What's the Deal with Lie Detectors?

The history and current world-view of lie detectors.

by Michelle Kaminsky, Esq.
updated May 02, 2022 ·  5min read

The bigger the scandal, the more likely you'll hear talk of lie detectors being used to tease out confessions or catch lies. But how reliable are lie detectors? Do they work? Can you beat them? When are they used? Are they admissible in court?

The Basics

First of all, what is a lie detector, and how does it work? A lie detector is also known as a "polygraph," literally "many writings." It is an instrument that measures physiological changes in blood pressure, heartbeat, respiration, and perspiration—reactions allegedly linked with the anxiety caused by lying—while a subject is asked questions.

The earliest form of what we now know as a lie detector was invented nearly a century ago by William Moulton Marston, a Harvard psychology student at the time. Marston devised a systolic blood pressure test that measured a subject's cardiovascular reaction to questions.

However, as Marston's machine really only consisted of one test, the first "polygraph," measuring galvanic skin response in addition to blood pressure, was developed years later by Dr. John A. Larson of the University of California and used by the Berkley Police Department.

In a typical lie detector test, rubber tubes are attached to the subject's chest and abdomen, metal plates to the fingers, and a blood pressure cuff to the arm to measure respiratory, sweat glands, and cardiovascular activity, respectively.

A usual screening starts with a pre-test interview to determine the answers for "control questions," which determine the base level of a subject's reactions. The examiner then explains the test process and often asks the subject to intentionally lie for a physiological response.

The actual test then begins with "irrelevant" questions (age, eye color, height), "probable lie" questions (have you ever stolen money?), and "relevant" questions (purpose of the test). The subject passes if his physiological responses during the "probable lie" questions are larger than those during the "relevant" questions. If this doesn't happen, the examiner may then interview the subject post-test if an admission is forthcoming.


Are the results reliable? According to the American Polygraph Association, a polygraph test can reveal deceptive answers in over 90% of cases when administered correctly, but this statement is in the minority. Even a quick Internet search comes up with many anti-polygraph testing groups, blogs, and research. Much of the information that doubts the accuracy of polygraph testing notes that recorded physiological responses may or not be related to deception as they may arise from various sources, including fear.

One such source is a 2003 Study by the National Academy of Sciences (NAS) that found no scientific evidence showing that the polygraph's physiological reactions are only related to deception.

The NAS report entitled "The Polygraph and Lie Detection" was presented to Congress and the Department of Energy and stated that "[a]lmost a century of research in scientific psychology and physiology provides little basis for the expectation that a polygraph test could have extremely high accuracy." Even more damaging, the report even expressed doubt that investing in technological development could improve accuracy.

Errors can occur during testing not only because of the difficulty of pinpointing anxiety caused by deception. Mistakes may also be made during the subject's preparation, the approach to the questions, and the interpretation of results.

Can You Beat the Test?

Aldrich Ames, the infamous spy for the Soviet Union, passed his lie detector tests by following his Soviet bosses' advice: "simply relax." More specific ways that a subject may alter results include taking sedatives to minimize anxiety, applying antiperspirant to counteract sweating, and even self-inflicting pain after each question so that the response remains constant. For tests that measure oxygen flow to the brain, simply holding your breath after each question can skew results.

When Are Polygraph Tests Used?

People do confess before, during, and (most frequently) after polygraph tests, so their use in police investigations is fairly common. Defense attorneys may choose to have their clients undergo polygraph testing to convince prosecutors to drop weak charges. However, no defendant or witness can be forced to take a polygraph test.

In the working world, private employers' use of polygraph testing is limited by the Employee Protection Act (EPPA) of 1998. In the private sector, pre-employment polygraph tests aren't allowed. During employment, the test can be administered only after certain criteria are met—one being that there must be an identifiable economic loss to the employer. An employee can also not be fired for refusing a lie detector test.

In addition to federal law, 20 states and the District of Columbia have their own laws regarding a private employer's use of polygraph testing.

Notably, police, public, and government organizations are not included in the EPA's restrictions. One government use that is quickly becoming the norm is to keep tabs on sex offenders on parole or probation. The 2nd Circuit Court of Appeals has recently approved the use of a polygraph test as a condition of probation for a convicted sex offender so long as the information requested is used for supervision, case monitoring, and treatment; the court noted that the test "produces an incentive, to tell the truth."

Are Polygraph Tests Admissible In Court?

It depends on where you are, as the United States Supreme Court has left it up to individual jurisdictions whether to outlaw polygraph use or set standards for admission, like the 11th Circuit in the federal court system has. On the other hand, in some states, even the mention of a lie detector test can be grounds to ask for a new trial.

In the jurisdictions where polygraphs may be admitted, it is usually where parties have agreed to the test's terms before the exam is administered. At least 29 states have laws requiring certification of examiners.

Continued Popularity

Although the science is questionable at best, polygraph testing continues to be popular in law enforcement and government agencies. More than the tests' alleged accuracy, this use seems to be because the fear of being caught in a lie is often enough to make people tell the truth.

Does that mean we'll never be able to tell for sure whether someone's lying? Maybe not. In recent years, major scientific advancements such as brain fingerprinting may soon make the polygraph test look like something out of the Middle Ages—and give us a whole new technology to debate.

Happy testing!

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Michelle Kaminsky, Esq.

About the Author

Michelle Kaminsky, Esq.

Freelance writer and editor Michelle Kaminsky, Esq. has been working with LegalZoom since 2004. She earned a Juris Docto… Read more

This portion of the site is for informational purposes only. The content is not legal advice. The statements and opinions are the expression of the author, not LegalZoom, and have not been evaluated by LegalZoom for accuracy, completeness, or changes in the law.