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polygraph ("lie detector")

"I don't know anything about lie detectors other than they scare the hell out of people."  --Richard Nixon

"For federal agencies, the polygraph is a way to get around discrimination laws. There is virtually no appeal you can make if you are failed by a federal polygrapher. The polygraph is a license to abuse power." --Robert Todd Carroll

In ancient China (circa 1000 BCE), suspected liars had to chew rice powder and spit it out. If the powder was dry the suspect was deemed a liar because, it was thought, lying increases fear and saliva flow decreases in response to fear (Vrij). During the Inquisition, suspected liars were made to chew bread and cheese. If they choked on the meal--due to a dry throat caused by fear--it might be their last (Segrave). The idea of a physiological response to lying being detected by a simple test is an old one and seems barbaric and primitive to us. Add a few electrical wires and some technical-sounding jargon and the idea of detecting a lie by physiological response becomes "scientific."

A polygraph is an instrument that simultaneously records changes in physiological processes such as heartbeat, blood pressure, respiration and electrical resistance (galvanic skin response or GSR). The polygraph is used as a lie detector by police departments, the FBI, the CIA, federal and state governments, and numerous private agencies. The underlying theory of the polygraph is that when people lie they also get measurably nervous about lying. The heartbeat increases, blood pressure goes up, breathing rhythms change, perspiration increases, etc. A baseline for these physiological characteristics is established by asking the subject questions whose answers the investigator knows. Deviation from the baseline for truthfulness is taken as sign of lying.

There are three basic approaches to the polygraph test:

  1. The Control Question Test (CQT). This test compares the physiological response to relevant questions about the crime with the response to questions relating to possible prior misdeeds. "This test is often used to determine whether certain criminal suspects should be prosecuted or classified as uninvolved in the crime" (American Psychological Association).

  2. The Directed Lie Test (DLT). This test tries to detect lying by comparing physiological responses when the subject is told to deliberately lie to responses when they tell the truth.

  3. The Guilty Knowledge Test (GKT). This test compares physiological responses to multiple-choice type questions about the crime, one choice of which contains information only the crime investigators and the criminal would know about.

Psychologists do not think either the CQT or the DLT is scientifically sound, but a majority surveyed by the American Psychological Association think that the Guilty Knowledge Test is based on sound scientific theory and consider it "a promising forensic tool."  However, they "would not advocate its admissibility [in court] in the absence of additional research with real-life criminal cases." One major problem with this test is that it has no controls. Also, unless the investigators have several pieces of insider information to use in their questioning, they run the risk of making a hasty conclusion based on just one or two "deviant" responses. There may be many reasons why a subject would select the "insider" choice to a question. Furthermore, not responding differently to the "insider" choices for several questions should not be taken as proof the subject is innocent. He or she may be a sociopath, a psychopath, or simply a good liar.

Is there any evidence that the polygraph is really able to detect lies? The machine measures changes in blood pressure, pulse, and respiration rate. When a person lies it is assumed that these physiological changes occur in such a way that a trained expert can detect whether the person is lying. Is there a scientific formula or law which establishes a regular correlation between such physiological changes and lying? No. Is there any scientific evidence that polygraph experts can detect lies using their machine at a significantly better rate than non-experts using other methods? No. There are no machines and no experts that can detect with a high degree of accuracy when people, selected randomly, are lying and when they are telling the truth.

Some people, such as Senator Orrin Hatch, don't trust the polygraph machine, even if used by an expert like Paul Minor who trained FBI agents in their use. Anita Hill passed a polygraph test administered by Minor who declared she was telling the truth about Clarence Thomas. Hatch declared that someone with a delusional disorder could pass the test if the liar really thought she was telling the truth. Hatch may be right, but the ability of sociopaths and the deluded to pass a polygraph test is not the reason such machines cannot accurately detect lies with accuracy any greater than other methods of lie detection.

The reason the polygraph is not a lie detector is that what it measures--changes in heartbeat, blood pressure, and respiration--can be caused by many things. Nervousness, anger, sadness, embarrassment, and fear can all be causal factors in altering one's heart rate, blood pressure, or respiration rate. Having to go to the bathroom can also be causative. There are also a number of medical conditions such as colds, headaches, constipation, or neurological and muscular problems which can cause the physiological changes measured by the polygraph. The claim that an expert can tell when the changes are due to a lie and when they are due to other factors has never been proven. Even if the device measures nervousness, one cannot be sure that the cause of the nervousness is fear of being caught in a lie. Some people may fear that the machine will indicate they are lying when they are telling the truth and that they will be falsely accused of lying. Furthermore, even the most ardent advocate of the polygraph must admit that liars can sometimes pass their tests. One need only remember the spy Aldrich Ames, who passed the polygraph test  several times while with the CIA. This lesson was lost on the FBI, however, who started requiring polygraph tests of its employees after spy Robert Hanssen was caught. Heretofore, the FBI had only used the polygraph on suspected criminals. Apparently, the FBI thinks that they could have prevented Hanssen's betrayal if only he had been made to take the polygraph.

