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Here’s the truth about lie detectors

Polygraphs are notoriously unreliable in detecting whether or not someone is telling the truth. So why do investigators still put so much faith in them?

You’ve seen polygraph tests a thousand times on crime dramas and daytime TV: a stubborn suspect gets hooked to the machine and their darkest secrets come spilling out, etched forever onto a roll of paper. Its results are often presented as fact, as black and white, but the truth about the lie detector is much more complex.

The machine was invented in Berkeley, California in the early 1920s by a rookie police officer called John Larson. He was a physiologist by training, and he wanted his machine to offer a more humane alternative to the ‘third degree’ – the brutal beatings that police would dole out to suspects to get information out of them.

At first, he had great success solving thefts at college dorms and some high-profile murder cases. But when the lie detector failed to find the truth in the 1922 case of Henry Wilkens, a San Francisco mechanic accused of plotting his wife’s murder, there were deadly consequences that shook Larson’s faith in his invention. Instead of ending the third degree, Larson had inadvertently created a tool of psychological torture, that he ended up viewing as a “Frankenstein’s monster”.

He spent the rest of his life trying to undo the damage.

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The polygraph measures pulse, breathing rate and sweat, and it is supposed to work by comparing a suspect’s bodily response to control questions (“Did you have breakfast today?”) with their response to questions about the crime (“Did you steal the watch?”). In theory, someone who is lying will be more stressed than someone who is telling the truth, and that stress will increase as they lie about their crime. Polygraph examiners pore over the charts for signs of an increased pulse, or a change in someone’s breathing.

But lying is complicated – people react in different ways, and there’s no single tell-tale sign of lying shown by everyone, all the time – no Pinocchio’s nose. That means, as Larson quickly realised, that there’s no way for a polygraph examiner to tell if someone’s heart is racing because they’re lying, or simply because they’re anxious about being falsely accused.

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It’s also largely subjective. Although digital scoring algorithms to score polygraph charts were created in the 1980s, they failed to catch on, which means many polygraph examiners make their assessments by feel and intuition, creating the perfect conditions for bias. Unsurprisingly, the rate at which suspects fail polygraph tests varies wildly depending on the individual examiner, with Black people disproportionately likely to fail a test, regardless of whether they are lying.

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Despite these problems, it didn’t take long for the lie detector to become a political tool. In the 1950s, it was used to interrogate political dissidents in the United States, and in the 1970s, polygraph tests were used to blackmail suspected homosexuals. The targets of new forms of lie detectors – powered by brain scans and AI – are likely to be migrants and asylum seekers.

Tremors in the blood book cover
Tremors in the Blood: Murder, Obsession and the Birth of the Lie Detector by Amit Katwala is out now (Mudlark/ HarperCollins, £20)

The polygraph has certainly helped to catch criminals awed by its supposed power, but it’s also put plenty of innocent people in prison and allowed murderers to walk free.

In fact, the polygraph has been repeatedly debunked in studies going back decades – one of them estimates its accuracy at just 65 per cent.

But that doesn’t matter, because the polygraph fills a niche in the criminal justice system, providing police officers with a pseudo-scientific way of extracting confessions.

There are still millions of tests a year. Lie detectors have never been admissible in court in the United States or the UK, but they have long been used as a way of avoiding an expensive trial, or as a means for governments to look tough on crime. That’s why their use is actually increasing in the UK, where polygraph tests are now being rolled out for use in parole decisions for convicted sex offenders and terrorists. British police forces are quietly expanding their use of lie detectors too – although there is a lack of transparency about when and where they are using them.

Ultimately, the polygraph remains popular not because it works, but because people think it works. It’s used as a psychological prop during interrogations, or a PR tool that plays well in the papers. But the truth is – a century on from John Larson’s first, fumbling experiments with the polygraph – there is still no such thing as a lie detector.

@amitkatwala

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