We’re relying more and more on DNA evidence to catch rapists and murderers, but there’s evidence that psychological bias plays a part in how this evidence is interpreted. Labs aren’t always as objective as we’d like them to be.

Recently, we’ve seen cases where DNA evidence freed innocent people from prisons, but sometimes, contaminated DNA evidence causes police to create a perpetrator in their minds who doesn’t really exist. This happened in Germany in 2007, when some contaminated swabs caused them to search for–as Vaughan Bell writes in the Observer–an "invincible, transsexual, border-hopping serial killer just to keep the story coherent with the genetic evidence."

What about fingerprints? Bell writes that, "Itiel Dror, a psychologist at University College London, found that forensic fingerprint examiners could come to different conclusions depending on what they knew about the case.

"In a landmark study, he gave five experts pairs of prints to compare" (these were prints that they had ALREADY looked at). "The examiners were told that the FBI had already mistakenly identified them as coming from the same person." Being informed that there was a mismatch caused only one of the five examiners to reach the same conclusion they had reached when studying the same prints before.

In the Observer, Vaughan Bell quotes Dror as saying, "The contextual influences are many and they come in many forms." Despite the evidence, most forensic scientists have rejected the suggestion that they are anything less than objective, or that their suspicion based on prejudice about a suspect’s lifestyle and beliefs.

Bell writes: "Justice, it seems, needs to be not just blind, but also psychologically validated."

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