior [seeAttention].
Vocalization and smiling. Other classes of behavior besides sucking and head turning have also been experimentally modified. The number of vocalizations uttered by a three-month-old infant in the presence of the experimenter was increased when the experimenter responded to each vocalization by simultaneously smiling, saying “tsk, tsk, tsk,” and touching the infant’s abdomen (Rheingold et al. 1959). Similarly, the rate of smiling in infants four months of age was increased by an adult’s picking the child up and talking to him following each smile (Brackbill 1958). Both are examples of instrumental conditioning.
Summary. These are illustrative studies, selected to show the varieties of responses, rein-forcers, and techniques used in recent studies. It can be predicted with confidence that many other responses of the infant will also yield to experimental modification. The rich accounts given by Piaget (1936) of the moment-by-moment learning of his own children supply models for experiments of the greatest variety. Such experiments will demonstrate under controlled conditions the processes by which the infant learns the nature of the environment in which he lives and discovers the different physical properties of things and people, the ways in which they respond to his own behavior, and the regularity or variability with which they respond.
In the first few months of life, the human infant gives positive responses to almost all social objects. By three months of age, he gives some signs of discriminating the strange person, not by negative or withdrawing responses, but by sobering, staring, and cessation of movement. By six months of age, negative and withdrawing responses appear in some infants. From an early age, then, the infant organism can differentiate between the familiar and the novel. To acquire this differentiation he had to be exposed to some set of stimuli sufficiently long for it to become familiar. In this sense, then, experience modifies his behavior, and the ability to discriminate between the strange and the familiar offers yet another example that his behavior is modifiable.
Although, in general, any organism tends to prefer what he has become accustomed to, the process is especially clear in the case of social attachments. In fact, it is a basic requirement; attachments are built up by familiarity. The institutionalized infant, if cared for by many different caretakers, smiles to all. The home infant, with fewer caretakers, in time smiles more to those he knows, less or not at all to strangers, from whom, indeed, he may withdraw; he shows a narrower, discriminated attachment.
Although some degree of familiarity is a basic factor in the genesis of social attachments (Scott 1963), it alone may not be sufficient; other attributes of the social object may also be required. Opinion (Bowlby 1958) and research on human infants (Schaffer & Emerson 1964) suggest that, in addition to the satisfying of primary biologic needs, the providing of “interesting” auditory and visual stimulation may be an important factor in the formation of social attachments.
Which experiences will become familiar to the infant, what the nature of his accustomed environment will be, and who the people will be with whom he has contact depend almost entirely on the caretaker, his mother. Observations of maternal behavior in homes (Rheingold 1960) revealed striking variations even among firstborn infants within a homogeneous socioeconomic group. Differences appeared in the number and kinds of toys, the extent of quiet and isolation for naps, the amount of auditory stimulation provided by radio, television, or phonograph, the number and nature of contacts with other people, the amount of time left alone, and the number and nature of excursions outside the home. These are only the simplest and most obvious measures of environmental stimulation to which an infant is exposed. More important still are the responses of the caretaker to such behaviors of the infant as his smiling, vocalizing, and crying. The environmental events supplied by the caretaker become the infant’s accustomed environment; only departures from the accustomed are strange.
Familiarity and strangeness constitute poles of but one of the important dimensions of the stimulation provided by the caretaker. Other dimensions are, at the simplest level, sheer amounts of stimulation and, at a more complex level, the kinds of stimulation. We can look to rearing practices, to maternal behavior—if maternal is taken in its generic sense and is meant to include any other member of the family or group who has commerce with the infant—for information on these dimensions. Although at the present we do not know the effects of different kinds of stimulation upon infant behavior, there is evidence on the effects of amounts of stimulation, more specifically, on the effects of less than average stimulation. The evidence comes from studies of what is called “maternal deprivation.”
Placing maternal deprivation in the category of stimulation which may modify the infant’s behavior serves three purposes. First, it establishes theoretical links with the work in general behavior theory on the effects of sensory deprivation; second, it structures maternal deprivation in terms which can lead to experimental operations; and third, it opposes enrichment to deprivation.
Massive deprivation as an experimental procedure is not to be entertained in human research. Certain short-term deprivations might be feasible but have not been experimentally employed. To date, knowledge about the effect of deprivation has come from descriptive studies of life situations in which infants were deprived of normal stimulation by force of circumstance. As scientific evidence, much of the inference on the damaging effects of institutionalization and other forms of maternal deprivation is open to criticism (Casler 1961; Yarrow 1961): the subjects were not assigned to treatments at random, and factors responsible for placement were unknown and unmeasured. The extent to which these factors affected the findings cannot be assessed. Furthermore, neither institutionalization nor maternal deprivation is a unitary variable, although each has so been considered. It is not now possible to ascribe the reported deleterious effects specifically to reduced stimulation, delayed relief of distress, or multiple caretakers, to mention a few likely factors.
Informative as deprivation studies might be, a more attractive alternative and one that is possible of experimental analysis is investigating the effects of increased environmental stimulation. Such studies can best be carried out in environments which now offer less than adequate stimulation; actually they can be carried out in any environment that the experimenter can control for even short periods of time. The study of long-term effects of complex variables upon complex behavior is still the goal of many investigators and must, of course, be the ultimate objective of a science of behavior. But at present, the study of the short-term effects of unitary and, hence, simpler variables upon unitary and simpler behaviors stands a better chance of providing useful data. Parenthetically, a theoretical question remains: Can studies of increased stimulation provide the same information on the process of learning as studies of stimulus deprivation? Until more data from controlled studies are available, we cannot specify the dimensions of adequate stimulation, to say nothing of optimal stimulation.
