Forensic Science
Forensic science
Forensic science reflects multidisciplinary scientific approach to examining crime scenes and in examining evidence to be used in legal proceedings. Forensic science techniques are also used to verify compliance with international treaties and resolutions regarding weapons production and use.
Forensic science techniques incorporate techniques and principles of biology , chemistry , medicine, physics , computer science, geology , and psychology .
Forensic science is the application of science to matters of law. Both defense and prosecuting attorneys sometimes use information gleaned by forensic scientists in attempting to prove the innocence or guilt of a person accused of a crime.
A basic principle of forensic science is that a criminal always brings something to the scene of a crime, and he or she always leaves something behind. The "some-thing" left behind is the evidence that detectives and criminalists (people who make use of science to solve crimes) look for. It might be fingerprints, footprints, tooth marks, blood , semen, hair, fibers, broken glass , a knife or gun, a bullet, or something less tangible such as the nature of the wounds or bruises left on the victim's body, which might indicate the nature of the weapon or the method of assault. Careful analysis of evidence left at the scene of a crime often can be used in establishing the guilt or innocence of someone on trial.
History
Archimedes, who proved that his king's crown was not pure gold by measuring its density , was perhaps the world's first forensic scientist. However, it was Sir Arthur Conan Doyle's fictional stories of Sherlock Holmes, written in the late nineteenth century, that first anticipated the use of science in solving crimes in the twentieth century. At about the same time, Sir Francis Galton's studies revealed that fingerprints are unique and do not change with age. As early as 1858, William Herschel, a British official in India, used imprints of inked fingers and hands as signatures on documents for people who could not write. Unknown to Herschel, contracts in Japan had been sealed by using a thumb or fingerprint for centuries.
During the 1890s, Scotland Yard, headquarters for the metropolitan police of London, began to use a system developed by a French police official named Alphonse Bertillon. The Bertillon system consisted of a photograph and 11 body measurements that included dimensions of the head, length of arms, legs, feet, hands, and so on. Bertillon claimed that the likelihood of two people having the same measurements for all 11 traits was less than one in 250 million. In 1894, fingerprints, which were easier to use and more unique (even identical twins have different fingerprints), were added to the Bertillon system.
Edmond Locard, a French criminalist, established the first laboratory dedicated to crime analysis in 1910. A decade later, crime labs had been established throughout Europe . The first crime lab in the United States was opened in Los Angeles in 1923, but it was 1932 before the Federal Crime Laboratory was established by the Federal Bureau of Investigation (FBI) under the direction of J. Edgar Hoover. Today, there are about 400 crime labs and nearly 40,000 people involved in forensic science in the United States alone.
Fingerprints
Although fingerprints have been used by crime investigators for more than a century, they remain one of the most sought after pieces of evidence. All human beings are born with a characteristic set of ridges on our fingertips. The ridges, which are rich in sweat pores, form a pattern that remains fixed for life. Even if the skin is removed, the same pattern will be evident when the skin regenerates. Some of the typical patterns found in fingerprints are arches, loops, and whorls.
Oils from sweat glands collect on these ridges. When we touch something, a small amount of the oils and other materials on our fingers are left on the surface of the object we touched. The pattern left by these substances, which collect along the ridges on our fingers, make up the fingerprints that police look for at the scene of a crime. It is the unique pattern made by these ridges that motivate police to record people's fingerprints. To take someone's fingerprints, the ends of the person's fingers are first covered with ink. The fingers are then rolled, one at a time, on a smooth surface to make an imprint that can be preserved. Fingerprints collected as evidence can be compared with fingerprints on file or taken from a suspect.
Everyone entering military service, the merchant marine, and many other organizations are fingerprinted. The prints are there to serve as an aid in identification should that person be killed or seriously injured. The FBI maintains a fingerprint library with patterns taken from more than 10% of the entire United States population. Each year the FBI responds to thousands of requests to compare samples collected as evidence with those on file at their library. The process of comparison has been improved in terms of speed and effectiveness in recent years by the development of automated fingerprint identification systems (AFIS) that allows police departments with computer access to search the collection.
