How does a polygraph lie detector work?

Question

I know it's based on your heartrate, blood pressure etc.

But if someone asked me questions whilst I was hooked up to a machine, it'd probably scream liar even if I was being truthful.

You know when someone's done something bad at work - eg stolen something - don't you always blush even though it's not you?!

So how can it distinguish the nervous from the liars?

Answer 1

I've always wondered that.  Also if it was relating to something so important, that you've heard asked of you a million times, it becomes such an issue that it affects you anyhow.

Answer 2

I didn't read the other answers, so disregard my answer if you already got a satisfactory one to your question!

A polygraph test will  measure your blood pressure, pulse, and sweat activity.

It's based on the principle that when you lie all of these measurements will increase.

You said that you'd be nervous if you had to take a lie detector. Which of course is true. But a polygraph measures your baseline at the time.

And of course the administrator will ask you test questions, like your name which you will answer truthfully, and test question which you'd answer un-truthfully.

So say your regular stats are at 100.

Your stats while taking the polygraph might be at 150.

But while lying they're at 200.

But some people are better at lying than others, so their stats won't change. That's why it's not admissible.

There's a test called a voice/stress analyzer, based on the concept that a your voice increases and it's more reliable than a polygraph!!!

next time when you ask someone a question they want to conceal the answer to, listen to their pitch of voice. It would usually be higher than the tone they used previously!!!

Edit: typically polygraph examinations take hours. You won't fail regardless of the truth. But it will come up non conclusive if you're a good liar.

And there's a scale, it's not black or white.

Think about it for a second, ask yourself have you ever thought I want to kill you?

Most of us have, but we never meant it.

There's a large gray area!!

Misty, you can't "beat"  polygraph, but you can't make the results non-conclusive.

Like I said there's a scale say 0-29. I'm not 100% sure. But that's what sticks in my head!

So if you score a 15 you score non conclusive!!

Answer 3

You go through some test questions first to get a baseline result.

They are not reliable though which is why evidence from them is not admissible in court

Answer 4

The polygraph is set up by asking the subject a series of questions to establish a baseline response to which the results of further questioning is compared. So if you're naturally a 'jumpy' person it's already taken account of in your results.

Answer 5

Latest news-http://ndtvnews.net/

Answer 6

Am not very sure...and i'm sure i'd fail one too regardless of the truth!

Methinks it relies not only one's physical reactions like heartrate & pressure that you mention but that there are also trickily worded questions (not unlike the ones some people out here frame!) that are repeated. The point being that if you answer in the affirmative to one & in the negative to another, you're lying.

The tricker part is that these Qs confuse more than anything!

i don't think its a very reliable or scientific method of ascertaining the truth. With a little yoga & meditation even the most guilty of them all can manage to control ones heartrate etc & you'd have innocents like you...blushing to their roots for nothing done wrong.

Answer 7

The person's breathing rate

The person's pulse

The person's blood pressure

The person's perspiration

Sometimes a polygraph will also record things like arm and leg movement.

Answer 8

Others have given the answer re establishing a baseline but did you know that it's possible to beat the lie dectector by pulling in the muscles of your bum.

Now, was I lying........ or not?

Answer 9

Here's the deal- they are so unreliable, caused by the facts that you have mentioned , they are not accepted as evidence in a court of law.-A company that I once worked for was having a security problem- Everyone had to take a polygraph test- one of the employees was shown to be lying when he told them his name. How reliable is that?