So far we have been considering only the amount and nature of stimulation. It is possible, however, that the timing of stimulation is also important. The learning of different skills does appear to proceed more rapidly at some ages than at others. That events occurring at these periods have lasting inhibitive or facilitative effects (the “critical period” hypothesis) has not yet been demonstrated in the human infant. A lively discussion of the implications of the hypothesis for one class of infant behavior, the formation of attachments, has been presented by Caldwell (1962).
The questions asked in research on the effects of enriched stimulation will depend in part on the temper of the times and the social values of the community; the nature of adequate or optimal stimulation will depend on what the culture dictates the desirable characteristics of its members shall be. That at the present time a large proportion of the population is not receiving adequate stimulation during infancy and childhood may be surmised from the poor academic performance of many children when first exposed to formal education.
We have presented an account of the kinds of behavior which have been modified, of the environmental events which modify, and of the processes by which modification occurs. The evidence indicates that learning begins early. The environment, social no less than physical, exerts its pressures. As a consequence, the infant learns the dimensions of his universe, guides his behavior to achieve his own ends, and behaves in an adaptive—and yes, an intelligent and socialized—fashion. The laws by which his behavior is modified, it is proposed, are the same as those by which the behavior of all organisms is modified.
The infant modifies the environment
The fourth general principle is that the infant modifies the environment in which he lives, in particular, that he modifies his social environment. It is, of course, obvious that each organism has an effect upon some part of the environment. Still, one so young and helpless as the human infant might be thought incapable of affecting his environment. It is especially these characteristics, however, which confer upon him extraordinary powers. He must, of course, be cared for. But even more, so great is his attractiveness that all members of his group pay him attention. He is small, lively, and responsive; his babyish ways charm caretakers and onlookers alike. Many small and immature creatures appeal to man; the small and immature of his own species appeal even more. Man is not alone in this respect; other mammals, especially the primates (Jay 1963; DeVore 1963), also find their young attractive. Who can say that a father does not look forward each evening to the welcome he receives from his young? Thus, not only does the infant affect the behavior of others, he may well have been one of the social forces responsible for the formation of the family.
Infants modify their social environments in still another way. Just as the people around him provide the environmental events that shape his behavior—by the stimulation they provide and the rewards they distribute—so, too, the infant shapes the behavior of his caretakers. His smiles produce an even greater display in the adult of the behavior that evoked the smile in the first place. His cries command a response. It follows, then, that the infant arouses and reinforces nurturant behavior in his parents; of men and women he makes fathers and mothers. An obvious statement? Yes, but the evidence is wanting in humans; animal studies (e.g., Harlow et al. 1963; Rosenblatt & Lehrman 1963) do show the contribution of the young to the “maternal condition.”
As is the case in any social interaction, so it is too in the interaction between child and parent; the response of one to the other may be inappropriate in nature or faulty in timing. Such insensi-tivity between partners often arises because of preconceived notions on the part of one about how the other ought to behave. Much of the literature on parent-child relationships explores the preconceptions of the parent about his role as parent. The more fixed his notions the less sensitive he may be to the behavior of the child. But it is time now to investigate also the other side, child-parent relationships. Infants, too, may be insensitive to the demands of their caretakers; at any rate, they are partners to the interaction and affect its nature.
That the infant modifies the behavior of others seems clear; the process, however, still awaits analysis.
In this account, the active and responsive nature of the human infant has been emphasized. Much that is customarily included in accounts of this topic has been omitted. There is, for example, no discussion of habit training, maternal attitudes, individual differences, or the effects of breast versus bottle feeding. These and other topics are deliberate omissions. The goal here is to formulate what is known about the behavior of the infant in such a way that it can be integrated with the larger body of knowledge about the principles governing the behavior of all organisms, both humans more mature than the infant and other species. Such a formulation should reveal more clearly the questions still awaiting answers and the research procedures most likely to supply the answers. The gain, one may anticipate, will be in the contribution of knowledge about the origins of behavior to the science of behavior.
Harriet L. Rheingold
[Directly related is the entryDevelopmental psychology. Other relevant material may be found inAffection; Intellectual development; Learning, article onLearning in children; Mental disorders, article onchildhood mental disorders; Sensory and motor development; Socialization.]
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The damaging effects of early institutional care on the human infant, particularly in the first six months of life, have been well established (Bowlby 1951). The viability of institutionalized infants is reduced: they are less resistant to disease, and their physical development is hampered. In later life their capacity for affective relationships and their capacity for abstract thinking appear to have been damaged by this experience of early institutionalization.
Maternal deprivation. Perhaps the most striking of the studies dealing with institutionalization was carried out by Spitz (1946). He compared two institutions, a prison nursery and a foundling home, equated for level of medical care. In the former, delinquent mothers cared for their own babies. In the latter, there was one nurse available for every seven infants after they had been weaned. The mortality rate in the prison nursery, over a three-and-a-half-year period, with 122 cases each year, was zero. The mortality rate in the foundling home during a two-year period was 37 per cent.
The immediate social consequence of these observations was early placement of institutionalized infants in foster homes, to minimize the amount of institutional experience. The question remains, however, what neural and physiological mechanisms could have mediated these effects. We shall consider here experimental work undertaken since about 1950. We shall further consider recent discoveries about brain function that may constitute a framework in which both experimental evidence and observations on maternal deprivation can be incorporated.
Early experience and stress. Weininger showed (1954)that postweaning handling of the infant laboratory rat by the experimenter for a few minutes a day increased the adult rat’s resistance to heart damage and stomach ulceration when it was subjected to the severe stress of 48 hours of immobilization. In a subsequent experiment, Long (1955)showed that handled rats survived starvation an average of 53.8 hours longer than comparable unhandled rats of the same weight.
