An instrument used to measure physiological responses in humans when they are questioned in order to determine if their answers are truthful.
Also known as a "lie detector," the polygraph has a controversial history in U.S. law. First developed in the late nineteenth century, its modern incarnation is an electromechanical device that is attached to a subject's body during an interview. The discipline of polygraphy is based on the theory that by recording involuntary physiological changes in the subject, the polygraph yields data that can be interpreted to determine whether the subject is telling the truth. Supporters of the scientific validity of the polygraph claim that results are approximately 90 percent accurate. For much of the twentieth century, however, polygraph evidence was inadmissible in criminal cases on grounds of unreliability. Polygraph evidence was admissible in civil cases, however, and it was also used widely in law enforcement, government, and industry.
Polygraphy uses a variety of formats. Until the 1950s the format was the relevant/irrelevant (R/I) test; it rested on the now discredited belief that a subject produces a specific identifiable physiological response when lying. The R/I test has been replaced by the control question (CQ) format, the only format routinely used in forensic tests. Typically, a trained examiner fits a subject with sensors to measure respiration, heart rate and blood pressure, and perspiration, which the polygraph records using pens on graph paper. The examiner asks a series of questions, including control questions that are designed to provoke anxiety and denial. Later, another examiner compares these answers with answers pertaining to the matter at hand. This is known as numerical CQ testing. So-called global CQ testing includes a more subjective component: one examiner scores the test while also factoring in the subject's observable physical responses, such as movement, expression, and voice.
In U.S. courts, the use of the polygraph was first addressed in 1923. In refusing to admit polygraph evidence in a murder case, the Court of Appeals for the District of Columbia created a legal standard that would last for nearly 70 years (Frye v. United States, 54 App. D.C. 46, 293 F. 1013 ). This standard came to be known as the Frye rule, or general acceptance test. To be admissible in court, novel SCIENTIFIC EVIDENCE first must have gained general acceptance in...