The Lie Detector Paradox

Vaughan Bell, of Mindhacks, with an interesting piece in the Guardian on ‘Lie Detectors.' Although highly fallible, suspects are more likely to tell the truth when wired up to them. Bell explores why and whether this means we should trust the results.

“It turns out,” Bell writes, “that polygraphs have a sort of placebo effect, where people are more truthful because they believe that they work. In fact, studies show that people are more truthful when wired up to a completely bogus ‘lie detector’ look-alike.”

The name “lie detector” is misleading in many ways. First, the polygraph doesn't actually detect lies but, instead, measures arousal. It is based on the idea that we will be a little more stressed, with fleeting changes in blood pressure, sweat gland activation and respiration, when answering questions with lies compared to giving truthful responses. The majority of tests involve comparing responses to control questions that the interviewee will respond truthfully to (“Are you sitting in a chair?”) with responses to investigation-relevant questions (“Did you handle the money?”).

The “lie detection” part comes from an interpretation of the differences in arousal between these types of answers. But physiological differences may arise for many reasons, not just from intentional deception – I may become more stressed if I worry that I won't be believed, or if the question concerns something that is naturally arousing – perhaps even just a question that contains highly emotional words.

Because there is no pattern of arousal that is unique to deception, the decision to classify a set of responses as untruthful is inevitably a leap from the shaky ground of ambiguous data into the fog of inference. As a result, techniques to “beat” a polygraph are simple and effective. The simplest strategy seems to be to increase arousal during the control questions, rather than trying to reduce arousal during deception, to eliminate any difference.

Bell concludes:

So if it leads to more information, shouldn't the police be using it? For those who want to base offender monitoring on a technique that relies on ignorance for its validity, it is unfortunate that none of these details is secret as they've been discussed openly in scientific and lay forums for years. Any form of risk management that relies on an offender not knowing about Google is inherently flawed, but perhaps more importantly, we have a responsibility to ensure that the police are not basing public safety on methods that are so easily fooled.

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Still curious? Reviews of the scientific evidence by the National Research Council in the US and the British Psychological Society in the UK have indicated that the polygraph has an accuracy of about 85% when evaluating genuinely guilty people. “Unfortunately,” says Bell, “the accuracy is probably nearer to 50% (with results here varying greatly across studies) when attempting to do the same with genuinely innocent people.”