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Following a series of high-profile terrorist attacks in London and elsewhere in the UK, the British government has announced that they intend to introduce polygraph tests (also known as lie detector tests) for convicted terrorists released from prison on licence. The exact purpose of the polygraph testing regime is not clear, but seems likely the government expects the procedure to detect lies about behaviour in contravention of licence conditions, deter convicted terrorists from re-engaging in terrorist activity for fear of being caught out by the polygraph, and leverage confessions of illegal activity. Keeping the public safe and reducing the fear of crime are fundamental to societal health, and being seen to do something is crucial for improved public confidence. But is polygraph testing in this context-appropriate, and is it likely to be effective?
The polygraph measures ‘autonomic’ arousal while a person is being asked a series of questions. The autonomic nervous system is largely unconscious and regulates bodily functions such as heart rate, digestion and respiration. Indicators collected during a polygraph include skin conductivity, heart rate, blood pressure and respiration rate. The idea is that people will show larger physiological responses to questions when being deceptive compared to when answering honestly.
The most common test is the Control Question Technique, where responses to irrelevant questions are compared with responses to crime-relevant questions. An alternative is the Guilty Knowledge Test, where multiple-choice questions are presented that include information only a guilty person would know. Again, guilty people are expected to react more strongly (become more autonomically aroused) when the guilty information is presented in questions.
The polygraph is used in many security, law enforcement and employment contexts worldwide, but whether it is effective for detecting lies is debatable. Indeed, the scientific basis for the polygraph is weak, and many psychologists agree there is no robust empirical evidence that the polygraph reliably detects when people are lying or that it encourages reliable confessions. Both the British Psychological Society and the American Psychological Association (APA) are clear that polygraphs are unreliable, and that while the idea of a polygraph may be comforting, the APA states that ‘…the most practical advice is to remain sceptical about any conclusion wrung from a polygraph’.
Polygraph tests for convicted terrorists follow the introduction of compulsory polygraph tests for high-risk sex offenders in England. A pilot study with sex offenders on licence found that those who took the compulsory test made twice as many clinically significant disclosures about their behaviour (e.g., they had contacted victims or entered exclusion zones in contravention of their licence) than those who did not take the test. These disclosures sometimes triggered a change in the way that these offenders were managed, including recall to prison. Here the polygraph did not detect lies but instead leveraged confessions, typically during a pre-polygraph interview indicating that offenders believed they would be caught lying, so might as well confess to reduce the severity of possible punishment.
On the face of it, this is a good new story. But, while polygraph tests were compulsory for high-risk sex offenders who met the study criteria, participation in the research element of the trial was not. So, offenders who agreed to take part may have been more motivated to make disclosures because they wanted to change irrespective of the polygraph. Further, the numbers of disclosures, ‘deception indicated’ and ‘no deception’ outcomes decreased as the number of polygraph tests for each offender increased. Therefore, participants were either becoming less concerned by the threat of the polygraph over time, realised it was ineffective, learned to ‘cheat’ the test, which is relatively easy, or they may have changed their behaviour as the research implies, we simply do not know.
What does this trial indicate for other types of offender? If we accept that the polygraph might be a useful deterrent for sex offenders (which is disputed by many), can the same be said of terrorists? One of the risks of using a polygraph is the problem of false negatives. In the case of the latter, someone who gets a null result is badged as innocent, and so not a threat in which case the authorities turn their attention elsewhere. If a terrorist gets through, an unreliable procedure, their potential threat may be ignored just as the continuing threat of the 2019 London Bridge attacker was underestimated despite an electronic tag and presumably a full risk assessment pre-release.
The UK Government is also considering use of the polygraph for domestic violence offenders. Yet people who assault their partners may not consciously intend to reoffend, so would not be ‘lying’ when they take a polygraph. Again, if they pass, they too are badged as ‘no threat’. The crimes of terrorism and domestic violence are very different, but share similar characteristics whereby to be effective the polygraph must be useful for detecting past deeds and future intentions.
The polygraph was not designed to detect intentions and as the literature reveals is not even reliable for detecting lies associated with past deeds. If the effect is one of deterrence, then how long will it take for terrorists and domestic abusers to realise that the polygraph is ineffective? Machines like the polygraph seem like a great idea, but we need to be careful in placing our faith in them because this might make things worse.