Many fingerprints found at crime scenes are not visible. These latent fingerprints, which are often incomplete, are obtained in various ways. The oldest and most frequently used method is to use a powder such as ninhydrin to dust the surface. The powder sticks to the oily substances on the print making the pattern visible. The print can then be photographed and lifted off the surface by using a tape to which the powder adheres. To search for fingerprints on porous materials such as paper , forensic technicians use fumes of iodine or cyanoacry-late. These fumes readily collect on the oils in the print pattern and can be photographed. Since 1978, argon lasers have also been used to view latent fingerprints. When illuminated by light from an argon laser , a latent print is often quite visible. Visibility under laser light can be enhanced by first dusting the print with a fluorescent fingerprint powder.
Fingerprints are not the only incriminating patterns that a criminal may leave behind. Lip prints are frequently found on glasses. Footprints and the soil left on the print may match those found in a search of an accused person's premises. Tire tracks, bite marks, toe prints, and prints left by bare feet may also provide useful evidence. In cases where the identity of a victim is difficult because of tissue decomposition or death caused by explosions or extremely forceful collisions, a victim's teeth may be used for comparison with the dental records of missing people.
Genetic fingerprints
The nuclei within our cells contain coiled, thread-like bodies called chromosomes. Chromosomes are paired, one member of each pair came from your father; the other one from your mother. Chromosomes are made of deoxyribonucleic acid, often called DNA. It is DNA that carries the "blueprint" (genes) from which "building orders" are obtained to direct the growth, maintenance, and activities that go on within our bodies.
Except for identical twins, no two people have the same DNA. However, we all belong to the same species ; consequently, large strands of DNA are the same in all of us. The segments that are different among us are often referred to as junk DNA by biologists. It is these unique strands of DNA that are used by forensic scientists. Strands of DNA can be extracted from cells and "cut" into shorter sections using enzymes. Through chemical techniques involving electrophoresis , radioactive DNA, and x rays , a characteristic pattern can be establishedthe so-called genetic fingerprint. Because different people have different junk DNA, the prints obtained from different people will vary considerably; however, two samples from the same person will be identical. If there is a match between DNA extracted from semen found on the body of a rape victim and the DNA obtained from a rape suspect's blood, the match is very convincing evidence-evidence that may well lead to a conviction or possibly a confession.
Although genetic fingerprinting can provide incriminating evidence, DNA analysis is not always possible because the amount of DNA extracted may not be sufficient for testing. Furthermore, there has been considerable controversy about the use of DNA, the statistical nature of the evidence it offers, and the validity of the testing.
Genetic fingerprinting is not limited to DNA obtained from humans. In Arizona, a homicide detective found two seed pods from a paloverde tree in the bed of a pickup truck owned by a man accused of murdering a young woman and disposing of her body. The accused man admitted giving the woman a ride in his truck but denied ever having been near the factory where her body was found. The detective, after noting a scrape on a paloverde tree near the factory, surmised that it was caused by the accused man's truck. Using RAPD (Randomly Amplified Polymorphic DNA) markers—a technique developed by Du Pont scientists—forensic scientists were able to show that the seed pods found in the truck must have come from the scraped tree at the factory.
DNA analysis is a relatively new tool for forensic scientists, but already it has been used to free a number of people who were unjustly sent to prison for crimes that genetic fingerprinting has shown they could not have committed. Despite its success in freeing victims who were unfairly convicted, many defense lawyers claim prosecutors have overestimated the value of DNA testing in identifying defendants. They argue that because analysis of DNA molecules involves only a fraction of the DNA, a match does not establish guilt, only a probability of guilt. They also contend that there is a lack of quality control standards among laboratories, most of them private, where DNA testing is conducted. Lack of such controls, they argue, leads to so many errors in testing as to invalidate any statistical evidence. Many law officials argue that DNA analysis can provide probabilities that establish guilt beyond reasonable doubt.
