It's an almost mythical machine, a bundle of wires and metal with the ability to read minds.

On the surface, the lie detector could solve a myriad of problems. Cheating husband? Dishonest employee? Potential terrorist? Just hook them up to the polygraph and let science be the judge.

A polygraph machine can't actually tell what a person is thinking, of course.

But it does measure heart rate and blood pressure, respiratory rate and sweatiness. It's through these responses that the examiner determines whether a subject is answering truthfully.

While some praise the polygraph for the investigative latitude it provides, critics say there is no magic bullet, and that all too often the lie detector gets it wrong.

'Mythological device'

"The polygraph is the translation of a mythological device into a technological idiom," says Steven Aftergood of the Federation of American Sciences told the Boston Globe.

He adds: "What the polygraph measures is not truth and deception, but perspiration and respiration."

But police, investigators, and government agencies around the world still rely on lie detectors, even if they still aren't generally used in many courts.

HOW A LIE DETECTOR WORKS A polygraph works on the principle that a person who is lying will show signs of stress Pneumographs (1) measure breathing rate Galvanometers (2) test how much the subject is sweating Cuff (3) measures heart rate and BP which increase under stress The results from each instrument appear as wave patterns By comparing the patterns with those when the subject was definitely telling the truth, the examiner can spot a potential lie

In the US 18 of 50 states allow evidence gleaned from lie detector tests in some instances.

Part of the reason investigators rely on the polygraph is because of the popular belief that the test is rarely wrong.

Just the suggestion of a lie detector test could goad a suspect into a confession, or otherwise move an investigation forward.

"This belief is propagated by numerous television dramas that portray polygraph tests ... as accurately revealing hidden truths," three members of a group that studied polygraphs for the US National Research Council said.

"Unfortunately, the best available technologies do not perform nearly as well as people would like or as television programs suggest," the trio wrote in the journal Issues in Science and Technology. "This is unlikely to change any time soon."

Reliability questioned

The American Polygraph Association, which represents 2,600 examiners in 31 countries, unsurprisingly defends its profession and says that lie detectors have been used by cultures for centuries.

"Its origin stems back thousands of years because even ancient civilizations needed a way to determine the truth," it says.

"Those beginnings included a test in which the suspect person was given a mouthful of dry rice and if that person could salivate enough so as to swallow the moistened rice, he or she was deemed innocent or truthful to his or her response."

Professor Don Grubin of the University of Newcastle's forensic psychiatry department says there are two kinds of bad responses polygraph tests can produce.

These are false negatives, where the test misses someone who's lied, or false positives, when a person who's telling the truth is not believed.

"The false negative rate is probably lower than the false positive rate," Prof Grubin says. "In terms of the actual numbers, my estimate would be ... when you're saying someone is telling the truth, that's probably correct 90% of the time.

"When you say someone is lying, that's probably correct about 85% of the time, so it's slightly less accurate."

Professor Grubin says that lie detectors do provide the impetus for people to disclose facts either while the test is ongoing, or after a test. And he says they have more uses than just in investigative situations.

"There's a lot of applications, and I think that's one of the things people get confused about," he says, pointing to the use of polygraphs with sexual offenders in the US. "There are post-conviction and treatment uses.