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.
Fingerprint evidence rests on two basic principles:
Police officers use fingerprints to identify defendants by comparing prints found at a crime scene with prints already in police files. (Today, the FBI has a collection of prints that numbers in the millions.) 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.
Fingerprint experts can disagree about how many points in common are needed to declare a match between two sets of fingerprints. For example, some experts will declare a match based on only 12 points in common, whereas other experts may require up to 20 points in common before declaring a match.
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 almost impossible 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 fingerprint and other evidence, read The Criminal Law Handbook: Know Your Rights, Survive the System, by Paul Bergman and Sara Berman (Nolo).