Fingerprint Evidence in Criminal Cases

Fingerprint evidence, although sometimes not as high-profile as other high-tech crime-solving methods like DNA typing, is still very much used in criminal investigations and cases. While the principle that no two people can have the same fingerprints cannot be scientifically validated, fingerprint evidence is generally considered to be highly reliable and is particularly accessible to juries: You don't need a Ph.D. or a scientific lecture on genetics to understand that your own fingers contain a contour map of ridges and whorls that is completely unique.

Matching Fingerprints

Fingerprint evidence rests on two basic principles:

• A person's "friction ridge patterns" (the swirled skin on their fingertips) don't change over their lifetimes.
• No two people have the same pattern of friction ridges. Even identical twins have different fingerprints.

Police officers use fingerprints to identify defendants by comparing prints found at a crime scene with prints already in government databases. Fingerprinting algorithms used by the FBI have a matching accuracy of 99.6%.

People's fingerprints can be on file for a variety of reasons. For example, people may be fingerprinted when they are arrested or when they begin certain occupations. And it is increasingly popular for parents to ask local police departments or schools to fingerprint their young children, a grim reminder that children who are abducted or are the victims of other heinous crimes often cannot be identified otherwise.

How Fingerprints Are Found

Friction ridges contain rows of sweat pores, and sweat mixed with other body oils and dirt produces fingerprints on smooth surfaces. Fingerprint experts use powders and chemicals to make such prints visible. The visibility of a set of prints depends on the surface from which they're lifted; however, with the help of computer enhancement techniques that can extrapolate a complete pattern from mere fragments, and laser technology that can read otherwise invisible markings, fingerprint experts increasingly can retrieve identifiable prints from most surfaces.

The age of a set of fingerprints is difficult to determine. Therefore, defendants often try to explain away evidence that their fingerprints were found at crime scenes by testifying that they were at the scene and left the prints at a time other than the time of a crime.

To learn more about fingerprints and other evidence, read The Criminal Law Handbook: Know Your Rights, Survive the System, by Paul Bergman and Sara Berman (Nolo).