DAVID MANLEY: Have a seat, David.

DAVID POGUE: Wait, you're going to put me in the electric chair, already?

DAVID MANLEY: Well, not yet. If you'll…

DAVID POGUE: The whole idea of the polygraph is that lying can cause stress. In that case, my brain would signal my heart and lungs to work faster and harder, and I'd start sweating.

DAVID MANLEY: All right, for the purposes of the test, I need you to uncross your legs, put your feet flat on the floor.

DAVID POGUE: Next door, investigator Keith Gaines watches for these changes in me like a hawk.

KEITH GAINES (National Center for Credibility Assessment): This records respiration. We have sensors on the chest and on the abdomen that pick up the person's respiration during the test. This middle channel is recording perspiration across the surface of his fingers. And then your bottom trace is blood pressure and pulse rate.

DAVID MANLEY: All right, David. What I want to do now is I want to go over the questions with you that are going to be on the test.

DAVID POGUE: Wait what? You're going to tell me the questions before the test?

DAVID MANLEY: Yes.

DAVID POGUE: Well, doesn't that sort of give me an unfair advantage? I know what's coming.

DON KRAPOHL (National Center for Credibility Assessment): The purpose for reviewing the test questions is that we don't want to surprise the examinee with the test questions themselves, because the mere act of surprise can, in fact, cause reactions.

DAVID POGUE: But that's not what we see on TV and in the movies.

EDDIE (The Simpsons/Film Clip): Do you hold a grudge against Montgomery Burns?

MOE (The Simpsons/Film Clip): No. (Buzzer sounds.)

All right maybe a little…

DAVID POGUE: On the screen, questions get asked once and lies are caught right away.

MOE (The Simpsons/Film Clip): Good 'cause I got a hot date tonight. (Buzzer sounds.)

A date. (Buzzer sounds.)

Dinner alone. (Buzzer sounds.)

DAVID POGUE: Not so in the real world.

DON KRAPOHL: What we're very concerned with is reactions that occur with the same test question, repeatedly, over time.

DAVID POGUE: This is the real test?

DAVID MANLEY: This is the real test.

Did you plan with anyone to steal that money?

DAVID POGUE: No.

DAVID MANLEY: Did you steal that money from that wallet?

DAVID POGUE: No.

KEITH GAINES: He's reacting for some reason.

DAVID POGUE: Even though I think I'm doing a great job staying cool and collected, the sensors betray me.

KEITH GAINES: Right here, he was asked, "Did you take that money out of that wallet?"

DAVID POGUE: The yellow line that tracks my sweat response is literally off the chart.

The finger sensors seem to have picked up a small amount of sweat, blowing my cover.

This time, the polygraph looks like it detected my stress and my lies.

NITA FARAHANY: The polygraph is picking up your stress. It's picking up the degree to which you're responding stressfully, or it's difficult for you to make an answer. The idea is, the more you have to work to tell a lie, the more likely the polygraph is to register the difference from the baseline.

DAVID POGUE: But the polygraph doesn't always work with everyone. Some people, like this serial killer, have no trouble lying during a polygraph. They simply don't react the way most of us do.

NITA FARAHANY: Psychopaths don't have those physiological changes when they are lying. The very experience of being interrogated can be very stressful for some and not at all stressful for others.

DAVID POGUE: Innocent people can appear to be lying, even if they're just nervous, and that's one reason several states have banned its use as evidence in court.