Cite as: 523 U. S. 303 (1998)
Opinion of Thomas, J.
function of making credibility determinations in criminal trials. A fundamental premise of our criminal trial system is that "the jury is the lie detector." United States v. Barnard, 490 F. 2d 907, 912 (CA9 1973) (emphasis added), cert. denied, 416 U. S. 959 (1974). Determining the weight and credibility of witness testimony, therefore, has long been held to be the "part of every case [that] belongs to the jury, who are presumed to be fitted for it by their natural intelligence and their practical knowledge of men and the ways of men." Aetna Life Ins. Co. v. Ward, 140 U. S. 76, 88 (1891).
By its very nature, polygraph evidence may diminish the jury's role in making credibility determinations. The common form of polygraph test measures a variety of physiological responses to a set of questions asked by the examiner, who then interprets these physiological correlates of anxiety and offers an opinion to the jury about whether the witness—often, as in this case, the accused--was deceptive in answering questions about the very matters at issue in the trial. See 1 McCormick § 206.9 Unlike other expert witnesses who testify about factual matters outside the jurors' knowledge, such as the analysis of fingerprints, ballistics, or DNA found at a crime scene, a polygraph expert can supply the jury only with another opinion, in addition to its own, about whether the witness was telling the truth. Jurisdictions, in promulgating rules of evidence, may legitimately be concerned about the risk that juries will give excessive
9 The examiner interprets various physiological responses of the examinee, including blood pressure, perspiration, and respiration, while asking a series of questions, commonly in three categories: direct accusatory questions concerning the matter under investigation, irrelevant or neutral questions, and more general "control" questions concerning wrongdoing by the subject in general. The examiner forms an opinion of the subject's truthfulness by comparing the physiological reactions to each set of questions. See generally Giannelli & Imwinkelried 219-222; Honts & Quick, The Polygraph in 1995: Progress in Science and the Law, 71 N. D. L. Rev. 987, 990-992 (1995).
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