Weininger also found that the handling of the rat was related to increased skeletal growth and body weight. The studies of Ruegamer, Bernstein, and Benjamin (1954) showed that these gains were due to superior assimilation of food by the handled animal, rather than to a greater food intake.
Together with other results, these findings suggested that postweaning handling altered the balance of hormonal output from the anterior pituitary, decreasing the output of adrenocorticotrophic hormone (ACTH) under chronic stress and increasing the normal output of somatotropic (growth) hormone. Such an alteration of hormonal output from the pituitary could be mediated by the hypothala-mus, which is the final common pathway for neural influence on the pituitary (Harris 1960).
It should be noted here that a wide variety of noxious stimuli, ranging from solar radiation at the beach to examinations in medical school, have been shown to have the nonspecific effect of inducing a higher than normal release of ACTH from the anterior pituitary into the blood stream, thereby stimulating the release of adrenal cortical hormones, such as hydrocortisone in humans and dogs and corticosterone in rats, from the adrenal cortex. The end effect of this reaction, first discovered by Selye in 1936 (Selye 1950), is protein catabolism; protein reserves are converted into glucose for the short-term energy requirements of the organism.
At the same time, the sympathetic division of the autonomic nervous system stimulates release of epinephrine and norepinephrine (adrenaline and noradrenaline) from the adrenal medulla, through the action of the great splanchnic nerve. These powerful hormones further aid in the emergency reaction. The twin arms of the response to stress, i.e., the pituitary-adrenal response and the sympatheticoadrenal medullary response, occur together and are mediated by the posterior and medial hypothalamic zone.
The response to stress is therefore an expenditure of metabolic reserves to prepare the organism for an emergency situation. In the case of physical stress, such as extreme cold, this preparation is essential, and without the adrenal cortex the animal will quickly die. In the case of psychological stress, however, such as a medical examination or an international crisis, where a more differentiated response may be required, the stress reaction has no obvious value. Indeed, if prolonged it may lead to cardiovascular and renal damage and eventually to the death of the animal (Selye 1950).
In the case of emotional or psychological stress the organism may be said to be responding to psychological assault as if it were actually physical assault (Wolff 1953). The terms “psychological” and “emotional,” as used here, refer to the fact that in such stress the noxious stimulus must necessarily be mediated by the central nervous system—that without such neural mediation, the stress response will not occur. In general, psychological stress may be thought of as a noxious stimulus that does not threaten immediate tissue damage to the organism, for example, an insult or social rejection by a group. Psychological rather than physical stress may be considered to predominate in an industrialized and urban environment.
Extending the work of Weininger and others, Levine (1962) and others have since shown that preweaning stimulation of the infant rat can have marked physiological consequences in the adult, including a reduced plasma corticosterone response to chronic stress. Even noxious stimulation (for example, electric shock) appears to have this effect, which Levine believes is related to the earlier maturation of the pituitary-adrenal axis in the early-stimulated animal. Nowhere, however, have there been any results to indicate that early shock increases resistance to starvation or other forms of severe physical stress.
The mechanism operating in Weininger’s and Levine’s studies appears, on the whole, to be an alteration in the threshold of response to stress in the early-handled or early-stimulated animal. In general, the early-stimulated animal is less responsive to environmental change that does not involve tissue damage, such as being moved about in the laboratory, and on these grounds as well as others could be termed less reactive emotionally. Levine was able to show, however, that the early-stimulated animal responds more sharply (measured in terms of plasma corticosterone levels) to acute forms of stress, such as electric shock applied to the soles of the feet.
One can therefore conclude from these experimental studies that in all likelihood the experience of preweaning stimulation (usually removal from the nest for a few minutes) and postweaning handling have the common effect of raising the rat’s threshold of response to chronic environmental stress. As evidence of this, Weininger showed that adrenal weights of handled rats following 48 hours of immobilization were lower than those of unhandled rats undergoing the same stress.
Other studies (Yates et al. 1961) have shown that a hypothalamic homeostatic mechanism presets the pituitary-adrenal response, measured in terms of plasma corticosterone, to any given noxious stimulus, such as laparotomy (incision through the abdominal wall), for the rat. If the amount of corticosterone normally produced under such stress is injected into the blood stream before the stress, no further increase in plasma corticosterone level will occur in response to the actual stress.
A simple hypothesis to explain the results of a majority of studies is, therefore, that early handling and stimulation have set the hypothalamic “homeostat” for chronic stressors at a higher level, thus producing a higher threshold for stress. This could be accounted for by inhibition of the posterior hypothalamic zone, which mediates the response to stress, and by increased activity of the anterior zone.
Population density and stress. What consequences of an increased or decreased pituitary-adrenal response to stress would result respectively from depression or from elevation of the stress threshold? The results of an increased pituitary-adrenal response to environmental change can be most clearly seen in studies on population density and stress at the Philadelphia zoo and in the field (Christian & Lemunyan 1958; Christian & Davis 1964).
Christian has shown that as the density of a given animal population increases, two mechanisms come into play to inhibit further population growth: (a) an inhibition of sexual maturation, apparently due to inhibition of gonadotropin output from the anterior pituitary, and (b) a decrease in female lactation after birth of the young and hence a reduced survival rate among litters. Both of these effects are apparently related to increased pituitary-adrenal activity resulting from crowding, the second of the two mechanisms apparently being an extra-adrenal effect of ACTH.
A remarkable finding of Christian and his associates has been, however, that if the animal population being investigated has received tranquilizing drugs, such as reserpine or chlorpromazine, increasing population density has no effect on mortality—the two inhibitory mechanisms limiting population growth are silent. Further, these inhibitory mechanisms do not work with placid animal populations, no matter how crowded the individuals are. Finally, Christian has shown an inverse relation between social dominance and adrenal weight —the more dominant the animal in a competitive species, the less pituitary-adrenal activity results from crowding and competition.
