You’ve seen it on Law and Order or one of the other cops-and-lawyers shows.

The suspect claims he didn’t do it. “Then you won’t mind taking a lie detector test to clear your name,” suggest the detectives.

Next scene they’re telling their captain either “He passed the polygraph, so we’ve got to keep looking for the killer” or “He flunked the lie detector — too bad we can’t use it in court.”

But in real life, although lie detectors (the popular name for polygraph equipment) are used by many police departments, the courts have very good reasons to keep the test results from being submitted as evidence, say scientists who have studied them — namely that the lie detectors don’t detects lies.

“Polygraph testing is no better than the reading of entrails,” University of Toronto psychologist John Furedy told a recent meeting of the Ontario Skeptics, referring to the ancient Roman practice of studying animal guts to predict the future. “Similarly, in North America the polygraph is all too readily referred to whenever the truth concerns some specific act like the identity of a killer or rapist, even some much less clear-cut issue, like whether Clarence Thomas sexually harassed Anita Hill a decade ago.”

The polygraph carries the aura of being scientific, with the sensors measuring supposedly uncontrollable human responses, such as blood pressure and skin conductivity, and the results printing out on graph paper, overseen by a seemingly objective expert. The subject’s relative anxiety as he answers the questions is thought to be reflected in the polygraph’s output. Use this hyperlink to get some additional info.

But Furedy says his and other studies have shown it is virtually impossible for a polygraph to differentiate between an anxious-but-innocent party and an anxious-and-guilty party when the polygraph’s needles swing widely.

“There is no way of knowing whether this large response is due to an innocent person being nervous about the accusation, as against a guilty person being nervous about being caught.”

After dozens of studies by medical schools and psychology departments, universal correlation has been found between telling a lie and having a stronger heartbeat or having suddenly sweatier palms.

Moreover there are proven techniques for fooling the polygraph. An Internet survey turns up many tips for hiding one’s nervousness from the polygraph — whether you have a guilty conscience or are just naturally twitchy. One of the most popular techniques is clenching your sphincter while relaxing your buttocks. This anal puckering may sound a bit difficult to do at first but a half hour’s practice could help you evade that twenty-year jail stretch.

There’s a still greater problem with the test. A subject’s nervous response to questions about the misdeeds of which he is suspected can be rated only in comparison to his response to “control” questions. The examiner may first ask innocuous questions, regarding perhaps the subject’s name, age and work to find that person’s normal level of response. He will also ask questions concerning matters that might be expected to elicit guilty responses – such as asking about masturbation or about driving after drinking. Everyone is assumed to have engaged in these activities at some time but is likely to lie about them, thus giving the examiner an idea of the subject’s level of response when being moderately deceptive. Then when the questions concerning the real issue are made, the subject’s responses are compared to these control questions.

You can probably spot the flaw: Not everyone has the same level of guilt about every topic. It would take an extensive psychological profiling of a subject to reach conclusions about the significance of that person’s response to the control questions, before the responses to the serious test questions are compared.

So what should you do if you are pressured by a real-life Lenny Briscoe to take a test “to clear your name”?

Refuse, says Furedy.

If you do have to take a polygraph however and the examiner says the test has found you guilty or deceptive, “then you should leave immediately rather than try to defend yourself,” says Furedy.

That’s because although the test results may be wildly misleading, investigators can use the claimed conclusion to try to pressure you into confessing in the post-test interview phase – and anything you say then can be introduced in court.

This is one reason why police use polygraphs despite their unreliability. They can pressure nervous suspects into confessing or revealing damaging information.

Whether innocent or guilty.