Use of ‘junk science’ lie detector tests for sex offenders triples in three years

The number of lie detector tests being used on sex offenders in England and Wales has almost tripled in the last three years, leading to fears that victims’ safety is being put at risk.

The controversial tests are being used on prisoners out on licence, and are taken into account when deciding the level of restrictions put in place or whether someone should be put back behind bars.

But their validity is highly disputed among scientists and evidence produced using them is not admissible in the UK’s criminal courts. One MP condemned them as “junk science”.

Data obtained by The Independent under freedom of information (FOI) laws reveals that the number of lie detector tests used by the probation service on sex offenders rose from 634 in 2020 to 1,797 in 2023.

The tests measure physical reactions such as breathing, sweat levels, and blood pressure to see if someone is telling the truth.

Offenders could be asked whether they have had contact with children or are going to prohibited locations in sessions that can last up to more than four hours.

One of the reasons for the rise in lie detector tests is thought to be sentencing changes introduced by the Tory government, which resulted in more sex offenders being let out on licence.

Labour MP Kate Osborne, who sat on the Women and Equalities Committee until the election, said the use of lie detectors must not be “normalised”, warning they can enable sex offenders to manipulate the system.

The MP for Jarrow and Gateshead East said: “Lie detector tests are completely unreliable. It is appalling that they are being used in sex offence cases. This junk science puts women and victims at risk.”

Lie detectors have been used to monitor high-risk sex offenders released on licence since 2014 but have also started being used on terrorists and domestic abusers in recent years. 

Marion Oswald, a professor of law at the University of Northumbria, who specialises in law and digital technology, told The Independent they are wielded as a “risk assessment tool” and a mechanism to encourage offenders to disclose information about what they have been doing and potentially to make confessions.

She said: “If they are seen as not being deceptive under the test then they are likely to be downgraded on their risk assessment threat. 

“The concern is this may pose a risk to victims and women if people are downgraded inappropriately. They might have fewer conditions to comply with if they are seen as truthful, this might be things like the number of times you have to check in, whether you can go out at night, and access certain locations.”

Prof Oswald warned increasing “reliance” was being placed on lie detector tests and said their use is “shrouded by a lack of transparency”.

She added: “We have a very contested scientific method incorporated in a really serious context of sexual violence and domestic abuse and now other offending like terrorism.”

Kyriakos Kotsoglou, an associate professor at Northumbria University, who has also done research into lie detectors, said: “We predicted back in 2020 the proliferation of the polygraph and that we were on a slippery slope with regards to its usage because the probation service treats the polygraph as a silver bullet.”

A government fact sheet about lie detector usage on domestic abusers states: “Information gathered from a polygraph examination may also be shared with the police who are able to conduct further investigations that may or may not result in charges being made.”

Ms Osborne said she would ask the new justice secretary and the minister for women and equalities to look urgently “at the use of mandatory polygraph exams on high-risk domestic abuse perpetrators started by the last government”.

open image in gallery

Hannah Couchman, senior lawyer at legal charity Rights of Women, called for the government to stop using lie detector tests, warning they “lack an evidential basis and exacerbate discriminatory approaches”.

The barrister told The Independent that the former Conservative government’s rollout of lie detectors was part of their wider “tendency to introduce technological gimmicks as a cure-all to complex social problems”.

Ms Couchman added: “They are baseless and unscientific. Polygraphs are inherently discriminatory because a human has to analyse the data and then decide whether the person is lying or not, and nobody is immune to discriminatory and biased thinking.

“I would be concerned that anxiety could affect the results of that polygraph. It is a self-fulfilling prophecy. Polygraphs can’t detect lies, they can detect stress. There is nothing human bodies do and don’t do when they are lying.”

She accused the recently departed government of using lie detectors as a “fig leaf to hide accountability over the lack of action on violence against women and girls”.

“We are trying to quantify risk or assign it a numerical value when we know that risk is a subjective and complex concept,” Ms Couchman said.

A Ministry of Justice (MOJ) spokesperson said: “It is wrong to suggest test results alone can lead to a change in licence conditions.

“Rather, tests – which have an accuracy rate of approximately 90 per cent - are used to obtain invaluable information about the behaviour of offenders supervised by the Probation Service so we can put them back behind bars if they put a step out of line.”

The research referenced by the MoJ spokesperson was done by The American Polygraph Association - a body whose membership includes polygraph examiners themselves.

Prof Oswald added: "It is very difficult to measure the efficacy of a polygraph test as in real cases we often don’t ever know what the truth is so we can’t decide if the test is accurate or not."

A spokesperson for the Conservative Party has been contacted for comment.