It seems counter-intuitive, but in criminal trials lawyers for both the prosecution and the defense want jurors who know as little as possible about the issues involved in the case. For example, if you are an accountant and you are called for jury duty in a case involving embezzling by an accountant, you’ll be one of the first dismissed by the lawyers (in a legal process called voir dire). The reason is that lawyers on both sides want jurors whose opinions they can influence and mold to their benefit. Any expertise or specialized knowledge should come from one side or the other, not from the jurors’ own experience or education—and certainly not from television.
Experts are finding that TV crime dramas are having an increasing influence on juries. According to a recent article in The Economist, “Bernard Knight, formerly one of Britain’s chief pathologists, said that because of television crime dramas, jurors today expect more categorical proof than forensic science is capable of delivering.” It’s been dubbed the CSI Effect, and the most obvious symptom is “that jurors think they have a thorough understanding of science they have seen presented on television, when they do not.”
As they say, a little knowledge is a dangerous thing. For example, jurors who watch a lot of crime shows with cases which are solved by good quality forensic evidence (such as clear fingerprints) might expect to find fingerprints at the scene of a crime. If a defendant’s fingerprints are not found at the crime scene, a juror might conclude that he or she wasn’t there, whereas in fact there could be many reasons why a person might not leave identifiable fingerprints—for example the criminal could have worn gloves or simply not touched any smooth, print-revealing surfaces.
Another potential issue is jurors expecting more conclusive evidence than science can provide. In fictional crime dramas the evidence is usually presented as conclusive. Either the suspect’s DNA matched the sample found at the crime scene or it did not; either the eyewitness positively identified the suspect from a lineup or she did not. On TV and in the movies the answers are all very nice and tidy.
But in the real world, science does not operate on certainties. There is always a margin of error; in DNA tests it is often small; in eyewitness identification it can be huge. As detectives and psychologists well know, eyewitness testimony is often unreliable and must be corroborated with other evidence. There’s really no such thing as a 100% forensic match, and forensic experts often speak in terms of statistical likelihoods. Thus if jurors are expecting a perfect match of evidence (DNA samples, tire tracks, fingerprints, etc.), they may be unduly dismissive if the match is only 80% or 90%.
There is of course a good side to this: crime and medical dramas often do have some good science in them and can help educate the public. But it’s not always clear just how much artistic license the screenwriters take.