You can fool a lie detector test with just one simple movement - and get away with it

It's safe to say, if you find yourself in a situation where you're wired up to a polygraph machine, something has gone wrong.

And you could be in for a lot of trouble.

The accuracy of lie detector tests has been debated and speculated over for a long time.

One big flaw is the fact an honest person may be nervous - and a dishonest person may be non-anxious.

That said, while not one percent foolproof, the consensus seems to be lie-detector tests ARE still accurate.

In fact, if you wanted to beat one - and we're not recommending you do this - there is apparently one tiny thing you can do.

Doug Williams joined the police force in Oklahoma in 1972 and as part of his training, actually went to polygraph school.

He later became a certified expert in lie detector tests, Mamamia writes.

As time passed, however, he began to tire of the process and begin to wonder exactly how effective the tests were.

"I began to doubts in the tests after a while, I knew I could control my breathing but I didn't know for sure how to control the cardio and the blood pressure," he told radio show This American Life.

"It wasn't until my friend came in and started talking about the pucker factor and tightening up the anal sphincter muscle when he was under stress."

And that, as it turned out, proved to be the trick to duping the polygraph machine - clenching your anus.

"After he left, I just hooked myself up to the polygraph test...and tightened up my anal sphincter muscle like I was trying to stop my bowel movements and low and behold there was the most gigantic, wonderful, naturally occurring cardio rise, accompanying a GSR rise."

While it's very kind of Williams to share this information with the world, his candour actually got him prosecuted for obstruction of justice and mail fraud.

Still, as he puts it in his own words, "It takes an a-hole with a little training to beat an a-hole with a little training."