You know, just in case.

This is actually surprisingly simple. What happens is that a polygraph measures your body's level of physiological arousal (not that kind of arousal, get your mind out of the gutter!). Now, a trained spy or criminal can train themselves to feel no anxiety at all when they lie, thus no sweat or faster heartbeat or other arousal responses. A sociopath can do the same thing with no training at all, because, by their nature, it just doesn't matter to them.

But I'm going to assume you're neither spy nor sociopath, because if you were, you wouldn't need me anyway. So, when you have to take the polygraph test, take a thumbtack and hide it in your sock, so that it's not pressing into the sole of your foot, but you can bear down on it easilly. Now, when a polygraph test is administered, you'll first be asked a series of questions to center the test, a series of obvious but inconsequential questions like "Is your name John Smith?", and "Do you have brown hair?" For all of these questions, press down hard on the thumbtack with your foot when you answer. When the test begins in earnest, press the tack into your foot when you answer a question truthfully, but don't when you need to tell a lie. That's all there is to it.

Why this works: The pain which stepping on the thumbtack produces causes your body to react with the release of adrenaline and an arousal response, the same as lying would. When you step on the tack through all of the centering questions, you establish a higher baseline for yourself. When you step on the tack when you answer real questions truthfully, you conform to this baseline, and you lie without stepping on it, you're close enough to that baseline that even a skilled polygraph operator won't be able to tell the difference. Unless they notice you limping when you come in, which is more or less a dead giveaway.

Some of your more sophisticated outfits with experience administering polygraph tests know to look for this now, but with a bit more discipline, you can achieve the same affect by biting the inside of your cheek, or clenching your muscles as much as possible, though I'm told this takes some practice to pull off right.

Voters may like to look at the previous writeup before voting on mine.

If you don't have access to a thumbtack (What with you being in prison and all), you can use pretty much any other non-visible technique to create false highs when asked baseline questions.

Two techniques that can be put to quite effective use are biting down on your own tongue and clenching your buttocks as hard as you can.

The on-paper results of the lie detector test aren't the only thing you need to control, however. The FBI know that you can trick the lie detector, and thus don't rely solely on its outputted results. It is also intended as a psychological tool; They let you stew for a while locked in a plain room whilst they go and get the machine, then they hook you in, sticking electrodes all over your body to measure your responses.

With people who lack our knowledge, this is understandably stressful; they are hooked up to a machine that can see into their thoughts, a machine that can detect their lies easily. 'YOU CANNOT TRICK THE MACHINE, SON' it says, followed up with 'YOU COULD GET TWENTY YEARS FOR THIS, UNLESS YOU CO-OPERATE.'

It is, in other words, a tool for threatening suspects with discovery, in the hope they will crack and make a confession. Resisting the police trying to 'sweat' a confession out of you is half of beating the lie detector.

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