validity of inferences of deception with certain populations and in certain situations that have not been resolved by empirical research. These issues are raised later in the chapter; the relevant empirical data are discussed in Chapter 5. The other is that in the case of polygraph security screening, the empirical record necessary for an atheoretical justification of the test does not exist, and is unlikely to be developed, because of the difficulty of building a large database of test results on active spies, saboteurs, or terrorists.


This is the case even when the response reflects a change in the activation of a specific region of cortical tissue (see Sarter, Berntson, and Cacioppo, 1996).


Converging evidence is always important in making inferences using the subtractive method because this method assumes that components or processes can be inserted or deleted without altering other components or processes (e.g., relevant and control questions differ only because the relevant questions have special meaning to deceptive individuals). This may not be true in relevant-irrelevant and comparison question polygraph tests. In concealed information tests, when only those with the information can identify the relevant items, a differential physiological response provides the basis for a stronger inference.


Both terms are equal to P(deception AND physiological activity). Conditional probabilities show what proportion of a restricted sample have a certain property; thus they are ratios. The two conditional probabilities have the same numerator P(deception AND physiological activity), but different denominators p(deception) and p(physiological activity). With low base rates of deception and somewhat inaccurate tests, p(deception) can be orders of magnitude smaller than p(physiological activity), and so p(deception given physiological activity) can be orders of magnitude smaller than p(physiological activity given deception).


Tests that are less accurate than DNA matching can have diagnostic value for detecting deception even though they are imperfect. Chapter 7 discusses the policy issues raised by using such tests, either alone or in combination with other sources of information, in security screening and other applications.


If a test is 100 percent specific, the prosecutor’s fallacy is not a fallacy. For example, given the current state of DNA matching, finding blood with DNA that matches the defendant’s on the victim means it is virtually certain that the defendant was there and constitutes strong evidence against the defendant unless the defense has another reasonable explanation of how the blood got there.


Some of these threats to validity can be ruled out if the test design provides adequate standardization or other controls. Efforts to standardize the interview process and the specific relevant and comparison questions across examinations can be helpful in this regard, and there is some such standardization in some tests, such as the Test of Espionage and Sabotage, that are used in federal employee screening programs. In addition, the concealed knowledge test approach rules out the possibility that extraneous factors may elicit differential responses to relevant and comparison questions by innocent examinees because they have no way of knowing which are the relevant questions.


The effect might be different on concealed information tests. Examinees who do not have concealed information would not be able to respond differentially to relevant questions on these tests because they do not have the information needed to recognize those questions. Examinees who have concealed information, however, might respond differentially to relevant questions, with the possible result that the rate of false negative errors would be lower for stigmatized than unstigmatized groups.


According to signal detection theory, it would be appropriate for expectancies about the probability that an examinee is deceptive to be reflected in the decision about what

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement