Advertisement
Life

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.

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.

Advertisement
Advertisement

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.

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

This article is taken from The Big Issue magazine, which exists to give homeless, long-term unemployed and marginalised people the opportunity to earn an income. To support our work buy a copy!

If you cannot reach your local vendor, you can still click HERE to subscribe to The Big Issue today or give a gift subscription to a friend or family member. You can also purchase one-off issues from The Big Issue Shop or The Big Issue app, available now from the App Store or Google Play.

Advertisement

Learn more about our impact

When most people think about the Big Issue, they think of vendors selling the Big Issue magazines on the streets – and we are immensely proud of this. In 2022 alone, we worked with 10% more vendors and these vendors earned £3.76 million in collective income. There is much more to the work we do at the Big Issue Group, our mission is to create innovative solutions through enterprise to unlock opportunity for the 14million people in the UK living in poverty.

Recommended for you

Read All
What are ultra-processed foods? The health risks explained
Health

What are ultra-processed foods? The health risks explained

Here's where to get instant cost of living advice
money

Here's where to get instant cost of living advice

35 places where kids can eat free or for £1 during the May half term
Cost of living crisis

35 places where kids can eat free or for £1 during the May half term

Perfect electric family transport with Citroën ë-Berlingo
Advertorial

Perfect electric family transport with Citroën ë-Berlingo

Most Popular

Read All
Here's when people will get the next cost of living payment in 2023
1.

Here's when people will get the next cost of living payment in 2023

Strike dates 2023: From trains to airports to tube lines, here are the dates to know
2.

Strike dates 2023: From trains to airports to tube lines, here are the dates to know

Suranne Jones opens up about her 'relentless and terrifying' experiences of bullying
3.

Suranne Jones opens up about her 'relentless and terrifying' experiences of bullying

Arctic Monkeys team up with Big Issue to produce unique tour programme
4.

Arctic Monkeys team up with Big Issue to produce unique tour programme