The effects of population density on mortality and the consequent self-limitation in population growth are therefore due to competition and aggression among members of the species and the resultant increase in pituitary-adrenal activity. Christian was able to show that increasing population density causes renal disease in woodchucks, an effect that could be replicated by administering exogenous doses of ACTH to these animals.
These dramatic studies show the consequences of increased pituitary-adrenal activity (among them, lowered resistance to infection) in an animal population and further suggest that these effects depend on hypothalamic mediation of the noxious stimuli. If the hypothalamic response is inhibited, as through the action of tranquilizing drugs or the effects of early handling, then it can be presumed that the noxious effects of increasing population density will be reduced.
These considerations would imply, for the human species, a relation between early experience and survival in conditions of increasing population density and competition.
Early stress and life span. The work of Hardin Jones (1956) has suggested that early stress experience reduces the individual’s life span, whether this stress is in the form of childhood disease in humans or radiation in animals. Early stress appears to have a multiplicative effect—stress damage increases exponentially with a linear increase in time. Jones’s work has established a clear-cut relation in human populations between amount of childhood disease and life span. His figures show that reduction of childhood disease in Scandinavian populations has resulted in a significantly long life span for these populations in comparison with Americans. It would seem possible that the amount of early emotional stress experienced by the individual may be also related to life span and that those individuals with higher stress thresholds would be relatively more protected from emotional stress and would live longer, but this point remains to be demonstrated experimentally.
Threshold for stress. The work we have been considering here suggests that a reduced pituitary-adrenal response to emotional or psychological stress has protective consequences for the organism and may increase its resistance to severe physical stress. We may consider here that the organism is endowed with a fixed initial amount of “adaptation energy” (Selye 1950), or negentropy, for its lifetime. If these metabolic resources are used up early in the individual’s life span through excessive response to environmental change, leaking across a low threshold for stress, then one may expect that these reserves will be depleted when severe life-threatening stress is encountered at a later date.
For this reason the threshold for stress is crucial in any individual animal or person. If the threshold is set too low in an environment such as ours, where psychological rather than physical stress predominates—and where a pituitary-adrenal response and consequent expenditure of protein reserves may be considered inappropriate—then one can expect impairment of viability. Too high a setting in a physically stressful environment, as would be the case for a tame laboratory rat released in the natural habitat of the wild rat, would be equally fatal.
The actual threshold for stress may be considered to be set by a combination of genetic and experiential factors, the latter including the amount of prenatal stress experienced by the pregnant female. One mechanism by means of which early experience of the organism influences the threshold for stress may involve a direct effect on brain cells of blood plasma corticosterone (mouse) or hydro-cortisone (dog, monkey, man) induced by early stress. Howard (1965) has shown that exogenous corticosterone in mice inhibits brain as well as body growth in the first few days of life, and completely prevents the normal increase in forebrain deoxyribonucleic acid (DNA) found in untreated animals.
Neural mechanisms. Recent studies of brain stimulation in human (Sem-Jacobsen & Torkildsen 1960) and animal (Lilly 1960; Olds 1960; Howard 1965) subjects have revealed the existence of two great neural systems, closely related anatomically, running from the anterior end of the brain to the brain stem. The first, or positive, system, when stimulated electrically, elicits in animal subjects behavior (bar-pressing) that will continue the stimulation and elicits pleasant feelings in human subjects, according to reports from patients who experienced this stimulation as part of neurosur-gical procedure. The second, or negative, system, when stimulated electrically, elicits in the rat or monkey behavior that will stop the stimulation and, when stimulated accidentally in the human patient, elicits highly unpleasant feelings.
These two great systems converge on the hypo-thalamus, their final common pathway for influence on the endocrine system and the autonomic nervous system (Bovard 1961). The positive system is represented in the anterior and lateral hypo-thalamus, and neural activity in this region has parasympathetic autonomic and, in general, protein anabolic consequences, such as promotion of digestion. The negative system is represented in the posterior and medial hypothalamus, and neural activity in this region has sympathetic autonomic and, in general, protein catabolic consequences, such as breakdown of protein into glucose. This negative system mediates the response to stress.
The interesting point about these two great brain systems of opposite function is that from available evidence they appear to be reciprocally inhibitory—activity in one automatically inhibits activity in the other (Bovard 1962). In terms of their operation, we can now account for the effects of maternal deprivation (Bowlby 1951).
Brain mechanisms in maternal deprivation. In comparison with the normal infant, the absence of handling and affection for the institutionalized infant in Bowlby’s studies would mean an absence of stimulation of the positive system in the first few months of life, and hence relative dominance of the negative, or stress-mediating, system. This would result in a low threshold for stress in the institutionalized infant and, therefore, in an excessive pituitary-adrenal response to environmental change. The resultant depletion of metabolic reserves could be expected to lead to the reduced resistance to disease and physical stress found by Spitz (1946). Evidence from studies of animals (Eells 1961) and of humans suggests that noxious stimulation may be better than none at all, but that positive stimulation is preferable to noxious stimulation.
But as we have noted, the negative and positive systems are reciprocally inhibitory. Relative dominance of the negative system would therefore inhibit activity of the positive, or rewarding, system. This will result in a higher threshold for reinforcing stimuli, and hence such maternally deprived individuals would show a reduced capacity for responses to such positively reinforcing stimulation as affection and social approval. It would be more difficult to reach them by these means.