In California and many other states, the results of polygraph tests are inadmissible as evidence in a court of law. This may because polygraph tests are known to be unreliable, or it may be because what little benefit may be derived from using the polygraph is far outweighed by the potential for significant abuse by the police. The test can easily be used to invade a person's privacy or to issue a high-tech browbeating of suspects. Skeptics consider evidence from polygraphs no more reliable than testimony evoked under hypnosis, which is also not allowed in a court of law in California and many other states. Also, in 1998, the U.S. Supreme Court argued that Military Rule of Evidence 707, which makes polygraph evidence inadmissible in court-martial proceedings, does not unconstitutionally abridge the right of accused members of the military to present a defense (United States, Petitioner v. Edward G. Scheffer).

The American Civil Liberties Union strongly supported the passage of the Employee Polygraph Protection Act of 1988 (EPPA) which outlaws the use of the polygraph "for the purpose of rendering a diagnostic opinion regarding the honesty or dishonesty of an individual." The EPPA doesn't outlaw the polygraph across the board, however. Federal, state and local governments can still use the polygraph. The federal government can give polygraph tests to government contractors involved in national security projects. In the private sector, security and pharmaceutical firms can still use the polygraph on current or prospective employees. Furthermore, any employer can administer polygraph tests

...in connection with an ongoing investigation of an economic loss or injury to his/her business on these conditions: The employee under suspicion must have had access to the property, and the employer must state in writing the basis for a reasonable suspicion that the employee was guilty (ACLU).

The ACLU supported the EPPA not only because of the lack of evidence for the accuracy of the polygraph, but because of abuses related with its administration, including, but not limited to, the invasion of privacy.

For example, in order to establish "normal" physiological reactions of the person being tested, "lie detector" examiners ask questions that purposely embarrass, frighten and humiliate workers. An ACLU lawsuit in l987 revealed that state employees in North Carolina were routinely asked to answer such questions as "When was the last time you unintentionally exposed yourself after drinking?" and "Who was the last child that got you sexy?" Polygraphs have been used by unscrupulous employers to harass union organizers and whistle-blowers, to coerce employees into "confessing" infractions they did not commit, and to falsely implicate fellow employees (ACLU).

Why would so many government and law enforcement agencies, and so many private sector employers, want to use the polygraph if the scientific community is not generally convinced of their validity? Is it just wishful thinking? Do the users of the polygraph want to believe there is a quick and dirty test to determine who's lying and who's not, so they blind themselves to the lack of evidence? Perhaps, but there are other factors as well, such as the esoteric technology factor. The polygraph machine looks like a sophisticated, space- age device of modern technology. It can be administered correctly only by experts trained in its arcane ways. Non-experts are at the mercy of the high-tech, specially trained wizards who alone can deliver the prize: a decision as to who is lying and who is not.

Another reason for the polygraph's popularity is the pragmatic fallacy factor: it works! Case after case can be used to exemplify that the polygraph works. There are the cases of those who failed the test and whose lying was corroborated by other evidence. There are the cases of those who, seeing they are failing the test, suddenly confess. What is the evidence that the rate of correct identification of lying corroborated by extrinsic evidence is greater than the rate of identification of lying by non-technological means? There isn't any. The proofs are anecdotal or based on fallacious reasoning such as thinking that a correlation proves a causal connection.

On the other hand, it is possible that one of the main reasons so many government, law enforcement and private sector employers want to use polygraphs is that they think the test will frighten away liars and cheats who are seeking jobs, or it will frighten confessions out of those accused of wrongdoing. In other words, the users of the machine don't really believe it can detect lies, but they know that the people they administer it to think the machine can catch them in a lie. So, the result is the same as if the test really worked: they don't hire the liar/cheat and they catch the dishonest employee.

See also e-meter and ideomotor effect.

Michael Shermer Tests the Polygraph

further reading

reader comments

books and articles

Vrij, Aldert. 2008. Detecting Lies and Deceit: Pitfalls and Opportunities. 2nd ed. Wiley.

Carroll, Robert. 2007. criminal profiling: cold reading for cold cases.

Davis, R. C., "Physiological Responses as a Means of Evaluating Information," in A. D. Biderman & H. Zimmer, eds., The Manipulation of Human Behavior (Wiley, 1961).

Jussim, Daniel. "Ouija Board Justice" in Florida: the Polygraph Comes to School, in Civil Liberties, No. 358 (summer-fall 1986).

Lykken, David Thoreson. A Tremor in the Blood: Uses and Abuses of the Lie Detector (Plenum Press, 1998).

Maschke, George W. and Gino J. Scalabrini. The Lie Behind the Lie Detector [pdf format]

Morris, Roberta, "The Admissibility of Evidence Derived from Hypnosis and Polygraphy," in Psychological Methods in Criminal Investigation and Evidence, ed. by David Raskin (Springer Publishing Co., 1989.)

