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Polygraphs are often used in fiction, but also sometimes in court or by government agencies and corporations. Physiological reactions from the subject are recorded and an operator interprets them to decide whether the subject is telling the truth or is lying.

The basic premise sounds plausible at first glance, but how reliable are polygraphs really? Are there scientific studies on the accuracy of lie detection using polygraphs?

How do they compare to experiences interrogators using just their observations, without recorded physiological data, to judge if the suspect is truthful?

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    A few weeks ago I saw a tv broadcast of popular science, where they could spot a lie under computer tomography. If I remember correctly, they tested just volunteers, which didn't refuse an answer. However, in civilized countries torture is forbidden, and nobody is forced to incriminate himself. At least this was common sense, in western democratic systems a decade ago. But I can imagine, that, from a scientific viewpoint, lie-detectors could be improved over time, with very uncomfortable consequences.
    – user unknown
    Commented Feb 28, 2011 at 2:04
  • Unfortunately, the main thing a polygraph can't judge is whether the examiner is qualified.
    – Daniel R Hicks
    Commented Jan 18, 2018 at 1:12

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Nope, polygraphs cannot tell whether you're lying. 1, 2

The polygraphs try to measure nervousness by registering sweating, breathing, blood pressure/pulse, and (I think) light trembling. They always calibrate the polygraph for each individual by asking questions the subject has no reason to lie about, to generate what is called a honest baseline.

The concept has some damning flaws, as no such physiological traits have been shown to consistently correlate with lying. Not all people get nervous about lying! And quite a lot of people would be nervous in an interrogation even when they were telling the truth.

There are actually a few signs that have been seen to correlate with lying, such as going at great lengths to avoid self reference ("then this happened", rather than "then I did this"), but nothing of the kind that could be picked up by a polygraph.

Richard Wiseman has this to say on lie detection through body language (from the book 59 Seconds):

For successful lie detection, jettison the behavioural myths surrounding the Anxiety Hypothesis, and look for signs more commonly associated with having to think hard. Forget the idea that liars have sweaty palms, fidget and avoid eye contact. Instead, look for a person suddenly becoming more static and cutting down on their gestures. Also, learn to listen. Be on guard for a sudden decrease in detail, an increase in pauses and hesitations and a sudden avoidance of the words 'me', 'mine' and 'I', but an increase in 'her' and 'him'. If someone suddenly becomes very evasive, press for a straight answer.

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  • I think we can correctly say that the polygraph is a tool that helps assessing whether someone is lying - but it's an instrument that needs to be gauged and used amidst other factors by a trained professional.
    – Sklivvz
    Commented Feb 27, 2011 at 13:44
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    @Sklivvz: One way of framing it would be that eyewitness testimonies are probably more fallible than polygraphs, but I'd still argue that with an eyewitness you have a reliability problem that's easy to factor in (i.e. "the person may be mistaken"), but here you've got a validity problem: sure the machine may be good at testing for anxiety, but the subject here is being accused for murder (say), not anxiety in court! I don't see the case where a polygraph can help assessment. If the trained professional would've arrived at the same conclusion without it, then what did it add? ...
    – David Hedlund
    Commented Feb 27, 2011 at 14:10
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    And if not, I would really prefer to go with what the trained professional's verdict would've been without it. This article tells of a study that yielded a 61% validity. That's really not a lot better than chance would give you. I definitely think those figures make the polygraph a poor fit for any use, even in combination with other measurements.
    – David Hedlund
    Commented Feb 27, 2011 at 14:12
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    @Sklivvz: I guess you're right that the only question here is whether or not it works, and to that, I can only say "it works a little, but yet significantly, better than chance." Whether or not that means they should be used is a political question and not a scientific one, and perhaps has no place here. I would like to think that a trained professional achieves substantially better than chance, especially as they can factor in the probability of what the subject is actually saying, which a polygraph can't, so I hope they're redundant, but hey, I'll drop the politics :)
    – David Hedlund
    Commented Feb 27, 2011 at 15:06
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    @Sklivvz - no, we can't correctly say that. It measures some reactions that are often associated with lying, but not lying itself. If we hook up a sociopath or someone who has calm control over those physical reactions, then we reach the completely wrong conclusion. Same with someone who has general testing anxiety, or is anxious in a self-fulfilling way about the accuracy of a polygraph. In those cases, we get the opposite of the correct answer, trained professional or not. This is why they are not admissible in US courts as evidence.
    – PoloHoleSet
    Commented Jan 18, 2018 at 23:14

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