Finally, emotional stress appears to interfere with abstract thinking by polarizing thought around the noxious stimulus, thus restricting the normal range and sweep of ideas. What the mechanism is here has not been established, but it may involve a functional reduction in the number of neurons available for thinking. If this is the case, the individual with a low threshold for stress could be expected to have difficulty thinking abstractly. The relative impairment of abstract thinking found by Goldfarb (1947) in children institutionalized before six months may be related to such a functional reduction of cortical activity resulting from a low stress threshold.
It is of interest in this connection that handled rats, with a higher threshold for stress, have been found to learn mazes faster and with fewer errors than unhandled controls (Bernstein 1957). The reason for this is apparently that handling dampens the fear response, which has been shown to inhibit exploratory behavior. The handled rat simply explores his environment more, and therefore learns more about it—including how to open the wire-mesh covers of the maze runways with paws and nose, which unhandled rats never learn.
These studies suggest that exploratory responses, in behavior or thought, are inhibited by emotional stress. A low stress threshold set by early experience could therefore inhibit the development of abstract thinking and would most probably limit innovations in thinking.
We have now considered the evidence linking the effects of early experience on viability to neuro-endocrine mechanisms, in particular the axis of the hypothalamus, the anterior pituitary, and the adrenal cortex. It is by means of this axis that the external environment influences the internal metabolic dispositions of the organism in the direction of either protein anabolism or protein catabolism. A crucial function of this neuroendocrine system is to set the threshold for the pituitary-adrenal axis’s response to stress, a setting apparently made in part on the basis of the organism’s experience of an early sample of what life is like. This setting in large measure determines the rate of expenditure of metabolic reserves in response to environmental change.
Everett W. Bovard
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Infancy, the period between birth and eighteen to twenty-four months, has fascinated parents, philosophers, and developmental scientists perhaps more than any other period of the lifespan. The study of infants allows us to understand the origins of physical and psychological life. Furthermore, during no other period of life are physical and psychological changes more pervasive and rapid than in infancy.
Around the turn of the twentieth century, William James, an influential philosopher, psychologist, and parent, remarked that the world of the infant is a "blooming, buzzing, confusion." Throughout the twentieth century, researchers devised ingenious methods to study the infant and found that James severely underestimated the infant. We know now that infants' capacities are quite sophisticated in several domains, including perception, cognition, and emotion. Furthermore, infants' capacities in these domains and others continue to develop in infancy and beyond. Below are summaries of some of the key findings that scientists have uncovered about infants in several domains. The boundaries between these domains are somewhat artificial and arbitrary, but they nevertheless allow for an orderly arrangement of some of what is known about the human infant.
The infant's physical structure and central nervous system undergo dramatic and rapid change during the first two years of life. The infant's weight doubles by five months of age, triples by twelve months of age, and quadruples by the age of twenty-four months. The infant's length does not change as rapidly as its weight, for the infant's length at birth is already 75 percent of what it will be at two years of age. Changes in length and weight are accompanied by transformations in the infant's body proportions. The head grows the fastest and matures the earliest, followed by the rest of the body downward (e.g., the neck, torso, legs). In addition, those parts that are closest to the center of the infant's body (e.g., the trunk) grow faster and mature earlier than do parts that are farther from the center (e.g., the hands). The rapid changes in infants' body proportions affect other domains of development, including perceptual, motor, cognitive, and emotional.
The physical structure of the brain develops rapidly as well. Although a human is born with almost all of the neurons that he or she will ever have, the human brain triples in weight by age three and quadruples in weight by age fourteen. Two primary reasons account for this dramatic change in the brain's weight and size. First, a fatty substance called myelin forms around a part of the neuron, causing substantial growth of the brain and increasing its neural conduction. Second, a part of the neuron called the dendrite branches multiple times, creating numerous synapses or connections with other neurons. It is for these reasons that a brain that weighs 370 grams (13 ounces) at birth will weigh 1,080 grams (38 ounces) by the age of three.
At one time, scientists assumed that the newborn's brain was "hard-wired" and that the environment played little, if any, role in its development. Researchers studying human and nonhuman species have provided overwhelming evidence that experience does, in fact, play a powerful and enduring role in the infant's brain development. The infant's experiences "mold" the brain by preserving active synapses and pruning less active or inactive ones. Interestingly, researchers have found sensitive periods in which the brain is affected by experience more so than at other times.
Perceptual and Motor Development
At birth, infants' sensory systems are available for processing perceptual information from the world and from their own bodies, but each system operates, for the most part, within a more limited range than later in infancy. Newborns' senses of taste and smell are particularly well established; in the first two weeks of life, infants can discriminate among sweet, sour, and bitter tastes, and can recognize the smell of their mother's milk. Infants' hearing is also relatively mature at birth, although the loudness threshold for detecting sound is ten to twenty decibels higher in newborns than in adults. Young infants are highly attuned to human voices, especially their mother's. In addition, infants localize sounds, and by the end of their first month, if not sooner, they differentiate speech sounds in a manner comparable to adults. In fact, one-month-olds across cultures differentiate speech sounds not evident in their particular native languages. Thus, young infants demonstrate a wider range of speech sound sensitivity than adults; by ten to twelve months, however, infants' sensitivity to speech sounds narrows and conforms to their native language.
From birth, infants demonstrate distinct preferences for the human face and are especially attuned to moving rather than static stimuli. By two months, infants' color vision is well established. At the same time, infants can process depth information by some cues, a capacity that continues to develop into the middle of the first year. Infants' overall visual acuity, however, is much more limited than that of adults. In the first two to three months, infants cannot discriminate fine visual detail and tend to focus on areas of a stimulus where the contrast between light and dark is greatest, such as the hairline or the eyes of a face. By six to eight months, however, visual acuity nears adultlike maturity.
