Physical evidence cannot be wrong. But the human interpretation of that evidence can be very wrong indeed

Jeroen Hofman/gallerystock

The police brush the crime scene for fingerprints. They crunch the numbers. One name tops the list as a match for the physical evidence: yours.

But what if you weren’t even there? In the absence of a cast-iron alibi, your protestations aren’t likely to do you much good. The idea put forward in the “bible of forensic science”, Paul L. Kirk’s 1953 manual Crime Investigation, still holds sway today: “Physical evidence cannot be wrong.”

But the human interpretation of that evidence can be very wrong indeed. According to New York’s Innocence Project, an organisation devoted to exonerating the wrongfully convicted, unvalidated or improper forensic science has contributed to half the convictions overturned thanks to DNA evidence in the US since 1989.

Some techniques still used in the prosecution of capital offences, such as the interpretation of bite marks, are based on junk science. “Studies have shown that experts can’t tell whether a bite mark is even human, let alone who left the mark,” says Alicia Carriquiry, head of the Forensic Science Center of Excellence at Iowa State University in Ames.

In other areas, such as the analysis of shoe prints and tool marks, matches can be identified more accurately, but what that means is still unclear. “Trying to compute probability without really understanding what the frequency of different types of shoes and sizes in the population is – it’s practically impossible,” says Carriquiry. Efforts are therefore under way on both sides of the Atlantic to create large footwear reference databases, allowing for more realistic calculations of the chances of getting a match.

But even when such databases exist, as …