Rise of polygraph testing sparks concern about police powers

Myron Standret / Alamy Stock Photo

Polygraph test
Data disclosed by police forces shows an expansion of polygraph testing to include interviews of suspects

Reports Mark Wilding for Liberty Investigates and Cahal Milmo for the i.


Police forces have rapidly expanded the use of controversial “lie detector” tests and may be exceeding their powers by using the technology to interview and gather evidence against unconvicted suspects, it can be revealed.

At least 14 police forces in England and Wales now use polygraph tests — often described as ‘lie detectors’ — with at least 25 police officers and staff members trained to use the interrogation technique, according to figures obtained by Liberty Investigates using freedom of information (FOI) law.

Thirteen of the forces conducted more than 700 tests between them in 2022— a five-year high — while another, which could only provide figures by financial years, said it conducted almost 100 tests in 2022-23.

Among the forces for which data has been obtained, Essex Police carried out the highest number of tests (99), followed by Suffolk (93 for the financial year 2022-23) and South Yorkshire (87).

The science behind polygraph testing is highly contested and results are inadmissible as evidence in UK criminal courts. The tests have been used by the probation service to monitor convicted sex offenders across England and Wales since 2014, and have more recently been adopted as a condition for those released on licence following terrorism and domestic abuse convictions.

In the King’s Speech laying out its legislative programme last month, the Government unveiled plans to expand polygraph use and increase existing powers available to probation officers to monitor convicted sex offenders and terrorists. But the extent of parallel use of the technology by UK police has until now been unclear.

Documents obtained from police forces by researchers at Northumbria University sheds further light on the extent to which it is being deployed – revealing that polygraph testing is now being used when interviewing suspects, sparking concerns about the legality of the practice.

The Northumbria University researchers uncovered hundreds of occasions when suspects who have not been convicted of an offence have been subject to polygraph testing. Dr Kyriakos Kotsoglou and Professor Marion Oswald said police forces appear to be using polygraphs in “semi-evidential and semi-investigatory ways” and ”as an interrogation tool”, warning that officers could draw the wrong conclusions from test results and be “led in the wrong direction”.

“There is a high risk that such uses could be subject to challenge as not being in accordance with law.”

Dr Kyriakos Kotsoglou and Professor Marion Oswald

Kotsoglou and Oswald also raised questions about the legal validity of polygraph tests being used for pre-conviction interviews. “There is a high risk that such uses could be subject to challenge as not being in accordance with law,” they added. “Urgent action should be taken to clarify the legal basis for the many different uses disclosed and those still hidden or redacted … and to ensure appropriate independent scrutiny and oversight.”

The researchers examined policies disclosed by police forces, noting that the independent representatives suspects might usually have access to in police interviews, such as solicitors or medical assessors, are excluded from polygraph testing. Referring to examples that included polygraphs being used to assist the police with bail and child contact decisions, they said: “Although some testing is described as ‘voluntary’, this must be questionable because of the significant consequences of ‘failing’ a test or of refusal to take part.”

Despite the controversial nature of polygraph testing, and the lack of a clear legal framework for its use beyond the monitoring of convicted offenders, the College of Policing is in the process of setting up a National Polygraph School and has predicted a significant expansion of the technology’s use among forces.

A meeting paper considered by the College of Policing’s senior leadership team last year, obtained using FOI, describe how the school is intended to help forces defend themselves against legal challenges related to polygraph testing.

Noting the likelihood that police use of polygraphs will continue to increase, the documents state: “There is also an increased likelihood that use of polygraph will be challenged in the courts.” The documents note: “Currently there are no requirements or standards for police polygraph use.”

Kotsoglou and Oswald said: “The College of Policing’s work provides an ideal opportunity to conduct a full and independent review of all uses of polygraph testing involving police forces. Attention must be paid to scientific validity and to the evidential and procedural safeguards surrounding these uses.”

David Tucker, head of crime at the College of Policing, said: “The college is working closely with the National Police Chiefs’ Council to establish a polygraph school so that policing in England and Wales has access to standardised learning and development in the use of the tool that is tailored to their operating environment.

“In addition, the college is developing advice and guidance so that forces using the tool have a consistent basis on which to do so whilst recognising the relevant legal provisions.

“Polygraph is not used as evidence in court by police, it is to be used as a tool to assist officers to make judgements about the risk a suspect may pose. Polygraph should also not be used in isolation, but as one of a number of sources of information to assist decision making.”

A version of this article was published with the i.