OPENING a new training centre in forensic science (pictured above) at the University of Glamorgan in South Wales recently, 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. And when it comes to the gulf between reality and fiction, Dr Knight knows what he is talking about: besides 43 years' experience of attending crime scenes, he has also written dozens of crime novels.

The upshot of this is that a new phrase has entered the criminological lexicon: the “CSI effect” after shows such as “CSI: Crime Scene Investigation”. In 2008 Monica Robbers, an American criminologist, defined it as “the phenomenon in which jurors hold unrealistic expectations of forensic evidence and investigation techniques, and have an increased interest in the discipline of forensic science.”

Now another American researcher has demonstrated that the “CSI effect” is indeed real. Evan Durnal of the University of Central Missouri's Criminal Justice Department has collected evidence from a number of studies to show that exposure to television drama series that focus on forensic science has altered the American legal system in complex and far-reaching ways. His conclusions have just been published in Forensic Science International.

The most obvious symptom of the CSI effect is that jurors think they have a thorough understanding of science they have seen presented on television, when they do not. Mr Durnal cites one case of jurors in a murder trial who, having noticed that a bloody coat introduced as evidence had not been tested for DNA, brought this fact to the judge's attention. Since the defendant had admitted being present at the murder scene, such tests would have thrown no light on the identity of the true culprit. The judge observed that, thanks to television, jurors knew what DNA tests could do, but not when it was appropriate to use them.

Cops and robbers

The task of keeping jurors' feet on the ground falls to lawyers and judges. In one study, carried out by Dr Robbers in 2008, 62% of defence lawyers and 69% of judges agreed that jurors had unrealistic expectations of forensic evidence. Around half of respondents in each category also felt that jury selection was taking longer than it used to, because they had to be sure that prospective jurors were not judging scientific evidence by television standards.

According to Mr Durnal, prosecutors in the United States are now spending much more time explaining to juries why certain kinds of evidence are not relevant. Prosecutors have even introduced a new kind of witness—a “negative evidence” witness—to explain that investigators often fail to find evidence at a crime scene.

Defence lawyers, too, are finding that their lives have become more complicated. On the positive side, they can benefit from jurors' misguided notion that science solves crimes, and hence that the absence of crime-solving scientific evidence constitutes a reasonable doubt and grounds for acquittal. On the other hand they also find themselves at pains to explain that one of television's fictional devices—an unequivocal match between a trace of a substance found at a crime scene and an exemplar stored in a database, whether it be fingerprints, DNA or some other kind of evidence—is indeed generally just fiction.

In reality, scientists do not deal in certainty but in probabilities, and the way they calculate these probabilities is complex. For example, when testifying in court, a fingerprint expert may say that there is a 90% chance of obtaining a match if the defendant left the mark, and a one in several billion chance of a match if someone else left it. In general DNA provides information of a higher quality or “individualising potential” than other kinds of evidence, so that experts may be more confident of linking it to a specific individual. But DNA experts still deal in probabilities and not certainties. As a result of all this reality checking, trials are getting longer and more cases that might previously have resulted in quick convictions are now ending in acquittals.

Criminals watch television too, and there is evidence they are also changing their behaviour. Most of the techniques used in crime shows are, after all, at least grounded in truth. Bleach, which destroys DNA, is now more likely to be used by murderers to cover their tracks. The wearing of gloves is more common, as is the taping shut—rather than the DNA-laden licking—of envelopes. Investigators comb crime scenes ever more finely for new kinds of evidence, which is creating problems with the tracking and storage of evidence, so that even as the criminals leave fewer traces of themselves behind, a backlog of cold-case evidence is building up.

The CSI effect can also be positive, however. In one case in Virginia jurors asked the judge if a cigarette butt had been tested for possible DNA matches to the defendant in a murder trial. It had, but the defence lawyers had failed to introduce the DNA test results as evidence. When they did, those results exonerated the defendant, who was acquitted.

Mr Durnal does not blame the makers of the television shows for the phenomenon, because they have never claimed their shows are completely accurate. (Forensic scientists do not usually wield guns or arrest people, for one thing, and tests that take minutes on television may take weeks to process in real life.) He argues that the CSI effect is born of a longing to believe that desirable, clever and morally unimpeachable individuals are fighting to clear the names of the innocent and put the bad guys behind bars. In that respect, unfortunately, life does not always imitate art.