While lie detector tests are commonly associated with daytime TV – like The Jeremy Kyle Show – their use is becoming more common in criminal proceedings in the UK.

Officially known as ‘polygraphs’, these tests aren’t admissible as evidence in criminal courts in England and Wales. However, they’ve been used as a condition for the release of sex offenders since 2014, and the release of convicted terrorists since 2021. The latter came after the 2019 London Bridge attack in which Usman Khan killed two people and injured three more. Khan was released in 2018 on license following a prison sentence for terrorism offences.

At present, polygraph tests aim to assess how likely someone is to re-offend, so that officers can focus stretched resources on those considered most dangerous.

What is a polygraph test?

A polygraph test is a machine that measures a person’s physiological responses while they give answers to questions. These tests can’t guarantee that someone is being truthful or deceptive. However, they can detect typical stress responses to telling lies, such as increased blood pressure and heart and breathing rates – which may indicate someone is lying.

Critics of the polygraph argue that not everyone will have the same bodily response when they’re lying, making the test unreliable. For example, not everyone who lies feels stressed or nervous. Though, according to research, polygraphs can accurately predict deception in 80-90% of cases.

While polygraph results alone can’t be used to send someone back to prison, they can flag concerns about behaviour and encourage further investigations. They may also be reason enough to add extra conditions to someone’s license or recall someone for a further polygraph test ahead of schedule.

How do polygraph tests work?

How do polygraph tests work

Polygraphs are used in various countries such as the UK, Russia, China, and Japan – but the technology involved is largely the same.

The process is fairly lengthy, lasting for about an hour, and the individual being tested is attached to several sensors – usually via the fingers, palm, chest, and stomach. These sensors measure and record various physiological indicators like blood pressure, pulse, respiration, and skin conductivity while the person is asked a series of straightforward questions by a polygraph examiner.

The aim is to remove any outside distractions and make the person being tested feel as comfortable as possible before they begin answering. Interviewers will ask several control questions and compare these with the answers to key questions. Individuals will also have an opportunity to explain the responses to the questions during a post-test interview.

How many UK police forces are using polygraph tests?

At least 14 police forces in England and Wales are currently using polygraph tests, with the most being carried out in Essex and South Yorkshire.

What is the impact of polygraph testing so far?

What is the impact of polygraph testing so far?

Research carried out by the University of Kent in 2010/11, suggests that polygraph testing may increase the likelihood of convicted sex offenders disclosing information relevant to their management, supervision, treatment, or risk assessment. As a result, probation practitioners can take more actions to protect the public from harm.

Over 7,000 tests of individuals convicted of sexual offences have taken place since January 2014, with two-thirds of these leading to significant disclosures.

To look at one example from the government website…

J is a 47-year-old man convicted of sexually abusing young boys. He served a nine-year custodial sentence and was subjected to mandatory polygraph testing as part of his licence conditions. His licence also stated that he should have no contact with children (anyone under the age of 18).

During polygraph testing after his release, J was asked whether he’d made contact with anyone under the age of 18. He said no, but the test suggested signs of possible deception. This led the polygraph examiner to contact the police. When the police went to check on J at home, they found three young boys and another adult in his property.

J was immediately recalled to custody and police carried out further investigations.

Is the use of polygraph testing being expanded?

As mentioned, polygraph testing is already used as a condition for the release of sex offenders from prison – and to monitor terror suspects.

However, since 2021 police in England and Wales have also been trialling the use of polygraphs on domestic abuse perpetrators who have been released from prison. The pilot is set to last three years; after which the findings will be evaluated, published, and laid before parliament to decide whether to roll it out to all probation regions.

People who meet the eligibility criteria for the pilot are tested within 16 weeks of being released from custody. They then undergo additional testing every six months unless they fail the polygraph examination (suggesting they might be lying), in which case, they may be asked to undergo more frequent testing.

Probation officers involved in the study have been trained as accredited examiners to the standards set by the American Polygraph Association (APA).

The Met Police is also looking to use polygraphs to vet recruits and uncover corrupt police officers. This has come after a string of recent cases surrounding corrupt officers, including Sarah Everard’s killer Wayne Couzens and serial rapist David Carrick. Some – who’ve turned to polygraph testing to try and prove their innocence – have also queried why it can’t be used as admissible evidence in court in cases where it’s one person’s word against the other.

For example, Kevin Duffy, 70, was convicted of sexually assaulting a child, and a polygraph test says he was telling the truth about not committing the crime. But because the results weren’t taken into account by the judge, Duffy was jailed for more than nine years.

Sky News also reported that, according to the University of Northumbria, a non-statutory testing regime is being carried out by some police forces on suspects in criminal investigations. For example, if someone is arrested on suspicion of committing online child sex offences, a polygraph test may be used to assess whether they should be allowed to have contact with children.

Can you cheat a polygraph test?

Can you cheat a polygraph test

In the UK, beating a polygraph examination is possible with practise – but is highly unlikely when administered by a qualified and accredited polygraph examiner.

Despite various suggestions on how to fool a polygraph test, including taking drugs, altering breathing patterns, or having certain medical conditions, the reality is that experienced examiners are trained to detect deception effectively.

This can be seen in the case of double child killer, Colin Pitchfork, who was recalled to prison in 2021 after it was believed that he was using breathing techniques to beat the lie detector.

According to a Sky News report, probation services have carried out more than 8,800 polygraph tests in the last 10 years, and police have conducted more than 4,600. Around 60 to 70% of tests have resulted in someone revealing relevant information

See a polygraph test in action…

See Sky News reporter Henry Vaughan try out the polygraph test to see whether he can cheat it.

Final thoughts…

While polygraph examinations may have some effectiveness in prompting admissions or confessions and deterring undesired behaviour, their accuracy and reliability remain questionable.

However, research into polygraphs is ongoing, with new technology being tested and developed all the time. The most recent is the ‘polygraph in a box’, which is thought to be as accurate as testing by any polygraph examiner – but it’s quicker, the subjects are asked questions by a person from a recorded video, and it only requires 15 minutes of training to use.

Professional polygraph examiner Don Cargill, who invented the new technology, says it could be useful in police vetting or for the Border Force in determining a person’s age or country of origin.

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