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The Polygraph and Lie Detection
pretest interview, and his or her initial interpretations of a chart. Despite such difficulties, it is important to distinguish between the use of the polygraph as a diagnostic test of deception, in which the charts are scored and decisions are made on the basis of the score, and its use as part of an interrogation procedure.
Purposes of Polygraph Testing
As we note at the beginning of this chapter, polygraph testing and interviewing are used for three main purposes: event-specific investigation, employee screening, and preemployment (or preclearance) screening. These different purposes are reflected in different kinds of questions that are asked in polygraph tests.
For an event-specific investigation, the polygraph is used to investigate a specific incident, such as a crime or a specific act of sabotage or espionage. In this case, it is possible to ask relevant questions that are highly specific, such as “Did you plant the bomb that exploded at location X on June 12?” or “Was the murder committed with a knife?” Relevant questions like these are highly specific to a known event about which a guilty person may have a strong motive to lie or to conceal information.
For employee screening, the polygraph is used with current employees who may have committed acts prohibited by their employer or by law, but there is usually no specific known act that is the focus of the examination. Relevant questions in a security screening context might include “Have you released classified information to any unauthorized person?” or “Have you had any unreported contacts with a foreign government representative?” Some analysts believe that such questions, because they do not refer to specific past events, are more similar to comparison questions than are the relevant questions that can be asked in an event-specific investigation. For this reason, it has been argued that it is inherently more difficult to discriminate deception from truthfulness in a screening context (Murphy, 1993).
For preemployment screening or preclearance screening of employees being considered for new job assignments, the polygraph is used to try to determine the potential for future acts. For example, when someone is given a polygraph examination as part of an application to do intelligence work or for a new assignment that requires access to classified information, the employer’s concern may be with the potential that the person may commit an act in the future that he or she is not at present in a position to commit. In this situation, “relevant” questions can only be about unspecific past acts that are different in kind from the ones of greatest concern. Deception can be inferred from the polygraph in the same way it is done in screening current employees. However, in making