Infants' perceptual development is inextricably linked to their motor, or action, development. Like adults, infants' action in the world guides their perception of the world, just as their perception of the world guides their action in it. For example, the ability to perceive through touch the size, texture, and hardness of objects develops over the first six to nine months of life in parallel with changes in how infants manually explore objects. At birth, infant action is limited, rather inflexible, and reflexive, largely because they lack control of their heads and trunks. When provided with postural support, however, newborns demonstrate rudimentary reaching abilities, directing their arms in the general direction of objects. Within the first weeks of life, infants are able to support their own heads, facilitating gaze and the scanning of the environment. By three to four months, infants begin using vision to guide their reaching efforts, resulting in successful contact with objects. With experience in visually guided reaching, infants increasingly coordinate their grasping of an object with the movement of their arm toward the object. Between five and seven months, infants develop sufficient trunk control to support their independent sitting, which in turn provides a solid position for head and arm activity.
Infants around seven to nine months begin to move themselves in the environment by means of crawling, which opens up a whole new world of exploration for them. Between eight and twelve months, infants begin to stand, first by using furniture or other objects to support themselves and then by establishing postural control to support independent standing. Walking soon follows as infants move into their second year. With each new motor transition, infants gain new means of perceptually apprehending the world. At the same time, infants rely on their perceptual development to establish more efficient means of acting on the world. For example, infants must rely on what they see and feel when crawling or walking over surfaces in order to continuously update their action and make their action fit the ever-changing demands of the environment.
Enormous individual differences mark the timing of developmental changes in both perceptual and motor development. Increasingly, developmental psychologists have shifted their focus away from simply documenting infants' perceptual and motor milestones toward understanding how these changes occur and how the domains of perception and action constantly and seamlessly interact to produce unique, individual pathways of development.
Cognition, comprised of mental processes such as conceiving, reasoning, memory, and symbolization, organizes humans' action in and perception of the world and is the foundation of humans' status as psychological beings. Speculation over the developmental origin of cognition has fueled philosophical inquiry for millennia. The modern study of infant cognitive development takes its own origin from the theory and research of Swiss psychologist Jean Piaget.
According to Piaget, infants are born with no mental framework in place and only gradually construct a conceptualization of the world through their experience with it. The experience of the infant is initially bound by the immediacy of perception and action, a "sensorimotor intelligence." Piaget argued that newborns have no concept of self or object and simply experience a wash of sensations. Only rudimentary schemas for interacting with the world are available to them. They will, for example, grasp objects placed in their hands, suck on objects that contact their mouths, and visually track moving objects. As infants apply these and other basic perception-action schemas to different objects and situations, they gradually adjust their action to the vast complexity of the world, increasingly accounting for specific objects and events. In the process, a more generalized and abstract sense of the world and infants' place in it emerges. By the end of infancy, the child has built a primitive understanding of objects and events as independently existing in time and space. For Piaget, a consequence of the infant period is an emerging representational ability, captured in the consolidation by eighteen to twenty-four months of an "object concept," which allows the child to conceive of an object's existence even when it is no longer available to the senses (e.g., out of view).
The study of representation in general—and the object concept in particular—has remained at the forefront of research in infant cognition. Examining infant search behavior, Piaget established six developmental stages through which infants pass before establishing a mature object concept. For example, during the third stage, around four to eight months, infants will search out a partially covered toy. But if the toy is completely hidden, infants will not search for it, as if it no longer exists. By the fourth stage, around eight to twelve months, infants will search for a completely covered toy, but when the toy is then hidden in another location, infants search exclusively at the initial hiding site, as if the toy's existence coincided with that particular location in space. More recent studies, using infant looking behavior, suggest that even younger infants have formed certain expectations about objects and their physical properties. For example, infants expect an object to stop moving when it contacts a solid barrier and expect that two inanimate objects must come in contact with one another for one object to set the other in motion. Young infants seem to apply these expectations even to events that occur out of view. When three-month-olds see a ball roll behind a screen, and then the screen is lifted to reveal a barrier to the ball's path, they look much longer, as if surprised, at the event when the ball is revealed resting at the other end of the barrier, having seemingly moved through the solid barrier. Whether these expectations constitute conceptual understanding of objects has been a source of debate.
Studies of infant imitation and memory have further contributed to the understanding of infant representational ability. Newborns will imitate the action of an adult sticking out his tongue, even though they cannot see themselves imitate the action. Some psychologists have argued that newborns must possess an abstract representational system for linking their unseen facial movements with what they see the adult doing. Around nine months, representation is clearly in place as infants, having only observed an adult play with a toy in a particular way, will imitate the adult when given a chance twenty-four hours later to play with the toy. In this deferred imitation, infants must represent what they have seen twenty-four hours earlier and must recall from memory the representation. Prior to nine months, infants can retain memories for weeks or even months, but they retrieve those memories only if sufficient cues are present to allow them to recognize the familiarity of an event.
Emotions pervade infants' daily lives in that they are the means by which infants accomplish their goals, as well as the primary medium through which communication occurs. Newborns display general patterns of distress and excitement. Later in the first year, other emotional expressions develop such as joy, surprise, sadness, disgust, anger, and fear. For example, infants begin to smile at others around six to eight weeks of age and begin to show wariness of strangers, as well as separation distress, between seven to nine months of age. More complex emotions, including embarrassment, pride, shame, and guilt, become evident in the last half of the second year.
Emotional expressions are displayed not only by the infant but also by the caregiver. Developmentalists have found that infants begin to detect adults' emotional displays (vocal and facial) by the age of two months. For example, young infants are able to distinguish between a smiling face and one that appears to be frowning. It is not until the middle of the first year that infants begin to understand the meaning of facial displays (i.e., understanding that the displays are emotional). Beginning around ten months of age, infants use the emotional displays of others to regulate their own behavior toward events they encounter in the world, a phenomenon called social referencing. For example, infants at the end of the first year will avoid ambiguous objects toward which their caregivers act disgusted, while they will approach and touch objects toward which their caregivers smile.