Evidence and tools used in forensic science
Long before DNA was recognized as the "ink" in the blueprints of life, blood samples were collected and analyzed in crime labs. Most tests used to tentatively identify a material as blood are based on the fact that peroxidase, an enzyme found in blood, acts as a catalyst for the reagent added to the blood and forms a characteristic color . For example, when benzidine is added to a solution made from dried blood and water , the solution turns blue. If phenolphthalein is the reagent, the solution turns pink. More specific tests are then applied to determine if the blood is human.
The evidence available through blood typing is not as convincing as genetic fingerprinting, but it can readily prove innocence or increase the probability of a defendant being guilty. All humans belong to one of four blood groups–A, B, AB, or O. These blood groups are based on genetically determined antigens (A and/or B) that may be attached to the red blood cells. These antigens are either present or absent in blood. By adding specific antibodies (anti-A or anti-B) the presence or absence of the A and B antigens can be determined. If the blood cells carry the A antigen, they will clump together in the presence of the anti-A antibody. Similarly, red blood cells carrying the B antigen will clump when the anti-B antibody is added. Type A blood contains the A antigen; type B blood carries the B antigen; type AB blood carries both antigens; and type O blood, the most common, carries neither antigen. To determine the blood type of a blood sample , antibodies of each type are added to separate samples of the blood. The results,
| Antibody added to sample | Results of test indicates blood type to be | |
| anti-A | anti-B | |
| — | — | O |
| + | — | A |
| — | + | B |
| + | + | AB |
which are summarized in the table, indicate the blood type of the sample.
If a person accused of a homicide has type AB blood and it matches the type found at the crime scene of a victim, the evidence for guilt is more convincing than if a match was found for type O blood. The reason is that only 4% of the population has type AB blood. The percentages vary somewhat with race. Among Caucasians, 45% have type O, 40% have type A, and 11% have type B. African Americans are more likely to be type O or B and less likely to have type A blood.
When blood dries, the red blood cells split open. The open cells make identification of blood type trickier because the clumping of cell fragments rather than whole red blood cells is more difficult to see. Since the antigens of many blood-group types are unstable when dried, the FBI routinely tests for only the ABO, Rhesus (Rh), and Lewis (Le) blood-group antigens. Were these blood groups the only ones that could be identified from blood evidence, the tests would not be very useful except for proving the innocence of a suspect whose blood type does not match the blood found at a crime scene. Fortunately, forensic scientists are able to identify many blood proteins and enzymes in dried blood samples. These substances are also genetic markers, and identifying a number of them, particularly if they are rare, can be statistically significant in establishing the probability of a suspect's guilt. For example, if a suspect's ABO blood type matches the type O blood found at the crime scene, the evidence is not very convincing because 45% of the population has type O blood. However, if there is a certain match of two blood proteins (and no mismatches) known to be inherited on different chromosomes that appear respectively in 10% and 6% of the population, then the evidence is more convincing. It suggests that only 0.45 X 0.10 X 0.06 = 0.0027 or 0.27% of the population could be guilty. If the accused person happens to have several rarely found blood factors, then the evidence can be even more convincing.
Since handguns are used in half the homicides committed in the United States and more than 60% of all homicides are caused by guns, it is not surprising that ballistic analysis has been an important part of the work performed in crime labs. Comparison microscopes, which make it possible to simultaneously view and compare two bullets, are an important tool for forensic scientists. When a bullet is fired, it moves along a spiral groove in the gun barrel. It is this groove that makes the bullet spin so that it will follow a straight path much like that of a spiraling football. The striations or markings on the bullet made by the groove and the marks left by the firing pin are unique and can be used to identify the gun used to fire any bullets found at the scene of a homicide. Similarly, tool marks, which are often left by burglars who pry open doors or windows, can serve as useful evidence if comparisons can be made with tools associated with a person accused of the crime. Particularly incriminating are jigsaw matches-pieces of a tool left behind that can be shown to match pieces missing from a tool in the possession of the accused.
In the event that bullets have been shattered making microscopic comparisons impossible, the fragments may be analyzed by using neutron activation analysis . Such analysis involves bombarding the sample with neutrons to make the atoms radioactive. The gamma rays emitted by the sample are then scanned and compared with known samples to determine the concentration of different metals in the bullet-lead. The technique can be used to compare the evidence or sample with bullet-lead associated with the accused.
