two electrodes on the fingers or palm (skin resistance measurements can give misleading indications of magnitudes of response). Skin conductance responses can be elicited by so many stimuli that it is difficult to isolate specific psychological antecedents. Respiration is easily brought under voluntary control, so it is unlikely by itself to be a robust indicator of any psychological state an examinee is trying to conceal. Variations in respiration can produce changes in heart rate and electrodermal activity. Therefore, respiration needs to be monitored to determine whether cardiovascular and electrodermal responses to relevant and comparison questions are artifacts of other changes. (Appendix D provides more detail about current knowledge of cardiovascular, electrodermal, and respiratory response systems.)
The physiological responses measured by the polygraph do not all reflect a single underlying process such as arousal. Similarly, arousing stimuli do not produce consistent responses across these physiological indicators or across individuals. This knowledge implies that there is considerable lack of correspondence between the physiological data the polygraph provides and the underlying constructs that polygraph examiners believe them to measure. On theoretical grounds, it is therefore probable that any standard transformation of polygraph outputs (that is, scoring method) will correspond imperfectly with an underlying psychological state such as arousal and that the degree of correspondence will vary considerably across individuals. Little is known from basic physiological research about whether there are certain types of individuals for whom detection of arousal from polygraph measures is likely to be especially accurate—or especially inaccurate.
Polygraph theories assume that differences in physiological responses are closely correlated with psychological differences between examinees’ responses to relevant and comparison questions on the polygraph test. This assumption will be less plausible to the extent that a polygraph testing procedure gives an examiner discretion in selecting the relevant and comparison questions for each examinee. It is reasonable to expect that if a polygraph test procedure gives examiners more latitude in this respect, the results are likely to be less reliable across examiners, and more susceptible to examiner expectancies and influences in the examiner-examinee interaction.
Given the imperfect correspondence that can be expected between polygraph test results and the underlying state the test is intended to measure, inferences from polygraph tests confront both logical and empirical issues.