How does a polygraph (lie detector) function?

You often hear about polygraphs in criminal investigations, and sometimes individuals applying for specific jobs, such as those with the FBI or CIA, are required to undergo polygraph testing. The purpose of the lie detector is to determine whether the individual is being truthful or deceptive when responding to particular questions.

During a polygraph examination, four to six sensors are connected to the individual. The polygraph records the signals from these sensors onto a single moving paper strip, also known as a 'graph'. These sensors generally monitor:

Occasionally, a polygraph may also record other factors such as movement in the arms and legs.

At the beginning of a polygraph test, the examiner asks a few basic questions to establish the baseline for the person's physiological responses. Once this is done, the key questions under investigation are posed. Throughout the questioning, the machine records the person's responses on the moving paper.

During and after the test, the polygraph examiner examines the graphs to determine whether there were significant changes in vital signs for any of the questions. Typically, a noticeable shift (for example, a quicker heart rate, increased blood pressure, or heightened perspiration) suggests that the person is not being truthful.

When a skilled polygraph examiner conducts the test, they can often detect deception with considerable accuracy. However, due to the subjective nature of the interpretation and the fact that people may react differently to lying, polygraph tests are not foolproof and can be deceived.