Control questions in a polygraph test are broad questions about common human failings — things like “Have you ever stolen anything?” or “Have you ever lied to someone who trusted you?” — that the examiner uses as a physiological yardstick. Your reactions to those questions get compared against your reactions to questions about the specific incident under investigation. The test’s conclusion about whether you’re being truthful comes out of that comparison, which is why the technique is formally called the Comparison Question Test, though examiners still use the older name Control Question Test interchangeably.
What a Control Question Looks Like
Control questions are deliberately vague and cover behavior most people would rather deny. In a burglary investigation, a typical control question might be “Have you ever stolen anything in your life?” In a fraud case, it could be “Have you ever cheated anyone?” The examiner writes them broadly on purpose. Nearly everyone has taken a pen home from work or shortchanged a friend, so nearly everyone feels some discomfort answering “no.”
That discomfort is the whole point. The examiner is not trying to catch you on your past. The examiner is trying to generate a measurable physiological reaction that can be used as a benchmark against your reaction to the questions that actually matter.1National Academies Press. The Polygraph and Lie Detection – Appendix A: Polygraph Questioning and Techniques
Alongside control questions, the exam also includes relevant questions (“Did you take the missing funds?”) and irrelevant filler questions (“Is today Tuesday?”). The filler questions absorb the early jitters of each round and are not scored. The real work happens in the pairing of control and relevant.1National Academies Press. The Polygraph and Lie Detection – Appendix A: Polygraph Questioning and Techniques
How the Comparison Decides Truthful or Deceptive
The reasoning behind the test runs like this. An innocent person, the theory goes, should feel more anxious about the vague control questions (where they have something small to hide) than about the relevant questions (which they can answer honestly). A guilty person should react more strongly to the relevant questions, because those hit close to what they actually did. The examiner reads the gap between your control and relevant reactions and calls the test truthful, deceptive, or inconclusive.1National Academies Press. The Polygraph and Lie Detection – Appendix A: Polygraph Questioning and Techniques
Everything the polygraph tells the examiner rests on that assumed pattern. If it doesn’t hold, neither does the result.
The Two Kinds of Control Questions
Probable-Lie Questions
These are the classic version. The examiner asks something like “Have you ever stolen anything in your life?” and phrases the setup so you’ll answer “no,” even though the answer is almost certainly a lie. The small discomfort of that lie is expected to register on the instruments, and that reading becomes the comparison baseline.1National Academies Press. The Polygraph and Lie Detection – Appendix A: Polygraph Questioning and Techniques
The weakness of probable-lie questions is that they lean on the examiner’s skill. The question has to bother you enough to produce a reaction, but not so specifically that you connect it to the investigation. Different examiners get different results from the same person.
Directed-Lie Questions
Directed-lie questions were designed to fix that inconsistency. Here, the examiner tells you plainly that the answer will be a lie and instructs you to give it anyway. A common example: “During the first 20 years of your life, did you ever tell even one lie?” You know it’s a lie, the examiner knows it’s a lie, and your physiological response to giving that known lie becomes the baseline.1National Academies Press. The Polygraph and Lie Detection – Appendix A: Polygraph Questioning and Techniques
The advantage is that every examiner can use the same directed-lie questions regardless of the case. The disadvantage is that you know exactly which questions are the controls, which makes it easier to game them.
How the Examiner Chooses Your Control Questions
Control questions are not pulled off a shelf. Before the polygraph instruments are turned on, the examiner spends 30 minutes to two hours in a pre-test interview. During that conversation, the examiner explains the procedure, reviews your medical history, and goes over the details of the investigation. Where the test format allows examiner discretion, the interview also feeds the choice of comparison questions tailored to you.1National Academies Press. The Polygraph and Lie Detection – Appendix A: Polygraph Questioning and Techniques
The examiner then pairs each control question with a relevant question so responses can be compared directly. In a burglary case, “Did you break into that warehouse on June 12?” might be paired with “Have you ever stolen anything?” The pairing matters because scoring happens within each pair, not across the test as a whole.
How Reactions Are Scored
Polygraph results are not called on gut feeling. Examiners use a numerical scoring system that assigns values from −3 (strong indication of deception) to +3 (strong indication of truthfulness) at each comparison point. The examiner evaluates each relevant question against its paired control question, scores the relative size of the physiological reactions, and adds up the scores across multiple rounds. A total far enough into positive territory is scored truthful; far enough negative is scored deceptive; anything in the middle is inconclusive.
Scoring happens across each recorded channel — heart rate, blood pressure, breathing, and skin conductivity. One chart might show a bigger breathing response to the relevant question while another shows a bigger sweat response to the control. The examiner weighs the channels together, which layers a second round of judgment on top of the already subjective question design.
