Lie detector tests have been used for hundreds of years and are still based on the same principles of lying. A polygraph test records deviations from a person's normal behavior. When a person is stressed or under pressure, their body activates the fight or flight response. This causes a spike in adrenaline, increase in heart rate, deepened breathing and perspiration.
In order to conduct these tests, a person is asked a set of control questions. Their standard levels are derived from simple obvious questions. Once the control questions have established a "baseline for truth" the examiner then begins asking uncomfortable questions looking for a change in the monitored areas.
Many scientists claim that these tests are not valid because they are based on trickery rather than science. Another common criticism is that they do not follow the basic scientific principles. Because the individual interpretation of the test is subjective and people react differently to lying, the results of this test are not easily replicated. Other scientists believe that the test is, "inherently biased against the truthful." Genuinely honest people tend to answer the control questions without hesitation and with little physical reaction. However, in return it leaves more room for small physical disturbances to be confused and interpreted as a lie.
Scientists also argue that since these tests have no pure scientific substance, they can be easily fooled. The test focuses on a person's physiological reaction to the control questions, so by altering ones reactions to the assumed correct responses, a person is able to effectively lie. Popular ways to fool the test include: doing mental mathematics, altering ones breathing pattern, biting the side of the tongue or simply thinking exciting thoughts. By distracting the mind from the uncomfortable and sometimes incriminating questions, a person is able to effectively pass the test, proving that the test is inconstant and not credible.