Lie detectors are now a common device in police investigations, and sometimes a person applying for a job will have to undergo a polygraph test (eg, certain government jobs with State security require polygraph tests). the objective of a polygraph machine is to see if the person is telling the truth or lying when answering certain questions. The polygraph machine provides indicators when someone lies.
When an individual takes a polygraph test, 4 to 6 sensors are attached to them. A polygraph is a device in which the multiple (poly) indicators from the signals from the sensors are recorded on a single strip of moving paper (graph). the sensors record the individual’s:
- Breathing rate
- Blood pressure
Sometimes a polygraph will also record things like arm and leg movement.
When the polygraph test 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. 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. Generally, a significant change; eg fast heart rate, higher blood pressure, increased perspiration, indicates that the person is lying.
When a well-trained examiner uses a polygraph, they 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.