How Does a Lie Detector (Polygraph) Work?

lie detector
Does it work? Andrew Rich / Getty Images

­Y­ou hear about lie detectors all the time in police investigations, and sometimes a person applying for a job will have to undergo a polygraph test (for example, certain government jobs with the FBI or CIA require polygraph tests). 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:

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  • 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.

When the polygraph test starts, the questioner asks three or four simple questions to establish the norms for the person's signals. Then the real questions being tested by the polygraph are asked. 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 heart rate, higher blood pressure, increased perspiration) indicates that the person is lying.

When a well-trained examiner uses a polygraph, he or she can detect lying with high accuracy. However, because the examiner's interpretation is subjective and because different people react differently to lying, a polygraph test is not perfect and can be fooled.

Originally Published: Apr 1, 2000

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Polygraph FAQ

How accurate is a lie detector test?
The estimated accuracy of the polygraph test stands at 87 percent.
Can an innocent person fail a polygraph test?
The polygraph tests whether a person is lying or not by tracking breathing rate, blood pressure, perspiration and heart rate. An innocent person may fail the test out of pure nervousness. This is why these results are often dismissed in court.
Are lie detectors ever wrong?
According to the National Research Council, there is evidence suggesting that lie detector results can be fabricated.