Polygraph, also known as a lie detector test, is a well-established technique in the United States, used for assessing, treating, and managing sexual offenders in prison and community settings. In the UK, its use is not yet widely accepted in the legal system, but it has been increasingly recognized as an effective tool for promoting more truthful responding from offenders. This article provides an overview of what a polygraph is, how it works, and its history, legal status, and scientific acceptance.

What is a Polygraph and What Does it Measure?

Polygraph instruments collect physiological data from at least three systems within the human body, including respiration, sweat gland activity, and blood pressure measurement. The polygraph examination consists of three phases: a pre-test interview, a chart collection phase, and a test data analysis phase. The test works on the assumption that almost all people have a fear response associated with lying, particularly about matters of significant personal importance to them. Changes in the individual’s physiological responses associated with specific questions enable the polygraphist to conclude with considerable accuracy whether the examinee is likely being honest or deceptive when providing answers.

Types of Examinations

There are three principal types of polygraph examinations used in the treatment of sex offenders: (1) the Sexual History Examination (SHE), which obtains a fuller and more accurate account of an offender’s sexual history, any unidentified paraphilia interests, and offence behaviour; (2) the Instant Offence test, which focuses on the elements of denial (either partial or total); and (3) a Maintenance test, which focuses on an offender’s compliance with treatment and adherence to conditions mandated by the court.

History, Legal Status, and Scientific Acceptance

Polygraph is well established in the United States in the assessment, treatment, and management of sexual offenders, both in prison and community settings. In the UK, polygraph is not yet widely accepted in the legal system, but its perceived utility has assisted in the assessment, treatment, and supervision of sexual offenders. Key studies in the UK have shown that polygraphed individuals consistently make relevant disclosures regarding treatment and supervision issues at significantly higher rates than non-polygraphed offenders.

Current Employment of Polygraph

The use of polygraph has increased substantially in adult community sex offender treatment programmes in the United States, from 29% to 70% between 1992 and 2002. Following the mandatory polygraph pilot study in the UK, all high-risk sex offenders are required to undertake a polygraph. Polygraph has also been introduced into police services in the UK, with an evaluation of its use with convicted individuals or those suspected of committing a sexual offence. Furthermore, the government is currently planning to use the polygraph to assess convicted terrorists released under licence in the UK, to bolster other important public protection efforts.


The use of polygraph in assessing, treating, and supervising sexual offenders has continued to be described as “a lightning rod for controversy”. However, its perceived utility has assisted in the assessment, treatment, and supervision of sexual offenders, and it continues to hold a significant place in this area of work. Its assistive capacity has given rise to its employment with police services, and its inclusion in the Domestic Abuse Bill (2020).