The truth about lie detectors

Why is the UK government keen to introduce polygraphic tests for sex offenders, when they’re about as reliable as reading tea leaves?

Nathalie Rothschild

Topics Politics

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‘I have been following the use of polygraphic testing for post-release sex offenders in the US for more than four years. And I’ve yet to find any evidence that there is a scientific basis for such tests.’

Stephen Fienberg, Maurice Falk University Professor of Statistics and Social Science at Carnegie Mellon University, conducted an extensive study of lie detectors for the US National Academy of Sciences and found that they are a ‘blunt instrument’ indeed. So he’s surprised to hear that UK home secretary John Reid has proposed introducing lie detector tests for released sex offenders in Britain as a way of determining whether they’re likely to re-offend. Fienberg tells me he has no time for sex offenders, and does not want to coddle criminals, but ‘people have rights, and if they are to be subjected to a test that may accuse them of a [potential] crime, then we have the responsibility to make sure the test has scientific credibility’.

Commonly known as ‘lie detectors’, polygraphs monitor a person’s physiological reactions to certain lines of questioning, such as their increased heart rate or sweating. So contrary to the impression given by their popular name, polygraph tests do not detect lies; they do not tell truth from fiction. Rather they test reactions and produce graphs which an examiner then analyses to assess whether or not the subject has displayed deceptive behaviour. In America, where the use of polygraphic tests is more widespread, some sceptics have likened the technology to witchcraft or ‘reading tea leaves’ (1). So why is the home secretary proposing the introduction of such crystal-ball methodology here?

It’s reported this week that Reid wants to introduce compulsory lie detector tests to assess whether child sex offenders are at risk of re-offending after their release from jail. Under a new legal amendment to the National Offender Management Service Bill, probation officers would be permitted to subject these offenders to polygraphic tests. Any method that assesses the likelihood of someone committing a future offence must necessarily involve speculation and subjective interpretation. So can lie detectors, based on technology that measures only physiological reactions and involves data analysis, really tell if someone is likely to harass or harm children again? Or is this just an official version of fortune-telling?

A lie detector test involves a series of questions. It starts with control questions, usually requiring yes or no answers, and which are normally common knowledge – such as ‘Is January the first month of the year? or ‘Have you ever done something you’re ashamed of?’. This enables the tester to establish a baseline of physical reading. Then specific questions more directly related to the suspected crime, or future crime, are asked; for instance, ‘Have you ever behaved inappropriately towards children?’. The idea is that if the specific questions elicit stronger responses than the control questions, then the subject may be attempting deception. The polygraph produces charts which the examiner analyses before giving a decision as to the subject’s truthfulness.

Never mind the discomfort and intimidation of being hooked up to a machine with tubes and wires – and knowing that this machine and the examiner’s subjective interpretation of the result could determine your future – the subject’s physiological responses can be affected by a number of factors. Even needing to go to the toilet can impact on whether the polygraphic test points to deceitfulness.

Fienberg tells me that, while he is unaware what particular questions will be asked in polygraphic tests for released sex offenders in Britain, in the US ‘they are not asking things that are verifiable’: ‘The questions relate to “deviant fantasies”, so they presume that there’s a connection between this and “high-risk behaviour”. The questions are about things believed to be related to propensity for sex offence.’

Even if the technology could be shown to be foolproof, the data produced still has to be analysed by an examiner who has to make a judgement call. A nervous subject can produce false positives. Conceivably, if a subject trusts the methodology, they can have their own view of truth, their own beliefs about their actions, challenged by the results. Also, if the subject is indeed being deceitful, then who’s to say he or she is not also cunning enough to fool the lie detector?

‘You are lying, madam!’: Emily Hill on trash TV’s love affair with lie detectors

Lie detector tests may be a new addition to the great British paedophile hunt, but they have long been a staple of reality TV talk shows such as Jerry Springer, Trisha and The Jeremy Kyle Show, which run lie detector and DNA result ‘specials’ (for extra booing).