Raskin, David, "Polygraph Techniques for the Detection of Deception," in Psychological Methods in Criminal Investigation and Evidence, ed. by David Raskin (Springer Publishing Co.,1989.)

Segrave, Kerry. 2003. Lie Detectors: A Social History. McFarland.

Shneour, Elie. "Lying about polygraph tests," Skeptical Inquirer Spring 1990 (vol.14, no.3).

Zelicoff, Alan P. "Polygraphs and the National Labs: Dangerous Ruse Undermines National Security," Skeptical Inquirer, July/August 2001 (vol 25 no 4)


National Academy of Sciences report says polygraph testing too flawed for security screening - News and Comment Skeptical Inquirer, Jan-Feb, 2003 by Kendrick Frazier    (Read the whole report)

Too Hot of a Potato: A Citizen Soldier's Encounter With the Polygraph by George W. Maschke


"Polygraph Testing and the DOE National Laboratories" by  Steven Aftergood in Science Magazine.

Aldrich Ames letter to Aftergood 

"Psychologists Surveyed On Lie Detectors Say Most Are Not Valid Not Scientifically Sound and Can Be Easily Deceived" - American Psychological Association

"Lie Detector Testing," ACLU Briefing Paper

British Columbia Civil Liberties Union Briefing Paper

The Legal Implications of Graphology by Julie Spohn, Washington University Law Quarterly 1977.


11th Circuit Court of Appeals, US vs. Gilliard on admissibility of polygraph to support claim of innocence

"The Psychophysiologist as Innocent Bystander: Ethical Mismatch" by  Robert J. Barry

"Truth or Consequences" -A polygraph screening program raises questions about the science of lie detection by Tim Beardsley

How to Sting the Polygraph

Laboratory for Scientific Interrogation - Scientific Content Analysis

Truster & Computer Voice Stress Analyzer

The truth about the polygraph by Susan McCarthy (Salon.com)

Lies, damned lies and polygraphs by Jeff Stein (Salon.com)

Polygraph test

"Polygraph tests are degrading, don't work-- and should be banned," in Civil Liberties, No. 355 (fall 1985).

news stories

Glitch in widely used polygraph can skew results Interesting headline: suggests that if the glitch were fixed, the polygraph would be a reliable lie detector, which is a lie.

Feds expand polygraph screening, often seeking intimate facts By Marisa Taylor McClatchy Newspapers

Last year, more than 73,000 Americans across the country submitted to polygraph tests to get or keep jobs with one of the fifteen federal agencies that use a device virtually no scientist thinks is any better than torture for determining whether someone is telling the truth. The CIA, the FBI, the National Security Agency, the Department of Energy, and the Postal Inspection Service are just a few of the high profile federal groups that rely on a pseudoscientific device to weed out the liars and cheats trying to weasel their way into government positions where they can steal our secrets and bring down whatever regime was voted in during the last election

As polygraph screening flourishes, critics say oversight abandoned By Marisa Taylor McClatchy Newspapers

This article explores the abuses of the polygraph by federal agencies who can work in secret knowing there is no requirement for them to reveal anything about any test they give and there is no appeal the average citizen can make if he or she is “failed” by the polygrapher. Taylor suggests that some people think that agencies like the CIA use the polygraph because it allows them to discriminate against ethnic groups and individuals without fear of repercussions.

A Russian A.T.M. With an Ear for the Truth ... Russia’s biggest retail bank is testing an ATM machine that uses voice-analysis software to help assess the honesty of credit applicants. The bank is using technology popular with the former KGB. Despite the hype, voice analysis cannot reliably detect when a person is lying. Like the polygraph, voice analysis measures indications of stress, but it cannot detect the cause of the stress. The technology might scare away a few ignorant applicants, along with those who don't care to be fingerprinted, submit to a 3-D facial scan, or produce a valid passport.

If neuroimaging can reliably discern truth from falsehood, should brain scans be admissible evidence in court cases? Do fMRIs represent a better, more accurate technology that can detect whether a person is telling the truth? Not yet, but that hasn't deterred a few eager beavers from bringing brain scans into court as evidence that someone is or is not telling the truth.

Ohio judge admits polygraph evidence

Pseudoscience applied to scientists by Peg Brickley, The Scientist 3/26/03

Lie Detector Roulette by Brendan I. Koerner November/December, 2002 Issue Mother Jones

Polygraph Testing Too Flawed for Security Screening - press release summary from the National Research Council of their 18-month study; to Read the whole report click here

Lie-Detecting Devices: Truth or Consequences? by Ariana Eunjung Cha,Washington Post, August 18, 2002

Thermal camera captures guilty faces  (New Scientist) January 2, 2002

Brain scans can reveal liars (New Scientist) November 12, 2001

The Love Detector

Last updated 27-Oct-2015

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