Former FBI agent Clint Van Zandt comments for MSNBC. Excerpt:
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A polygraph does not really separate truth from lies; or the honest from the liar. It simply provides information concerning any change in physiological response in areas such as respiration, heartbeat and blood pressure, this while the person being tested undergoes questioning.
As a retired FBI agent, I don’t want to take anything away from my former law enforcement colleagues who practice the art (vs. science) of detecting truth, but I have been less than confident in the statistical success rate of the polygraph, having seen killers “pass” the test, and honest people “fail.”
You see, the prevailing theory behind the polygraph is that when someone tells a lie, they become nervous about their lies and their nervousness causes changes that can be noted in their breathing, their heartbeat, their perspiration and their blood pressure. An initial baseline is established by asking questions of the person being tested whose correct answers are known to the polygraph operator (or forensic psychophysiologist). Deviation from the known baseline for truthful answers is then taken as an indication of deception.
But what about psychopaths, sociopaths or just damn good liars? If the old adage is correct, i.e., “If you believe it yourself it then passes as truth,” or if you have learned to control of your bodily reactions, why can’t you “pass” a lie detector test even if you are “lying?”
And what about the opposite: What if you are completely innocent but nervous, angry, sad, embarrassed or just fearful of a test whose results may affect your entire life? Or what if you have a cold, a muscular problem, a headache, or if you’re simply constipated? Can these purely non-voluntary bodily symptoms or conditions affect the physiological changes that are being measured against “the truth baseline?” And what if you are nervous? Is the nervousness due to the fact that you know you’ve done something wrong and may get found out by the polygraph, or are you simply nervous for any number of other reasons–all totally unrelated to your complicity in some suggested criminal act?
The ACLU, an organization with which I do not normally side, supported the passage of the 1988 Employee Polygraph Protection Act that outlawed the use of the polygraph “for the purpose of rendering a diagnostic opinion regarding the honesty or dishonesty of an individual.”
Does the polygraph, in the hands of a trained, competent individual really allow the operator to detect deception on the part of the person being tested? Well, yes and no. Try betting your life or your career on that one!