Polygraphs can detect lies

Polygraphs can detect lies
	MISLEADING
	"A polygraph, or lie detector, can reliably tell whether a person is lying by measuring changes in their breathing, heart rate, blood pressure and sweating."
Domain	Crime & Public Safety
Tactic	Pseudoscience / Junk Science;
Era	Historical
First seen	1921
Origin	Instruments developed by John Larson and Leonarde Keeler in Berkeley, California, in the 1920s, building on William Moulton Marston's blood-pressure deception test
Last reviewed	2026-10-27
	Further reading: books on Crime & Public Safety

The claim that polygraphs can detect lies is widely believed and reinforced by television and film, where lie detector tests are often presented as decisive. A polygraph measures physiological responses, including breathing, pulse, blood pressure and skin conductance, while a person answers questions. Examiners interpret changes in these signals as signs of deception. But the polygraph does not detect lies; it detects arousal, which can be caused by fear, anxiety, anger or surprise as well as by lying. A comprehensive review by the US National Research Council in 2003 concluded that polygraph testing has weak scientific underpinnings and is especially unreliable for screening employees. Most US courts do not admit polygraph results as evidence, and federal law bars most private employers from using them.

The claim

In a typical test, an examiner asks a series of questions: relevant questions about the matter under investigation, control or "comparison" questions designed to provoke a reaction in anyone, and neutral questions. The theory is that a deceptive person will react more strongly to relevant questions, while a truthful person will react more strongly to comparison questions. Examiners score the charts and conclude whether the person showed deception.

Origin

In 1915 William Moulton Marston, a psychologist who later created the comic book character Wonder Woman, developed a test based on changes in systolic blood pressure. In 1921 John Larson, a medical student working with the police department in Berkeley, California, built an instrument that continuously recorded blood pressure and breathing. Leonarde Keeler refined the device and added skin conductance measurement, and the modern polygraph took shape. In 1923, in Frye v. United States, a federal appeals court refused to admit evidence from Marston's test, establishing the influential "Frye standard" that scientific evidence must be generally accepted in its field.

What the evidence shows

No unique lie response. There is no physiological pattern specific to lying. Innocent people may react strongly because they are nervous or afraid of being disbelieved; guilty people may stay calm.
National Research Council review. In 2003 a committee of the National Academies reviewed the scientific evidence at the request of the US Department of Energy. It found that polygraph tests can discriminate lying from truth-telling at rates above chance in specific-incident investigations with naive examinees, but well below perfection. It concluded that the theoretical basis was weak and that the research base was of low quality. For screening large numbers of employees to find rare spies or saboteurs, it warned, the polygraph would produce many false positives while missing some real threats.
Countermeasures. People can be trained to beat polygraphs, for example by deliberately raising their arousal during comparison questions.
Notable failures. Aldrich Ames, a CIA officer who spied for the Soviet Union and Russia, passed two polygraph examinations while spying. Gary Ridgway, the Green River Killer, passed a polygraph in 1987, years before he was identified as a serial killer. Conversely, innocent people have failed polygraphs and been pressured into false confessions.

Legal status

The Employee Polygraph Protection Act of 1988 prohibits most private employers in the United States from using lie detector tests for pre-employment screening or during employment, with limited exceptions. Most state and federal courts exclude polygraph results or admit them only if both parties agree. In United States v. Scheffer (1998), the US Supreme Court upheld a military rule banning polygraph evidence, noting the lack of scientific consensus about its reliability. Federal agencies, including intelligence and law enforcement agencies, nonetheless continue to use polygraphs for screening, and police use them in investigations.

Why it persists

The polygraph's value may lie less in detecting lies than in persuading people that it can. Examiners have long known that the test can prompt admissions and confessions, because subjects believe it will reveal the truth. This "bogus pipeline" effect, together with dramatic portrayals in popular culture, sustains belief in the device. Its scientific trappings, charts, sensors and trained examiners, add authority.

Lessons for readers

Physiological arousal is not the same as lying.
A scientific-looking instrument can still rest on weak science.
Ask about false positives as well as accuracy, especially when testing large groups for rare problems.
Be skeptical of confessions obtained under pressure from a test the subject was told is infallible.
Courts' treatment of evidence can be a useful guide to its reliability.

Sources

National Research Council, The Polygraph and Lie Detection (National Academies Press, 2003).
Ken Alder, The Lie Detectors: The History of an American Obsession (Free Press, 2007).
Frye v. United States, 293 F. 1013 (D.C. Cir. 1923).
United States v. Scheffer, 523 US 303 (1998).
Employee Polygraph Protection Act of 1988, 29 USC §§ 2001–2009.