Lie Detector Tests: Accuracy, Your Rights, and Cost

A lie detector test, known more formally as a polygraph, is an examination that records your heart rate, breathing, blood pressure, and sweat response while you answer a scripted set of questions. The examiner looks for physiological spikes on the “relevant” questions and uses them to form an opinion about whether you were truthful. The scientific case for the technique is weak, most courts refuse to accept the results, and federal law bars most private employers from requiring one in the first place.¹²

How a Lie Detector Test Works

The polygraph instrument tracks three physiological channels at the same time: cardiovascular activity, respiration, and electrodermal response.³ Rubber tubes around your chest and abdomen measure how deeply and quickly you breathe. A blood pressure cuff on your upper arm picks up heart rate and blood volume changes. Small sensors clipped to your fingertips register tiny shifts in sweat, which change the electrical conductivity of your skin. Many modern setups add a pressure-sensitive pad on the chair to catch deliberate movements that might distort the readings.

Federal law uses this three-channel setup as the minimum standard for anything called a polygraph.⁴ The broader statutory term “lie detector” also sweeps in voice stress analyzers, psychological stress evaluators, and any similar device used to render an opinion on honesty.

Important to understand: the machine does not detect lies. It detects stress-related physical reactions, and the examiner infers deception from the pattern of those reactions. Every criticism of the technology flows from that gap between what it measures and what it claims to show.

How Accurate Lie Detector Tests Really Are

The American Psychological Association puts it directly: polygraph tests “are not a reliable and valid way to detect deception.”¹ A 2003 review by the National Academies of Sciences concluded that the underlying science is “scanty and scientifically weak” and that “almost a century of research in scientific psychology and physiology provides little basis for the expectation that a polygraph test could have extremely high accuracy.”⁵

The numbers behind that judgment: specific-incident polygraphs, the kind used in criminal investigations, correctly identified deception about 70% of the time. Across multiple field studies, truthful people were wrongly flagged as deceptive roughly 19% of the time. In practical terms, if you take a polygraph and you are telling the truth, you still face roughly a one-in-five chance of being told you failed. The polygraph industry’s own meta-analysis claimed 89% accuracy, but that work was neither peer-reviewed nor independent.¹

A 2019 review of research published after the National Academies report found that study quality had not meaningfully improved and that the original conclusions still hold.¹ Screening polygraphs used for employment or security clearances perform worse than specific-incident tests, because the questions are more open-ended and the person being tested has less context about what the examiner is actually looking for.

What Happens During the Test

The appointment has three parts. The pre-test interview, which can take as long as the test itself, is where the examiner reviews your background, explains the equipment, and walks through every question you will be asked. You have the right to see all questions in advance, and no relevant question can be sprung on you during the recorded session.⁶

During chart collection, with the sensors attached, the examiner asks three types of questions in sequence. Relevant questions address the actual issue under investigation. Comparison questions are designed to produce a mild stress response from a truthful person, giving the examiner a baseline. Irrelevant questions (“Is today Wednesday?”) carry no diagnostic weight and simply help settle you between the meaningful ones.⁷ The examiner usually runs through the full sequence several times.

Afterward, the examiner scores the charts and discusses the results. If nothing on the charts suggests deception, that conversation is short. If the examiner reads deception, the post-test interview can stretch to 45 minutes or more as they probe for admissions.

Your Rights if an Employer Asks You to Take One

If you are facing a polygraph requested by an employer who is legally permitted to administer one, federal law gives you a specific set of protections. These apply at every stage of the examination.⁶

You must receive reasonable written notice of the date, time, and location, along with written notice of your right to consult a lawyer or employee representative before each phase.
You must be allowed to review every question before the test begins.
You can stop the test at any time, for any reason.
The examiner cannot ask about your religious beliefs, racial opinions, political affiliations, sexual behavior, or union activities.
The examiner must refuse to run the test if a physician has provided written evidence that you have a medical or psychological condition likely to cause abnormal responses.
You must be told whether cameras, two-way mirrors, or recording devices are present.

Before testing, you have to sign a written notice confirming that the test is voluntary, that it cannot be required as a condition of employment, and that you have legal remedies if the law is violated.⁶ These rights cannot be waived by contract.⁸

When Employers Can and Cannot Require a Test

The Employee Polygraph Protection Act makes it illegal for most private employers to require, request, suggest, or cause an employee or job applicant to take any lie detector test.² The ban runs end to end. An employer cannot use results to make hiring decisions, cannot fire or discipline you for refusing, and cannot even ask about the results of a test taken somewhere else.⁹ Retaliation against anyone who files a complaint, testifies in a proceeding, or exercises a right under the Act is also prohibited.

