How
can they tell if you’re lying?
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:
- The person's breathing rate
- The person's pulse
- The person's blood pressure
- The person's perspiration
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.
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