The Predictive Power of the Polygraph: The Lies Lie Detectors Tell

To the Editor.—  We were particularly interested by the recent report on the polygraph,¹ and commend the Council on Scientific Affairs for presenting a comprehensive overview of the current state of knowledge. The report drew attention to the problem of false-positive and false-negative errors in the interpretation of polygraph records. We recently attempted to quantify this problem by reviewing the published literature to determine the sensitivity and specificity of the polygraph and then using these data to calculate the predictive value of polygraph screening for the detection of liars.²We found the predictive power of the polygraph to be poor, since it supplies little new information over the "prior probability" that a subject is telling a lie. For example, if an investigator screened a group comprising 50% liars by flipping a coin, diagnosing a "liar" every time it came down heads, he would detect the actual liars in