At the scene of a recent crime, a police officer dusted the murder weapon for fingerprints. Later, back at the forensic lab, the cop used a computer to cross-reference the evidence against all the fingerprints on file in the department's database. The computer rapidly ticked through thousands of images until it landed on a match. "We got him," the cop said, as he left the lab to go make an arrest.

For television viewers, it's a familiar scene on popular shows such as "Crossing Jordan," "NCIS," "Cold Case," and "CSI: Crime Scene Investigation" and its two spin-offs. The problem is that no such fingerprint-matching technology exists. It's fiction. But it illustrates why some say the current spate of television crime dramas is giving people unrealistic expectations about forensic evidence. That's having a very real effect on the American legal system.

Prosecutors, district attorneys and academics have spent the past few years debating the "'CSI' effect"--the idea that members of a jury, weaned on a diet of procedural police shows, now have misguided notions about the kind of physical evidence to expect in a trial. Jurors have come to demand more hard forensic data, the theory goes, and they're becoming less likely to convict a defendant without it.

In terms of conviction rates, the "CSI" effect is only anecdotal. But juries are definitely developing inaccurate ideas regarding forensic data, says Jane Campbell Moriarty, a professor at the University of Akron School of Law and the author of "Misconvictions: When Law and Science Collide," due out next year. "Jurors start to believe that 'CSI' is real," Moriarty says. They expect to see the evidence laid out in unequivocal black-and-white terms, just like on TV. "They're getting a very skewed view about what forensic science even is."

For law enforcement officials, the debate over the "CSI" effect is moot. The mere perception of it is already changing the criminal process at every level. Attorneys have begun asking jurors during the voir dire process about whether they watch shows such as "CSI." Some judges have begun specifically instructing jurors not to expect the kind of forensic evidence they've seen on TV.

And the desire for more hard evidence is putting greater demands on crime labs. "I think [these shows] have had a big effect in a number of ways," says Tami Aker, commander of the crime lab unit for the Minneapolis Police Department. "They use equipment on TV that doesn't exist. You don't go to a crime scene, get a blood sample, and then put that in a computer and someone's face pops up on the screen. It just doesn't work like that." Lab technicians also are finding themselves asked to produce results more quickly. "It's put a lot of pressure on us to be faster, and you can't be faster," Aker says. "You're not going to get results in an hour. It's typically going to take more like 30 to 60 days."

Forensic TV shows even seem to be changing the way crimes are committed in the first place. Criminals are working harder to cover their tracks, removing evidence after they commit a crime or using bleach, which destroys DNA, to clean up the crime scene. In December, when a suspect in Baltimore confessed to a murder and to taking elaborate steps to destroy evidence, police investigators asked him where he'd learned his techniques. His reply: "Just 'CSI,' I guess."

It's not all bad news. The current focus on forensic evidence is pressuring crime labs to improve their standards, which is a good thing. And the number of students interested in careers in forensic science has skyrocketed along with the popularity of these crime shows. "And that's great," says Aker. "But I always tell those kids, 'It's not like on TV. The kinds of jobs you see on those shows are very few and far between.'"