Violations carry civil penalties of up to $26,262 per infraction as of the most recent inflation adjustment.¹⁰ Affected employees can also sue privately for reinstatement, promotion, and recovery of lost wages and benefits, and courts may award attorney’s fees to the prevailing party. The statute of limitations for a private claim is three years from the alleged violation.⁸

The ban is broad but not absolute. Federal, state, and local government agencies are fully exempt, which is why the CIA, FBI, and various law enforcement agencies routinely polygraph applicants and employees.¹¹ Three categories of private employers also keep limited polygraph authority:

Security services whose primary business is providing armored car personnel, security alarm installation and maintenance, or uniformed and plainclothes security staff may test prospective employees.¹¹
Employers authorized to manufacture, distribute, or dispense federally controlled substances may test employees or applicants who would have direct access to those substances.¹¹
Any private employer may test a current employee reasonably suspected of involvement in a workplace incident that caused the employer an economic loss, if the employee had access to the property at issue. The employer must give the employee a written statement describing the specific incident and the basis for suspicion before testing.¹²

Even inside these exemptions, every procedural right in the previous section still applies. The exemption lets the employer ask; it does not strip away your protections during the exam.⁶

Medical Conditions and Medications That Affect Results

Because the polygraph measures stress responses rather than lies, anything that changes how your body produces those responses can skew the result. Researchers describe the test as an “indirect measurement of deception-induced stress,” which means any condition affecting the autonomic nervous system can introduce error.¹³

Alcohol use disorder, rheumatoid arthritis, and various forms of dysautonomia can all produce abnormal readings unrelated to honesty. A range of common prescriptions, including beta-blockers, anti-anxiety medications, and drugs that influence heart rate or sweating, can shift results in either direction. Pregnancy is particularly disruptive: blood volume rises by roughly 50%, heart rate increases 10 to 20 beats per minute, and respiratory patterns change, all of which directly interfere with what the polygraph measures. Most examiners recommend waiting at least six to eight weeks after delivery before testing.

If any of this applies to you and you are facing an employer-requested test, get written documentation from your physician before the appointment. Under federal law, the examiner must refuse to run the test on that showing.⁶

Whether the Results Can Be Used in Court

Most courts refuse to admit polygraph evidence. Fewer than half the states allow it in criminal trials, and several of those permit it only when prosecution and defense agree in advance. The rest either exclude it outright or leave it to the trial judge.

Federal courts do not automatically exclude polygraph results, but the evidence rarely survives scrutiny. Under the Supreme Court’s decision in Daubert v. Merrell Dow Pharmaceuticals, Inc., judges act as gatekeepers for scientific evidence, weighing both relevance and reliability.¹⁴ Federal Rule of Evidence 702 requires expert testimony to rest on sufficient facts, reliable methods, and a sound application of those methods.¹⁵ Given the documented skepticism about polygraph validity, most federal judges keep the results out.

Military courts go further. Military Rule of Evidence 707, adopted in 1991, imposes a blanket prohibition on polygraph evidence in courts-martial. The rule excludes test results and examiner opinions, and it also excludes any reference to whether a person offered to take, refused to take, or actually took a polygraph. The only exception is for statements made during the session itself, which may come in if they would otherwise qualify as evidence.¹⁶

Cost of a Private Lie Detector Test

If you are arranging a polygraph on your own, outside of any employer or law enforcement context, expect to pay somewhere between $300 and $2,500. The range reflects differences in examiner experience, location, the complexity of the issue being tested, and whether you need a formal written report. Specific-issue tests focused on a single question usually cost less than broader screening exams. Travel, expedited scheduling, and later courtroom testimony typically carry additional fees.

When choosing a private examiner, look for someone credentialed by the American Polygraph Association or licensed by your state if your state requires it. A careful examiner reduces procedural error, which matters because procedural error compounds a technology that is already of limited accuracy to begin with.

1
American Psychological Association. Do Lie Detectors Work? What Psychological Science Says About Polygraphs
2
Office of the Law Revision Counsel. 29 USC 2002 – Prohibitions on Lie Detector Use
3
The National Academies of Sciences, Engineering, and Medicine. The Polygraph and Lie Detection – The Instrument, the Test, and the Examination
4
Office of the Law Revision Counsel. 29 USC 2001 – Definitions
5
The National Academies of Sciences, Engineering, and Medicine. The Polygraph and Lie Detection – Conclusions and Recommendations
6
Office of the Law Revision Counsel. 29 USC 2007 – Restrictions on Use of Exemptions
7
The National Academies Press. The Polygraph and Lie Detection – Question Techniques
8
Office of the Law Revision Counsel. 29 USC 2005 – Enforcement Provisions
9
U.S. Department of Labor. Employee Polygraph Protection Act
10
U.S. Department of Labor. Civil Money Penalty Inflation Adjustments
11
Office of the Law Revision Counsel. 29 US Code 2006 – Exemptions
12
U.S. Department of Labor. Fact Sheet 36 – Employee Polygraph Protection Act of 1988
13
PubMed Central. Beyond the Polygraph: Deception Detection and the Autonomic Nervous System
14
United States Department of Justice. Criminal Resource Manual 262 – Polygraphs Introduction at Trial
15
Legal Information Institute. Federal Rules of Evidence Rule 702 – Testimony by Expert Witnesses
16
Air Force Judge Advocate General’s Corps. The Admissibility of Polygraph Evidence in Court-Martial Proceedings