How Does A Lie Detector Work?

When a person takes a lie-detector test, four to six sensors are attached to the person. It is a machine in which the multiple signals from the sensors are recorded on a single strip of moving paper (or graph).

The sensors usually record the person's breathing rate, pulse, blood pressure and perspiration. Sometimes it may also record arm and leg movement.

When the 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 lie detector are asked. Throughout questioning, all the person's signals are recorded on the moving paper.

Both during and after the test, the examiner can look at the graphs and can see whether the vital signs show change 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."

Published in The Hindu on June 13, 2002.