The Polygraph: How It Works and How to Beat It - Interesting Engineering (2024)

In November 1985, when a pudgy, bespectacled young man named Mark Hofmann walked into the office of University of Utah professor Charles Honts, he appeared calm and relaxed. Hofmann was in Honts’ office to take a polygraph examination.

Hooked up to the “lie detector” machine, Hofmann was asked about his involvement in the grisly bombing murders of a young father named Steve Christensen and a beloved grandmother named Kathy Sheets. Proclaiming his innocence, Hofmann passed the lie detector test with flying colors.

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The reality was that Hofmann was guilty of both murders, along with dozens of other crimes, including the forging of hundreds of thousands of dollars worth of rare documents. Following his convictions for murder, Honts visited Hofmann in prison and asked him how he had beaten the polygraph machine.

Hofmann, who was a stickler for detail, replied that anticipating just such a contingency, he had set up a blood pressure monitor in his home and had practiced with it until he could control his blood pressure at will. Hofmann also said that he practiced self-hypnosis, and had been able to hypnotize himself into believing he was innocent while taking the test.

Hofmann was the subject of Netflix’s recent documentary Murder Among the Mormons, as well asSteven Naifeh and Gregory White Smith’s well-regarded 1988 book, The Mormon Murders.

How does a polygraph work?

A polygraph machine measures and records certain physiological characteristics including blood pressure, pulse, respiration, and skin conductivity, while a person is asked and answers a series of questions. The assumption is that deceptive answers cause physiological changes such as a person’s blood pressure to rise, the rates of their pulse and respirations to elevate, and their skin to become more conductive electrically through sweat.

The polygraph machine was adapted for forensic work in 1921 by John Augustus Larson, who at that time had a Ph.D. in physiology and was working as a police officer. Larson had first invented the machine years earlier, while he was a medical student at the University of California, Berkeley. Larson’s protege, Leonarde Keeler, added galvanic skin response to the machine in 1939, then sold the device to the U.S. Federal Bureau of Investigation (FBI).

A polygraph machine consists of two pneumographs, which are rubber tubes filled with air. These are placed around a subject’s chest and abdomen to measure respiration.A blood pressure cuff similar to that used in a doctor’s office is placed around the subject’s upper arm. Two plates called galvanometers are placed on the subject’s fingertips to measure sweat production.

Today, the old analog polygraph machines shown on TV and in movies have been replaced by laptop computers.Polygraphs are used by the FBI, the National Security Agency (NSA), and the Central Intelligence Agency (CIA) in hiring decisions. The Las Angeles Police Department (LAPD) and the Virginia State Police also use the polygraph to screen new employees.

How accurate are polygraph tests?

In 2003, the U.S. National Academy of Sciences (NAS) issued a report entitled, “The Polygraph and Lie Detection” which found that when a polygraph is used as a screening tool, “Its accuracy in distinguishing actual or potential security violators from innocent test takers is insufficient to justify reliance on its use in employee security screening in federal agencies.”

A report to the U.S. Congress by the Moynihan Commission on Government Secrecyconcluded that “. . . the polygraph is neither scientifically valid nor especially effective beyond its ability to generate admissions.”

In 1986, CIA officer and KGB mole Aldrich Ameswas facing a polygraph examination.He sought advice from his Soviet handler on how to beat the machine, and he received this simple advice: “Get a good night’s sleep, and rest, and go into the test rested and relaxed. Be nice to the polygraph examiner, develop a rapport, and be cooperative and try to maintain your calm.”

Ames passed this test easily and passed another one in 1991. Ames said, “There’s no special magic . . . Confidence is what does it. Confidence and a friendly relationship with the examiner . . . rapport, where you smile and you make him think that you like him.” Ames was the subject of the 1998 movie Aldrich Ames: Traitor Within, starring Timothy Hutton as Ames.

Cheating polygraph tests

The generally acknowledged tactics for beating a polygraph machine are to carefully control your breathing, and to artificially increase your heart rate during what are called “probable lie” or “control questions.”These include questions people are likely to lie about such as, “Have you ever stolen money?” “Have you ever lied to your parents?” “Have you ever cheated on a test?” or “Have you ever taken something of value from an employer?”

If you lie when answering these questions and you also induce pain, such as by biting your tongue hard, or if you task yourself with doing a tricky math calculation in your head, you will likely increase your blood pressure, heart rate, breathing, and sweat production, and the polygrapher will likely attribute your strong reaction to lying.

If you then lie in response to a relevant question, and your physical response is less than it was to the control questions, your answer will be interpreted as the truth. By becoming, in essence upset, during these control questions, it makes the responses to real questions, and possibly lies, indistinguishable from the truth.

Polygraphers are on to the tactic of putting something sharp in your shoe and pressing against it during the test to induce pain. You will most likely be asked to remove your shoes during a polygraph test.

Another tactic that can be used to beat a lie detector is to force yourself to think of something pleasant while lying, something such as a warm summer night at the beach. This will calm your physiological responses and mask a lie from the truth.

The reliability of a polygraph test

In United States v. Scheffer (1998), the U.S. Supreme Court left it up to individual jurisdictions whether polygraph results could be admitted as evidence in court cases.While many states do allow polygraph results to be admitted in court, this generally requires the agreement of both parties.

The Employee Polygraph Protection Act of 1988 (EPPA) generally prevents employers from using lie detector tests, either for pre-employment screening or during the course of employment, with certain exemptions.In Canada, with the 1987 decision of R v Béland, the Supreme Court of Canada rejected the use of polygraph results as evidence in court. In the Canadian province of Ontario, an employermay not use a polygraph.

Some states have laws prohibiting or restricting employers from using lie detectors in connection with employment, but most of these laws have been made obsolete by the federal antipolygraph statute. However, some state laws go further, for example, by prohibiting employers from even suggesting such a test, or by applying the federal law tostate and local government employees. In several states, the law does allow employees to volunteer to take a polygraph test, albeit with safeguards.

A 2004 report on the validity of polygraphs by the British Psychological Society found that the tests are likely to produce more false positives than false negatives. That means that more innocent people will wrongly fail the test than guilty people will wrongly pass.

Also, psychopaths and sociopaths don’t have the same physiological responses to lying as do normal people. For this reason, investigators often use what’s known as a Guilty Knowledge Test along with the polygraph test.This test assesses whether suspects are concealing “guilty knowledge” by measuring their physiological responses to a series of multiple-choice questions that contain specific information both related and unrelated to a crime.

For example, questions put to a bank robbery suspect could include the amount of money that was stolen along with other amounts, or the contents of a genuine ransom note that was passed to a teller among other notes created by the police. However, in order to use this test, the police must have access to information that only the guilty party would know.

Polygraphy is big business

In 2018, Wired Magazine reported that in the U.S., an estimated 2.5 million polygraph tests are given each year, with the majority given to police officers, firemen, paramedics, and state troopers. The average cost of each test is over $700, thus making the polygraph business a $2 billion a year industry.

So, while the actual value of the polygraph remains questionable, now you know how to deal with them.