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How do lie detectors work?

A person receiving a lie detector test (file picture)

A person receiving a lie detector test (file picture)

In police investigations - and often when someone applies for a job when national security can be compromised - a suspect or applicant will be subjected to a lie detector or polygraph test. They are now routine for U.S. government jobs with the FBI or CIA. The goal of a lie detector is to see if the person is telling the truth or lying when answering certain questions.

When a person takes a polygraph test, four to six sensors are attached to him. A polygraph is a machine in which the multiple ('poly') signals from the sensors are recorded on a single strip of moving paper ('graph). The sensors usually record a person's breathing rate, pulse, blood pressure and perspiration. They can also sometimes measure someone's arm and leg movements.

When the polygraph test starts, the questioner asks three or four simple questions to establish what normal signals are for the person being investigated. Throughout questioning, all of the person's signals are recorded on the moving paper.

Both during and after the test, a polygraph examiner can look at the graphs and can see whether the vital signs changed significantly on any of the questions. In general, a significant change - such as a faster heartrate, higher blood pressure or increased perspiration indicates that the person is lying.

But critics say that it easy to beat lie detectors - simply by 'lying with your body as well as your words'.  If a person substantially increases their respiratory rate, blood pressure and sweat level while answering standard questions, their answers to other questions (whether they are truths or falsehoods) will seem true.

 This was done in the film 'Ocean's Eleven', and is usually achieved by pressing down on a sharp object such as a drawing pin during routine questioning. This will cause more perspiration or a faster heart beat which in turn bamboozles the lie detector.

But while this countermeasure (if properly applied) can be effective, polygraphers have developed counter-countermeasures for it - the simplest being to simply make the subject remove his shoes. 

While both sides are in competition like this, the debate over the reliability of lie detectors continues.  That is especially the case when people who are mentally ill are subjected to polygraph tests -  because they do not know or cannot express the truth, the argument goes, they will hardly likely to be exposed as liars.

Sources: Howstuffworks.com and Livescience.com 

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