Polygraph tests, often called "lie-detector tests," are not admissible as evidence in the courts. This is because they have never been shown to produce reliable proof of whether the subject is lying.
Nevertheless, they continue to be popular as an employment screening tool and, sometimes, as a way to assess suspects and witnesses in criminal investigations. Many probation agents are also allowed to employ polygraphs to assess whether their clients are being truthful with them, though because they are not admissible in court a failed test cannot (alone) result in probation revocation.
It would be immensely convenient if lie-detectors worked. They seem to promise a fair, objective way of determining whether someone is telling the truth. They were first developed by psychologists at the beginning of the 20th century, then refined by law enforcement and private businesses in the 1920s.
The idea is that, when people lie, their bodies give off tell-tale signs in response to the emotional stress of doing so. The polygraph measures changes in the depth of the subject's breathing, their blood pressure and the electrical conductivity of their skin.
Unfortunately, there are several underlying assumptions that proved to be false. For one, it's not clear that everyone has a measurable emotional response when they lie. Among those who do, those responses are not uniform. And, it is thought that some people can beat the test by taking countermeasures such as controlling their breathing.
Indeed, when scientists attempted to verify the validity of polygraphy in large-scale, controlled studies, they found large numbers of false positives. That is, many people were rated as lying when they were not.
Why are polygraphs still used if they're invalid?
Recently, the Smithsonian magazine looked into the question of why employers -- especially federal security agencies -- still screen employees using polygraphs. The writer concluded that the test results are often useful for reasons of expediency. For example, a negative performance on a lie detector could give cover for firing an unwanted employee.
The CIA, for example, has been using polygraphs for employment purposes since the agency was founded in 1947. Even when Congress began investigating the validity of lie-detectors in the 1960s, the CIA vehemently defended their use.
Yet an internal CIA history from 1973 concluded that, "the precise yardstick for the measuring of security reliability of an individual continued to be elusive." In other words, the CIA couldn't define exactly what it considered a passing score on the test.
Polygraphs fell out of favor after it was shown that Aldrich Ames, a now-convicted Soviet spy, passed two polygraph tests despite having passed on U.S. secrets. However, they continue to be revived by people hoping they can provide a fair, technological way to sort out truth from lies.