Frye v. United States

A polygraph machine measures an examinee's rate of heartbeat and blood pressure, perspiration, and breathing with various sensing devices. Questions are asked of the examinee while he or she is connected to the machine, and polygraph examiners claim to be able to detect when an examinee is not telling the truth by analysis of these measurements.

The American Polygraph Association claims that polygraph examinations are highly accurate when properly conducted, citing studies finding an average accuracy of 98 percent. According to the association, most of the controversy regarding accuracy involves differing interpretations of tests that are inconclusive: critics label inconclusive tests as inaccurate, while the association does not. Some critics claim the polygraph has no validity, however, and that trained persons may easily and consistently "beat" polygraph examinations.

Polygraph examination results may or may not be admissible in court, depending on the jurisdiction; often they are deemed admissible when both parties to a case agree on such admissibility before the test is administered. Some jurisdictions have specific rules on admissibility, and in others admissibility is at the discretion of the trial judge.