Faith In "Scientific" Evidence Is Sometimes Misplaced

No two sets of fingerprints are the same. That statement or something to its effect has been repeated in so many television shows and movies that it is well-established in our culture. Yet, a review of the results from fingerprint examiners shows a shockingly high level of variation. When a group of examiners were asked to retest prints 7 months after they first analyzed them, the examiners reproduced the same result roughly 90 percent of the time. That means 10 percent of the examiners disagreed with their own prior work. Evidence, such as fingerprint analysis, that has the air of science around it may be given undue weight by judges and juries.

Junk science has a long and infuriating history in American courtrooms. Fingerprints are one form of pattern evidence that is often misrepresented by forensic examiners. Other forms include tire track analysis, shoe print analysis, and bite marks. Evidence that might suggest some likelihood is instead presented as incontrovertible proof. Instead of acknowledging that fingerprints exclude some percentage of the population from suspicion, forensic examiners have testified that the prints guarantee that a specific person committed a crime. There is no scientific basis for these claims and some jurisdictions have begun to examine just how much weight this evidence can be afforded.

The National Academy of Sciences concluded in 2009 that these types of evidence have not been proven to have the ability to link a specific individual or item to a crime. The group called for greater research to develop an accurate picture of the value of such evidence. Does a shoe print make it 90 percent likely it came from a defendant's shoe? Is it 99 percent likely? No one knows, at present, and any claims to the contrary have always been false.


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