Why do most psychologists believe that the polygragh is not dependable in detecting liars?
The difficulty that psychologists, and most of the scientific community, have with the polygraph test is its inability to accurately detect if someone is truly being deceitful.
The polygraph is designed to measure physiological responses that are supposed to indicate concern on the part of the person being tested; the premise is that people cannot alter their physiological responses.
A polygraph (popularly referred to as a lie detector) is an instrument that measures and records several physiological indices such as blood pressure, pulse, respiration, and skin conductivity while the subject is asked and answers a series of questions, in the belief that deceptive answers will produce physiological responses that can be differentiated from those associated with non-deceptive answers.
However, the difficulty is that the tests are not accurate; in fact, studies indicate that the polygraph is only 61% effective.
For example, some spies—even murder suspects—have been tested and passed the test, only for [government] officials to discover much later that the individuals in question were lying. Innocent people suspected of committing serious crimes have also been shown to be lying—the presentation of conclusive evidence has often been the only thing to remove suspicion from them.
The erroneous results are called "false positives," providing inaccurate feedback and, therefore, no reliable, conclusive results for the tests. A "false positive" can show innocence where there is guilt, and guilt where there is innocence.
This form of screening is still used by some government agencies (FBI, CIA), and by employers as part of their hiring process. Many foreign countries still use the lie detector (polygraph). However, it would seem, based on one report, that being rested and calm, while establishing a good rapport with the tester, allows the person being tested to pass without any indication of deceit, or criminal activity or intent.
It is for these many reasons that the scientific community, in general, frowns upon the use and reliability of this form of testing.
Although there are claims that 90% to 95% of polygraph tests are accurate, psychologists rate the test as only about 61% accurate. Critics claim that since the test does not result in perfect accuracy, then about 10% of the people taking the test are actually lying. In a court of law, critics claim, even a 90% possibility of truth is not the same as having a reliable eyewitness. Also, there is often about a 10% rate of false positives in people who are answering every question honestly, and that the test is biased against innocent people; thus, it would be unfair for people to accept undue consequences or punishment for "failing" the polygraph test. Also, people have been known to "beat" the test by "artificially augmenting responses to control questions. (Control questions are part of the pre-test Q&A given for computerized polygraph testing.)