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Why polygraphs fail innocent people: the science of false positives

By Neta Dan·Updated ·4 min read
The short answer

A polygraph measures arousal in the body, and there is no physical signal that belongs only to lying. The US National Research Council found that the states the test relies on, such as fear of being judged deceptive, can arise in honest people too, and that the method is "intrinsically susceptible to producing erroneous results". Honest people who are anxious, believed guilty, or from stigmatized groups may be at greater risk of a false positive.

In this guide
  1. The core problem: no signal belongs only to lying
  2. Four steps, four places to go wrong
  3. Which honest people are most at risk
  4. Why better equipment does not fix it
  5. What you can do about it
  6. Frequently asked questions
  7. Sources

The core problem: no signal belongs only to lying

People often imagine that lying produces a distinctive physical response, a kind of body alarm that the machine picks up. It does not work that way. The polygraph records general arousal: breathing, heart rate and blood pressure, and sweat on the skin.

The US National Research Council, in its major review of the science, found that the states a polygraph depends on, such as fear of being judged deceptive, can arise in the absence of deception. In other words, the body of an honest person can react in just the same way.

That single point explains most false positives. The rest of this guide shows where it plays out. For the overall numbers, see how accurate a polygraph really is. For a practical list of the usual triggers, see the top 10 reasons for failing a polygraph.

Four steps, four places to go wrong

1A questionHeard and interpreted in your own way
2An emotionFear, shame, anger or surprise, true or not
3A body reactionHeart, breathing, sweat, plus noise
4An opinionThe examiner reads the charts

Each step can introduce error that has nothing to do with whether you told the truth:

  • The question. A broad or ambiguous question touches memories and worries beyond what the examiner meant.
  • The emotion. Fear of not being believed is itself an emotion. It is the one the test is most likely to trigger in an honest person.
  • The reaction. Sleep, illness, pain, medication and the room all change readings.
  • The opinion. Scoring involves human judgment, and expectations can shape how a chart is read.

Which honest people are most at risk

The Council noted that some truthful people may show reactions that resemble deception, and that other factors add to the risk:

WhoWhy the readings may mislead
Anyone who fears not being believedThe fear of being judged deceptive is the same state the test is trying to detect.
Truthful people seen as likely guiltyBeing suspected or accused can create strong reactions to the relevant questions.
Members of socially stigmatized groupsQuestions or settings can provoke reactions unrelated to the honesty of the answer.
Anxious or unwell peopleAnxiety about being tested affects the physiological responses, as does poor physical condition.

None of this means the test is useless. It means a result must be read as an imperfect opinion, not as a verdict.

Why better equipment does not fix it

It is tempting to think a more sensitive machine would solve the problem. The Council concluded that polygraph testing is "intrinsically susceptible to producing erroneous results" and that improvements in technique would probably deliver only modest gains in accuracy.

The reason is that the weakness is not in the sensors. A perfect recorder still captures a reaction that fear can cause as easily as a lie. The Council also noted that polygraph research has not developed and tested theories of the underlying factors that produce the responses, which makes results hard to generalize from one setting to another.

What you can do about it

  1. Know where false positives come from.Understanding the four steps above makes a bad result easier to interpret, for you and for others.
  2. Reduce avoidable noise.Sleep, eat normally, disclose health and medication, and ask for unclear questions to be explained and agreed in the pre-test interview.
  3. Do not try to manage the machine.Deliberate attempts to control reactions are treated as countermeasures and can make things worse.
  4. If you fail honestly, act quickly.Write down what happened, ask what the result means, and look at your options. Here is what to do after failing a polygraph while telling the truth.

This guide is general information, not legal advice.

Frequently asked questions

Why do innocent people fail polygraphs?
Because the test measures body reactions such as heart rate, breathing and sweating, and honest people react to fear, stress and surprise just as dishonest people do. The US National Research Council found that the states the test relies on can arise in the absence of deception.
Is there a signal that only appears when someone lies?
No. The National Research Council found that the physiological responses measured are not uniquely tied to deception. Fear of being judged deceptive, for example, can produce the same responses in a truthful person.
What is a false positive on a polygraph?
A false positive is when an honest person is marked as deceptive. It is the main reason a failed result should not be treated as proof of anything.
Who is most at risk of a false positive?
The Council noted that truthful people from socially stigmatized groups, and truthful people who are believed to be guilty or likely guilty, may show responses resembling those expected of deceptive people. High anxiety and poor physical condition can also add risk.
Can better technology fix this?
The Council concluded that polygraph testing is intrinsically susceptible to producing erroneous results and that improvements in technique are likely to bring only modest gains in accuracy.

Sources

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