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The Polygraph and Lie Detection
Accuracy, or criterion validity, is essential for the overall validity of a test: no test that lacks it can be accepted as valid. However, it is not sufficient: additional evidence of validity is needed to give confidence that the test will work well with kinds of examinees and in examination settings that have not yet been tested. Thus, another critical element of validity is the presence of a theory of how and why the test works and of evidence supporting that theory. Construct validity refers to how well explanatory theories and concepts account for performance of a test. Users can have greater confidence in a test when evidence of its accuracy is supported by evidence of construct validity, that is, when there is a chain of plausible mechanisms that explain the empirical findings and evidence that each mechanism operates as the theory prescribes.
In the case of lie detection by polygraph, one theory invokes the following presumed chain of mechanisms. Lying leads to psychological arousal, which in turn creates physiological arousal. The polygraph measures physiological responses that correspond to this arousal: galvanic skin response, respiration, heart rate, and relative blood pressure. The measurements taken by the polygraph machine are processed, combined, and then scored to compute an overall index, which is used to make a judgment about the examinee’s truthfulness. The validity of psychophysiological detection of deception by the polygraph depends on validity all along this chain. Important threats to construct validity for this theory come from the fact that the physiological correlates of psychological arousal vary considerably across individuals, from the lack of scientific evidence to support the claim that deception has a consistent psychological significance for all individuals, and from the fact that psychological arousal is associated with states other than deception. We discuss these issues further in Chapter 3.
As just noted, evidence supporting the construct validity of the test is important to give confidence in its validity in settings where criterion validity has not yet been established. It is also important for refining theory and practice over time: according to the theory mentioned, better measures of psychological arousal should make a more valid test. And it is important for anticipating and defeating countermeasures: knowing the strengths and weaknesses of the theory tells practitioners which possible countermeasures to the test are likely to fail and which ones to worry about.
The strongest scientific basis for a test’s validity comes from evidence of both criterion validity and construct validity. Nevertheless, it may be possible to demonstrate that an appropriately selected set of physiological measures has sufficient accuracy in certain settings to have practical