Testing the Truth: Do polygraphs have a place?

August 3, 2020


3 min read

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What's going on here?

MPs have been debating the use of polygraph tests on those convicted of terrorism-related offences and released on licence (serving the remainder of their sentence in the community). The amendments to the Counter-Terrorism and Sentencing Bill would introduce mandatory polygraph testing on Very High/High risk Serious Harm terrorist offenders.

What does this mean?

This debate comes in the aftermath of the London Bridge attack, which was carried out by an offender on licence. One difficulty of deciding who can be released on licence is that offenders can lie about their intentions to convince probation officers that they pose a low risk. Polygraph tests could help with these decisions. The Home Office proposes that tests be carried out when deciding on release, 3 months post release, and every 6 months thereafter (unless closer monitoring is needed). 

The polygraph test is regarded by some as a magical mind-reading machine. It measures physiological changes in the body when an individual is asked certain questions, including changes in heart-rate, blood pressure, respiratory rate and sweat. The basic logic behind the test is that being deceptive is stressful. This stress causes changes in physical responses that people cannot perceive.

What's the big picture effect?

Since January 2013, the National Probation Service has used polygraph testing on sex offenders released on licence, but what might be the effects of using polygraph testing more widely in the criminal justice system? 

A review conducted by the US National Research Council in 2003 indicated that some polygraphs give accuracy of 80-90%. But many experts, including Professor Sophie Scott (a neuroscientist at University College London) assert that polygraph tests are not always scientifically reliable. Polygraphs may measure physiological changes well (as you experience different emotions) but experiencing emotion does not necessarily equate to lying. Anyone recounting traumatic experiences may appear to fail the test. Studies suggest that there are high false-positive results (with almost half of innocent people identified as liars) and evidence of people training to beat a polygraph. Professor Albert Vrij (psychologist at the University of Portsmouth) suggests that the science is shaky. Polygraphs were designed to test a person’s involvement in a known crime. In some cases, testing can involve open questions about future intentions and “to simply introduce the test in an entirely different setting is too far stretched”. Furthermore, for those who have genuinely reformed, or for innocent people implicated in terrorist activity, polygraphs might be dangerous. The use of a technique that cannot be used in UK courts as admissible evidence will likely appear discriminatory. Because the results of the test are unreliable in detecting lies, it does not rise to the level of reliability required for scientific evidence in a courtroom, so how does a person scrutinise or appeal the outcome?

Even if the technology is flawed, polygraph tests might still be useful. Forensic psychologist Theresa Gannon found that polygraph tests (regardless of their results) encouraged people to confess. Offenders were 24% more likely to disclose something of interest when using polygraphs in comparison to ordinary interviews. While disclosure often happened after the test had indicated deception, it could not confirm the veracity of these confessions or whether an offender felt that denial wouldn’t be believed. The polygraph could psychologically pressure offenders into disclosing self-incriminating information, even if that information is not true. 

Introducing polygraph testing for serious terrorism-related offences could make probationary decisions easier, but there are serious questions on the reliability of the technology. What’s more, using polygraphs also raises ethical concerns: is it fair to use the test to try to extract self-incriminating statements, especially in a country where the test cannot be relied upon in court?

Report written by Josie Laidman

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