US Attorneys > USAM > Title 9 > Criminal Resource Manual 260
prev | next | Criminal Resource Manual



The basic function of a lie detection device is to record signs of internal stress that a subject is thought to undergo when falsely responding to questions. A polygraph examination begins with a present interview and study of the witness. Even the best trained and most experienced polygraphers must have a thorough understanding of the factual context of the activities under investigation in order to prepare a series of simple, unambiguous questions. The pre-test interview allows the examiner to secure the confidence and cooperation of the subject, and to evaluate the subject's idiosyncracies that may affect the examination results. This procedure promotes the subject's belief in the infallibility of the machine and could augment his/her physical reactions by increasing his/her fear and anxiety over detection.

A polygraph examination can be administered either on location or at a specific site. The locale must have a minimum number of distractions. Today's machines generally consist of: (1) a cardiograph, monitoring pulse and changes in blood pressure; (2) a pneumograph, recording respiration rate by measuring chest expansions and contractions; (3) a galvanometer, displaying the skin's resistance to an electric current (this is normally attached to the palmal surface of the subject's hand); and sometimes a device measuring gross muscular movements. All responses are recorded in graphic form while the subject is undergoing questioning. Examiners employ different types of test questions to measure the subject's reactions. The most popular test uses true and false control questions so that a standard can be created with which to compare the subject's recorded reactions to essential questions. Examinations cannot be conducted without the voluntary cooperation of the subject.

Following the examination the administering polygraphist examines the results to determine whether the illustrated responses indicate deception. The amount of expertise the examiner possesses is extremely important in assessing the results of the examination. The examiner must not only interpret the tangible results of the test, like any forensic scientist would, but must also evaluate his/her own activities and procedures to uncover any factors that may have contributed to inaccurate test results. See generally 1 P. Giannelli & E. Imwinkelried, Scientific Evidence 219-222 (2d ed. 1993); C. Honts & B. Quick, The Polygraph in 1995: Progress in Science and the Law, 71 N.S.L. Rev. 987, 989-993 (1995).