conclude that it “works” for people like the examinees in situations like the mock crime. There would be many unanswered questions, including:

• Would the physiological responses be the same if the crime had been real?

• Would the test procedure perform as well if the deceptive examinees had been coached in ways to make it difficult for examiners to discriminate between their responses to relevant and comparison questions?

• Would the test procedure have performed as well if the examinees had been from different cultural backgrounds?

• Would the test procedure work as well for the people most likely to commit the target infractions as for other people (for example, are there systematic differences between these groups of people that could affect test results)?

• Would a polygraph test procedure that performs well in specificevent investigations perform as well in a screening setting, when the relevant questions must be asked in a generic form?

• Would different examiners who constructed the relevant and comparison questions in slightly different ways have produced equally good results?

Such questions can sometimes be answered by additional research, for instance, using different kinds of examinees or training some of them in countermeasures. But it is never possible to test all the possible kinds of examinees or countermeasures. A solid theoretical and scientific base is also valuable for improving a test because it can identify the most serious threats to the test’s validity and the kinds of experiments that need to be conducted to assess such threats; it can also tell researchers when further experiments are unlikely to turn up any new knowledge. In such ways, a solid scientific base is important for developing confidence in any technique for the psychophysiological detection of deception and critical for any technique that may be used for security screening.

Polygraph specialists have engaged in extensive debate about theories of polygraph questioning and responding in the context of a controversy about the validity of comparison question versus concealed information test formats. We are more impressed with the similarities among polygraph testing techniques than with the differences, although some of the differences are important, as we note at appropriate places in this and the following chapters. The most important similarities concern the physiological responses measured by the polygraph instrument, which are es-