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Question
How does a polygraph (a lie detector) work?
Answer
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 the person. 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:
When the polygraph tests starts, the questioner asks 3 or 4 simple questions to establish the norms for the person's signals. Then the real questions being tested by the polygraph are asked. All during 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 (e.g. - faster heart rate, higher blood pressure, more perspiration, etc.) 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.
Here are some interesting links:
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