Individual differences in infants' socioemotional development have fascinated developmentalists for decades, particularly infants' attachment styles and temperamental differences. Attachment refers to an enduring emotional tie that one person forms to another. It is a tie in which the infant takes another (typically the caregiver) as a protective figure, finding increased security in their presence, missing them in their absence, seeking them in times of stress or alarm, and using them as a secure base from which to explore. According to John Bowlby, the tendency for infants to form attachments is evolutionarily based, being evident across cultures and in other mammalian species. All human infants, even those who have been mistreated and abused, form attachments to others.
The attachment behaviors that infants display change with development. The very young infant can only cry when distressed, look to the caregiver if he or she is nearby, and be attractive to adults. In time, however, infants take on an increasingly active role. Older infants can deliberately signal to the attachment figure by, perhaps, calling for the caregiver. Furthermore, infants acquire the ability to remain in close proximity to their attachment figure with the onset of crawling between seven and nine months.
While all infants form attachments, there are individual differences in infants' attachments that have enduring socioemotional consequences. Mary Ains-worth devised a laboratory procedure (called "strange situation") consisting of a series of maternal separations and reunions designed to categorize twelve-month-old infants' attachment styles to their caregivers. According to this research, infants can be categorized into three groups depending on the attachment behaviors they display: (1) "securely attached" infants seek comfort from the caregiver during reunions and, once comforted, play with toys; (2) "insecurely attached-avoidant" infants avoid their mothers during reunion and focus their attention on their play; and (3) "insecurely attached-resistant" infants are ambivalent during reunion, first approaching the caregiver and then pushing her away. Through subsequent research a fourth category of infants was identified in which infants in the Strange Situation display "disorganized-disorganizing" attachment behavior, characterized by contradictory behavior toward their mothers during reunion (e.g., walking to the mother and then abruptly falling to the floor and rocking).
Attachment researchers have found a consistent relation between infants' attachment behaviors in the Strange Situation and infants' history of interaction with their primary caregivers. Mothers of "securely" attached infants tend to be sensitive and responsive to their infants' emotional signals, whereas mothers of "insecure" infants tend to ignore or to respond inconsistently to their infants' emotional signals. Infants who are classified as "disorganized-disoriented" tend to have caregivers who are frightening to, or frightened of, their infants. In sum, infants' history of interaction influences their quality of attachment, which, in turn, is related to their socioemotional development later in life.
In contrast to the individual differences in attachment thought to arise because of varying interactional histories, temperament researchers study enduring differences in emotionality, and behavioral responses to stimuli, that are due to constitutional factors. Various researchers have identified various temperamental attributes, but most researchers agree that the following are important components of temperament:
- Activity level—the typical pace or vigor of one's activities
- Irritability/negative emotionality—how easily or intensely upset one becomes over negative events
- Soothability—the ease with which one calms after becoming upset
- Fearfulness—one's wariness of intense or highly unusual stimulation
- Sociability—one's receptiveness to social stimulation
Behavioral genetic studies comparing identical twins to fraternal twins indicate that the components of temperament are moderately heritable. In addition, infants' temperament, particularly activity level, irritability, sociability, and shyness, endures to some extent into childhood and adulthood. One important point, however, is that temperament does not determine personality in later life. The study of temperament thus highlights a significant principle that cuts across all developmental phenomena; biology dynamically interacts with environment in the development of humans.
When taken together, some of the key findings in infants' physical, perceptual, motor, cognitive, and socioemotional development indicate that infants are qualitatively different from adults; infants are not simply miniature adults. Furthermore, development is characterized by several reorganizations both within and between domains, thereby precipitating changes in infants' perceptions of the world, as well as the way in which they act upon it. The period of infancy is likely to continue to generate fundamental and important questions that challenge scientists.
See also:STAGES OF DEVELOPMENT
Ainsworth, Mary S., M. C. Blehar, E. Waters, and S. Wall. Patterns of Attachment. Hillsdale, NJ: Lawrence Erlbaum, 1978.
Bowlby, John. Attachment and Loss, Vol. 1:Attachment. New York:Basic, 1969.
Fogel, Alan. Infancy: Infant, Family, and Society. Belmont, CA: Wadsworth, 2001.
Muir, Darwin, and Alan Slater, eds. Infant Development: The Essential Readings. Malden, MA: Blackwell, 2000.
Piaget, Jean. The Construction of Reality in the Child. New York: Basic, 1954.
Stern, Daniel N. The Interpersonal World of the Infant. New York:Basic Books, 1985.
Very early childhood, generally referring to the period up to age two. During this important formative period, children begin to develop habits and behavior patterns, and acquire many basic skills, including speech.
Compared to the young of other mammals, human infants are precocious in some ways—notably sensory development—and relatively helpless in others, such as physical strength and mobility. At birth , the average American infant weighs approximately 7.5 pounds (3.37 kg), although a baby born 28 weeks after conception may weigh as little as two pounds (0.9 kg). The average length of an American newborn is about 21 inches (53 cm).
Infants are born with several reflexes that are activated by particular stimuli, such as the grasping reflex when a finger is placed in the palm of a baby's hand. Other reflexes include rooting (turning the mouth toward the breast or bottle) and sucking. Many early reflexes— such as reaching and performing a step-like motion— disappear, only to reappear later. While the most important senses in human adults are vision and hearing , infants acquire much of their information about the world through touch . At birth, a baby's eyes and the pathways between the eyes and the brain are not fully developed; the eyesight of a newborn is estimated at 20-600 (an object viewed from 20 feet [609 cm] away appears as a distance of 600 feet [182 m] by an adult with 20-20 vision). The senses of newborns are particularly well adapted for bonding with their caregivers. Infants can see large objects close up and are especially interested in faces, and their hearing is most acute in the range of human speech.
