In an ideal world, a lie detector machine would measure some involuntary response which everyone has when they lie, but not when they tell the truth.  Unfortunately, there is no such response and, because of this, no such thing as a perfect lie detector.  The so-called lie detector tests you've heard of are usually polygraph tests.  How does this test work, and how accurate is it?

A polygraph generally measures four things.  Electrodes on your palm measure how much you sweat.  A rubber tube strapped around your chest detects changes in breathing.  A blood pressure cuff measures both pulse and blood pressure.

During the course of the test, the examiner usually asks about a dozen questions.  Most of these are neutral-like "What is your name?"-but mixed in are the important questions-"Did you kill your wife?" for example.  In theory, if a subject lies on an important question, his polygraph response will be different from the truthful responses to the neutral questions.  Does this really work?  It's true that a polygraph can measure emotional stress, and it's also true that MOST people have greater emotional stress when they lie.  Unfortunately, many people are likely to have an emotional response to the important question, whether they lie or not.  After all, "Did you kill your wife?" is more emotionally charged than "What's your name?"  This effect is so great, one study has shown that you've got a fifty-fifty chance of failing a polygraph test, even if you're innocent!  Because of these odds, lie detector tests are not allowed as evidence in court cases.