How accurate is polygraph test?

The answer to this question lies somewhere between "completely foolproof" and "worthless," depending on who you ask.

Polygraphs measure changes in respiration, heart rate, blood pressure and galvanic skin response (perspiration, basically) as the subject is asked a series of questions. The theory is that subjects will experience stress when trying to conceal information, and that the stress will be reflected in their physiological measurements.

Even the most outspoken proponents of the polygraph agree that the skill and integrity of the operator are critical to a reliable polygraph exam. Almost anyone can learn the mechanics of how to operate a polygraph, but the best polygraph examiners are also expert interrogators. They set the stage so that the subject is encouraged to believe that any attempt to lie will be discovered immediately, so there's no point in trying to beat the test. When subjects "confess" and recant a prior exculpatory statement, it's far more common for this to happen during a pre-exam or post-exam interview than during the test itself.

There are no consistent standards for polygraph operators. Some states have requirements for people seeking licenses as polygraph examiners, but a person who performs polygraph exams as an employee of a business or for a government agency may not require a license, so long as he doesn't ofter his services to the public at large. Since a subject is seldom privy to the credentials of the examiner, he takes the exam at his own peril.

There is another type of "lie detector," called a Computerized Voice Stress Analyzer or CVSA. These devices measure "microtremors" in the test subject's voice that are claimed to indicate deception. The time required to become certified as a CVSA operator is far less than the weeks of training for a polygraph course, and it is offered only to employees of organizations that have purchased CVSAs.

Scientific tests of the CVSA indicate that the device is no more reliable at detecting deception than the flip of a coin. Polygraph evaluations don't fare a lot better in these tests, but there is some documented science behind it.