In the first year, the shape and proportion of an infant's body are better suited to crawling on all fours than to walking erect. During the first three months of life, infants also lack the lower body strength and muscular control to support their weight standing upright. The urge to stand and walk upright is very strong, however, and babies work hard to accomplish this task. By seven to eight months, infants can usually stand holding on to a playpen or other object; at 10 or 11 months they can walk with assistance, and by 13 months they can usually take a few steps unaided.
As infants are developing physically, they are also developing cognitively in their ability to perform such mental processes as thinking, knowing, and remembering. The theory of childhood cognitive development developed by the Swiss psychology Jean Piaget describes four stages of increasingly complex and abstract thought that occur between birth and adolescence , each qualitatively different from but dependent upon the stages before it. The first, or sensorimotor, stage, (birth to approximately two years), is a time of nonverbal, experimental basic learning when infants gradually gain mastery of their own bodies and external objects. By sucking, shaking, banging, hitting, and other physical acts, children at this age learn about the properties of objects and how to manipulate them. The main goal at this stage is to achieve what Piaget termed "object constancy," or permanence: the sense that objects exist even when they are not visible and that they are independent of the infant's own actions. This sense forms the basis for the perception of a stable universe. The sensorimotor stage is followed by the preoperational stage (ages two to six), which involves the association of objects with words.
Infants are born with different temperaments. There are "easy babies," who are cheerful and seldom fuss; difficult babies, who are often irritable; and timid babies, who are wary when approaching new situations. Most people believe that temperament is inborn, although there is little hard evidence to prove it. Temperament's interaction with a variety of environmental factors, including parental expectations, determines the course of an individual's development. The most important aspect of an infant's socialization is forming secure attachments, primarily to parents or other principal caregivers. Attachment problems may have a negative effect on a child's normal development. Initially, infants will respond positively to all contact with adults, even though they recognize familiar faces and prefer their mother or other primary caregiver. By the age of three months, babies will begin to smile in response to outside stimuli, maintain eye contact, and vocalize, as distinguished from crying. Eventually, they will advance to what Piaget called the "secondary level" of concentration, at which they are aware of social changes in addition to objects and events. During this period, infants enjoy social contact and will fuss when left alone. They are able to distinguish their parents from other people, will smile and vocalize at familiar people, and will cry when those individuals are absent. At the age of six or seven months, when infants develop a conception of object permanence, an especially strong bond begins to form with the primary caregiver, usually the mother. This is accompanied by separation anxiety (distress at being separated from the primary caregiver) and stranger anxiety (shyness or fear in the presence of strangers). Such behaviors are an integral part of normal cognitive development and displays a healthy attachment to the primary caregiver.
During the second year of life, the infant's focus of socialization extends beyond the primary caregiver to the family unit as a whole and includes gaining some control over emotions and accepting discipline. In Erik Erikson's eight-stage theory of personality , the most important task in the first 18 months of an infant's life is establishing a basic sense of trust in the world, accomplished initially by the attachment formed with the primary caregivers. Sometime after his or her first birthday, an infant begins developing a tremendous need for autonomy, inevitably accompanied by a sense of doubt and shame brought on by learning to follow rules and social demands for self-control, including physical control (such as toilet training). The conflict between autonomy and doubt occupies much of a child's second year and continues into the third. Successfully negotiated, this stage leads to the emergence of independence and will power, and a sense of self-awareness—which appears to depend upon a combination of cognitive development, socialization, and linguistic skills—slowly develops during the second year of life.
Owens, Karen. The World of the Child. New York: Holt, Rinehart, and Winston, 1987.
Papalia, Diane E. A Child's World: Infancy through Adolescence. 5th ed. New York: McGraw-Hill, 1990.
Apart from their interest in infant mortality rates, sociologists rarely give much attention to infancy and usually do not treat it as a distinctive period in the life-cycle, subsuming it within the period of childhood. However, there is some sociological interest in psychological research on infant and child development, particularly because of its relevance to controversies about the relative importance of nature and nurture in explaining human behaviour.
Psychologists commonly regard the period of infancy as crucial to individual development and hence to adult behaviour. Sigmund Freud, emphasizing both innate tendencies and social and psychological experience, viewed the first five years of life as determining the individual's subsequent personality and emotional development, and regarded the experiences of the first year as essential to satisfactory ego development—ideas developed more fully by psychoanalytic theorists such as Melanie Klein. Other writers have focused on cognitive development. Jean Piaget's specification of stages of cognitive development, based on his detailed observations, has been especially influential. Recent research on infancy increasingly suggests that infants have considerable cognitive capacities—capacities which selectively regulate their experiences of the environment. See also NATURE VERSUS NURTURE DEBATE.
in·fan·cy / ˈinfənsē/ • n. the state or period of early childhood or babyhood: a son who died in infancy. ∎ the early stage in the development or growth of something: opinion polls were in their infancy. ∎ Law the condition of being a minor.
Minority; the status of an individual who is below the legal age of majority.
At common law, the age of legal majority was twenty-one, but it has been lowered to eighteen in most states of the United States. Infancy indicates the condition of an individual who is legally unable to do certain acts. For example, an infant might not have the legal capacity to enter into certain contracts. Similarly, infancy is a ground for annulment of a marriage in certain jurisdictions.
Although many states have lowered the age of majority for most purposes to eighteen, they frequently retain the right to mandate support of a child by a parent beyond that age in the aftermath of divorce.