Whether the Comparison Actually Works
The National Research Council published the most comprehensive review of polygraph science in 2003. Its findings were measured but not flattering. The Council found that polygraph chart features do correlate with deception above chance in controlled laboratory settings with untrained subjects, but errors are not infrequent and accuracy swings widely between studies. The middle range of accuracy indices came in between 0.81 and 0.91, and the Council warned against treating those as general performance figures. Many committee members believed the realistic upper bound was well below 0.90.2National Academies Press. The Polygraph and Lie Detection – Chapter 5: The Accuracy of Polygraph Tests
More fundamentally, the review flagged a hole under the whole CQT: no independent evidence has confirmed that relevant questions actually produce stronger reactions in deceptive people, or that control questions actually produce stronger reactions in truthful ones. That assumption is the basis of the test, and it hasn’t been empirically verified the way, say, a medical diagnostic would be.3National Academies Press. The Polygraph and Lie Detection – Chapter 3: Validity and Reliability
Why Control Questions Are the Weak Point
Control questions are the part of the test most vulnerable to countermeasures. A 1994 study found that subjects trained in simple physical tricks (biting the tongue, pressing toes to the floor during control questions) or mental ones (counting backward by seven) could defeat the polygraph roughly half the time. Physical and mental methods were about equally effective, and examiners struggled to detect either through the instruments or by watching the subject.4National Center for Biotechnology Information. Mental and Physical Countermeasures Reduce the Accuracy of the Concealed Knowledge Test
The mechanism is direct. If you artificially inflate your reaction during the control questions, the gap between control and relevant shrinks or reverses, and the examiner reads you as truthful whether you are or not. This is the built-in flaw of comparing one physiological response to another.
Whether a Failed Polygraph Can Be Used Against You
The scientific fog around polygraphs has shaped how courts handle them. In United States v. Scheffer (1998), the Supreme Court held that a blanket rule excluding polygraph evidence does not violate a defendant’s Sixth Amendment right to present a defense. The Court’s language was flat: “There is simply no consensus that polygraph evidence is reliable. The scientific community and the state and federal courts are extremely polarized on the matter.”5Legal Information Institute. United States v. Scheffer, 523 US 303
Neither the Federal Rules of Evidence nor the U.S. Code contains a specific provision on polygraph admissibility. Federal courts apply the Daubert standard for scientific evidence, and most have excluded polygraph results under it, though a trial judge retains discretion in narrow circumstances.6U.S. Department of Justice. Criminal Resource Manual 262 – Polygraphs Introduction at Trial
In military courts-martial, exclusion is absolute. Military Rule of Evidence 707 bars all polygraph-related evidence, including results, examiner opinions, and any reference to whether someone offered, refused, or took a test.7Air Force Judge Advocate General’s Corps. The Admissibility of Polygraph Evidence in Court-Martial Proceedings
State courts are inconsistent. Some allow polygraph results by stipulation (both sides agreeing beforehand); most either exclude them or leave the decision to the trial judge. Failing a polygraph almost never becomes admissible evidence against you, and passing one rarely helps.
Whether an Employer Can Make You Take One
If you work in the private sector, the Employee Polygraph Protection Act makes it illegal for your employer to require, request, or even suggest a lie detector test. The Act also prohibits firing, disciplining, or refusing to hire you for declining a test or exercising any right under the law.8U.S. Department of Labor. Employee Polygraph Protection Act
The protection is broad but not universal. The EPPA does not cover federal, state, or local government employers. It also carves out exceptions for certain private employers:9U.S. Department of Labor. Fact Sheet 36 – Employee Polygraph Protection Act of 1988
- Armored car, security alarm, and security guard firms may polygraph prospective employees who would protect facilities or materials affecting health, safety, or national security.
- Companies authorized to handle controlled substances may polygraph prospective employees who would have direct access to those substances.
- A private employer may request a polygraph from a current employee reasonably suspected of involvement in a workplace incident causing economic loss, provided the employee had access to the property under investigation.
Even inside the exceptions, the EPPA imposes testing standards on the examiner and limits what an employer can do with the results. Violations carry a civil penalty capped in the statute at $10,000, adjusted annually for inflation; as of early 2025, the inflation-adjusted maximum was $26,262 per violation.9U.S. Department of Labor. Fact Sheet 36 – Employee Polygraph Protection Act of 1988 Employees can also file private lawsuits within three years, seeking reinstatement, lost wages, and other relief.10Office of the Law Revision Counsel. 29 USC Ch. 22 – Employee Polygraph Protection