Autopsies can often establish the cause and approximate time of death. Cuts, scrapes, punctures, and rope marks may help to establish the cause of death. A drowning victim will have soggy lungs, water in the stomach, and blood diluted with water in the left side of the heart . A person who was not breathing when he or she entered the water will have undiluted blood in the heart. Bodies examined shortly after the time of death may have stiff jaws and limbs. Such stiffness, or rigor mortis, is evident about ten hours after death, but disappears after about a day when the tissues begin to decay at normal temperatures. Each case is different, of course, and a skillful coroner can often discover evidence that the killer never suspected he or she had left behind.
Modern crime labs are equipped with various expensive analytical devices usually associated with research conducted by chemists and physicists. Scanning electron microscopes are used to magnify surfaces by as much as a factor of 200,000. Because the material being scanned emits x rays as well as secondary electrons in response to the electrons used in the scanning process, the microscope can be used together with an x ray micro analyzer to identify elements in the surface being scanned. The technique has been particularly successful in detecting the presence of residues left when a gun is fired.
The mass spectrometer and the gas chromatograph have been particularly effective in separating the components in illegal drugs, identifying them, and providing the data needed to track down their source and origin. Thin layer chromatography (TLC) has proved useful in identifying colored fibers. Although many fibers may appear identical under the microscope, they can often be distinguished by separating the component dyes used in coloring the fabric. Fusion microscopy—using changes in birefringence with temperature—has also proved useful in identifying and comparing synthetic fibers found at crime scenes. In addition to using such physical properties as density, dispersion, and refractive index to match and identify glass samples, the plasma emission spectroscope has proven helpful in analyzing the component elements in glass as well as distinguishing among various types of glass found in windows, bottles, and windshields.
The role and impact of forensic sciences came to the forefront of public attention during the highly publicized and televised O.J. Simpson murder trial. Lawyers for both sides offered a wide variety of forensic evidence—and disputed the validity of opposing forensic evidence—before the controversial acquittal of Simpson.
In 2002, forensic science specialists played an integral role in the tracking and eventual identification of evidence (e.g. similarities in ballistics , psychological profiles, etc.) that allowed investigators link a nationwide a string of crimes that culminated in several snipers attacks in the Washington-Virginia-Maryland area.
A new and emerging area of forensic science involves the reconstruction of computer data. High speed and large memory capacity computers also allow for what forensic investigators term as "virtual criminality," the ability of computer animation to recreate crime scenes.
See also Antibody and antigen; DNA technology; Pathology; Toxicology.
Resources
books
Butler, John M. Forensic DNA Typing: The Biology and Technology Behind STR Markers Academic Press, 2001.
Lee, Henry C., and Charles D. Gill. Cracking Cases: The Science of Solving Crimes. Prometheus Books, 2002.
Nordby, Jon J. Dead Reckoning: The Art of Forensic Detection. CRC Press, 2000.
Sachs, Jessica S. Corpse: Nature, Forensics, and the Struggle toPinpoint Time of Death. An Exploration of the Haunting Science of Forensic Ecology. Perseus Publishing, 2001.
Saferstein, Richard. Criminalistics: An Introduction to ForensicScience. New York: Prentice-Hall, 2000.
other
Consulting and Ducation in Forensic Science. "Forensic Science Timeline." Norah Rudin [cited March 16, 2003]. <http://www.forensicdna.com/Timeline.htm.>.
Robert Gardner
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Birefringence
—Splitting of light into two separate beams in which the light travels at different speeds.
- Gas chromatograph
—A device that separates and analyzes a mixture of compounds in gaseous form.
- Mass spectrometer
—A device that uses a magnetic field to separate ions according to their mass and charge.
- Polymorphic
—Distinct inherited traits that are different among members of the same species. Blood groups, for example, are polymorphic, but weight is not.
- Scanning electron microscope
—A device that emits a focused beam of electrons to scan the surface of a sample. Secondary electrons released from the sample are used to produce a signal on a cathode ray tube where the enlarged image is viewed.