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.

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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.