For those of you not familiar with the world of daytime TV, Springer, Trisha and Kyle select some of the most hopeless members of society (alcoholics without teeth, obese benefit fraudsters with seven children, drug addicts who beat up their babymommas) and get them to air all their soiled laundry for the benefit of a TV audience who scream hate at them for 30 minutes or so.

After all the shouting, wailing and begging, the lie detector or DNA test is wheeled out to settle matters. Apparently it provides the definitive answer, the ‘truth’ as revealed by the god that is therapy. Not surprisingly, mistakes are made. On one episode of The Jeremy Kyle Show, as catalogued by a blogger on Digital Spy, a man called Hassan was accused of sleeping with his girlfriend’s best friend. Hassan and his girlfriend’s friend both had lie detector tests. Hassan’s test claimed that he was telling the truth when he said he hadn’t slept with the friend. But the friend’s test claimed that she was lying when she said she hadn’t slept with Hassan.

‘They quickly bypassed it, didn’t they!’ notes the blogger. ‘The girl who they said was lying tried to get the point across, but she was taken off the stage rather quickly.’ Kyle harangued the friend regardless, calling her an evil woman, and no doubt airing one of his favourite catchphrases: ‘You are lying, madam!’ So lie detector tests aren’t used in court (yet) because they’re too unreliable, but they are used on TV shows where unreliable results are just overlooked. Will the authorities follow suit? As Jeremy might say, ‘Tune in after the break to find out…’

The late David T Lykken, who was a professor of psychology at the University of Minnesota, also argued that polygraph methods are not scientific. He said: ‘An innocent suspect has nearly a 50:50 chance of failing a CQT [control question test] administered under adversarial circumstances, and those odds are considerably worse than those involved in Russian roulette.’ Lykken compared polygraphic tests with earlier rituals aimed at finding the truth, such as Roman priests reading entrails or the dunking of witches in medieval England (2). Senator Ted Kennedy has described them as ‘twentieth-century witchcraft’. Fienberg’s study for the National Academy of Sciences concluded that polygraphy was ‘insufficient to justify reliance on its use in employee security screening in federal agencies’. For Fienberg: ‘Polygraph testing has been the gold standard, but it’s obviously fool’s gold.’

And yet in the US, polygraphic interrogation is a common procedure for those applying for employment within federal, state and local law enforcement agencies, fire departments, national intelligence agencies, the US Army, Navy, Air Force and Marine Corps and in the US Department of Energy (3). Fienberg says the most common use of lie detectors in the US is in security screenings for government agencies or private companies applying for contracts with government agencies. The second most common use is in relation to sex offenders, followed by criminal investigations in general.

For Fienberg, the implementation of lie detector tests for post-release sex offenders would be a scandal, not based on a single scientifically credible study. He warns that Reid has chosen ‘the worse conceivable area in which to go ahead with polygraphic testing’.

Apart from the scientific unreliability of lie detector tests, Reid’s desire for such a ‘preventative measure’ also suggests a lack of faith in the justice system and disrespect for the rule of law. It will mean that the monitoring of former sex offenders, and other ex-convicts if the lie detector tests catch on, will be premised on the assumption that a court of law or parole board may have misjudged a defendant and we should always expect that he or she might commit a crime again in the future. In this context, ex-convicts will always remain ‘potentially criminal’ instead of eventually becoming rehabilitated and re-integrated into society. Reid’s measure would take us further down the route of ‘pre-crime’, where the authorities will presume that they can judge the likelihood of a future crime occurring and act to prevent it – on the basis of very dodgy science.

There is one prediction of the future that we can make, however: that the home secretary will continue pursuing the idea that most of us are guilty until proven innocent.

Nathalie Rothschild is reviews editor at spiked.

(1) Lies, damned lies and polygraphs, Jeff Stein, Salon, 1997

(2) See quote from Lykken’s Tremor In The Blood: Uses and Abuses of the Lie Detector

(3) The lie behind the lie detector by George W Maschke and Gino J. Scalabrini

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Topics Politics


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