DNA Testing, Fingerprints, and Polygraphs
| Author | Jeffrey Wilson |
| Pages | 453-456 |
Page 453
Viewers who watch police investigation shows on television often see intrepid experts solve crimes with the aid of a fingerprint on a doorknob or a strand of DNA from a hair miles from the crime scene. While the ease with which criminals are identified is exaggerated, the general picture is correct: Both DNA and fingerprints can help identify individuals through their unique markers—which means they can be useful tools both in identifying criminals and in clearing those who have been wrongly accused.
Polygraph machines, better known as "lie detectors," are also seen on television, but usually they can be found on older programs. The polygraph does not actually measure whether a person has made a true or false statement; in fact, it measures changes in breathing, blood pressure, and perspiration. A person who is lying, claim polygraph proponents, will breathe more rapidly, have a faster heartbeat, and sweat more profusely than one who is telling the truth.
All three of these tools have an established place in the criminal justice system, as well as other areas of society. When determining a person's culpability in committing a crime, law enforcement experts agree, the more corroborating evidence, the stronger the case. Even if a person confesses and witnesses to the crime come forward, having indisputable physical evidence helps guarantee that the right person will be called to account for the crime.
Fingerprints are the oldest and most accurate method of identifying individuals. No two people (not even identical twins) have the same fingerprints, and it is extremely easy for even the most accomplished criminals to leave incriminating fingerprints at the scene of a crime.
Each fingerprint has a unique set of ridges and points that can be seen and identified by trained experts. If two fingerprints are compared and one has a point not seen on the other, those fingerprints are considered different. If there are only mathing points and no differences, the fingerprints can be deemed identical. (There is no set number of points required, but the more points, the stronger the identification. Fingerprints can be visible or latent; latent fingerprints can often be seen with special ultraviolet lights, although on some surfaces a simple flashlight will identify the print. Experts use fingerprint powder or chemicals to set a print; they then "lift" the print using special adhesives.
The use of fingerprints for identification goes back to ancient times. In ancient Babylonia and China, thumbprints and fingerprints were used on
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clay tablets and seals as signatures. The idea that fingerprints might be unique to individuals dates from the fourteenth century. In 1686 the physiologist Marcello Malpighi examined fingerprints under a microscope and noted a series of ridges and loops. In 1823, another physiologist, Jan Purkinje, noted at least nine different fingerprint patterns.
The pioneer in fingerprint identification was Sir Francis Galton, an anthropologist by training, who was the first to show scientifically how fingerprints could be used to identify individuals. Beginning in the 1880s, Galton (a cousin of Charles Darwin) studied fingerprints to seek out hereditary traits. He determined through his studies not only that no two fingerprints are exactly alike, but also that fingerprints remain constant throughout an individual's lifetime. Galton published a book on his findings in 1892 in which he listed the three most common fingerprint types: loop, whorl, and arch. These classifications are still used today.
It did not take long for law enforcement officials to recognize the potential value of fingerprint evidence. Sir Edward Richard Henry, a British official stationed in India, began to develop a system of...
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