conductance at the skin surface).2 A polygraph examination includes a series of yes/no questions to which the examinee responds while connected to sensors that transmit data on these physiological phenomena by wire to the instrument, which uses analog or digital technology to record the data. Because the original analog instruments recorded the data with several pens writing lines on a moving sheet of paper, the record of physiological responses during the polygraph test is known as the polygraph chart.

A variety of other technologies have been developed that purport to use physiological responses to make inferences about deceptiveness. These range from brain scans to analyses of voice tremors; some evidence relevant to these techniques is discussed in this report.

Physiological Phenomena

The physiological phenomena that the instrument measures and that the chart preserves are believed by polygraph practitioners to reveal deception. Practitioners do not claim that the instrument measures deception directly. Rather, it is said to measure physiological responses that are believed to be stronger during acts of deception than at other times. According to some polygraph theories, a deceptive response to a question causes a reaction—such as fear of detection or psychological arousal— that changes respiration rate, heart rate, blood pressure, or skin conductance relative to what they were before the question was asked and relative to what they are after comparison questions are asked. A pattern of physiological responses to questions relevant to the issue being investigated that are stronger than those responses to comparison questions indicates that the examinee may be deceptive.

The central issues in dispute about the validity of polygraph testing concern these physiological responses. For example, are they strongly and uniquely associated with deception, or are there conditions other than deception that could produce the same responses? Does this association depend on particular ways of selecting or asking questions, and if so, do examiners ask the right kinds of questions and make the right comparisons between the physiological responses to different questions? Is the same association of deception with physiological response observable across all kinds of examinees in all kinds of physical and emotional states? Does it depend on factors in the relationship between examiner and examinee? Is it influenced by an examiner’s expectation about whether the examinee will be truthful? In Chapters 3, 4, and 5 we discuss in more detail the theory of the polygraph and two kinds of evidence on these questions. One comes from basic psychophysiological research on

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