The word polygraph comes form the Greek for “many writings.” The polygraph machine measures physiological information from the body: breathing, blood pressure, and perspiration. The faster the breathing, the higher the blood pressure, and the greater the amount of sweat, the more likelihood the person being tested is nervous.

Although it had been suggested in the nineteenth and early twentieth centuries that physiological changes could help determine whether a person was telling the truth, the first serious effort to apply this information came in 1920 when John Larson, a police officer in Berkeley, California, developed a device (which he called a polygraph) that could measure breathing and blood pressure. Larson believed that his invention could help determine whether a suspect was telling the truth. When the results of a polygraph test were included as evidence in a criminal case in 1923, they were challenged, and the D.C. District Circuit Court ruled in U.S. v. Frye that polygraph evidence needed to meet three criteria to be accepted: (1) that the general scientific community must acknowledge the test’s reliability, (2) that the person conducting the test must be qualified to do so, and (3) that it can be proven that correct procedures were followed. Known as the “Frye test,” it remained the judicial standard for 70 years.

During that time, scientists worked at refining Larson’s invention. Leonarde Keeler, who had worked with Larson, began developing more sensitive polygraph machines in the 1930s, even starting a polygraph school in 1948.

Through the years, polygraphs were used by law enforcement agencies, but they were not considered definitive. To begin with, the person who is hooked up to the polygraph would already be quite nervous, and to have tubes placed on the chest, a blood pressure cuff on the arm, and metal plates on the fingers would not relax most people. Moreover, there is a difference of opinion on the accuracy of polygraph tests. The American Polygraph Association has stated that inconclusive polygraph results are not the same as incorrect results. Yet typically inconclusive readings are figured in with incorrect ones when establishing a percentage of accuracy.

Polygraph experts continued to fine-tune the machines, and also developed a questioning technique that was intended to produce fewer incorrect readings (the subject is asked to respond “yes” or “no” to questions, and unrelated questions are mixed in with relevant ones; this is meant to eliminate nervous affect).

In 1975, federal judges were given more discretion about the admissibility of evidence under new “Federal Rules of Evidence.” Thus, a judge could allow a jury to consider polygraph results even if they did not pass the Frye Test. In 1993, the U.S. Supreme Court issued an opinion on Daubert v. Merrell Dow Phramaceuticals that definitively replaced the Frye standard. The court said that judges could admit certain scientific evidence as long as the theory behind it could be been tested, it had been subject to peer review and publication, the potential error was known, and the scientific community in general accepted the theory. In the 1998 case of U.S. v. Scheffer, the U.S. Supreme Court ruled that polygraph tests did not have to be admitted as evidence in military trials. (President George H.W. Bush had banned the admission of polygraph evidence from military trials in 1991, citing their unreliability.) But it did not ban polygraph evidence outright. Daubert grants judges the right to determine whether polygraph evidence can be used or ignored, so it is generally up to the judge.

The polygraph has also been used to pre-screen job applicants or to test employees to measure their truthfulness about such issues as drug use or theft. In 1988 Congress passed the Employee Polygraph Protection Act (EPPA), which prohibited business from using polygraph evidence to pre-screen employees or to test current employees, and which prohibited companies from disciplining or firing employees solely for failing a polygraph test. (Polygraphs can be used if an employer can show other evidence against an employee, but the employee still has the right to refuse.) EPPA does not apply to government workers.