Friday, July 20, 2012

Polygraphs and Sex Offenders

Just a short post on the news story that a pilot scheme which administers polygraph tests ('lie detector tests') to sex offenders on probation has been deemed successful, and the Ministry of Justice plans to roll it out nationwide.

The MoJ's research seems reasonable enough: the pilot scheme took place in the Midlands, and comparison groups of offenders were selected from other areas.  The assignment was not randomised, which is unfortunately all too common, but the comparison group was similar to the treatment group on the basis of the most obvious covariates (age, original offence, risk or reoffending, criminal history, etc.).  Based on a quick-ish reading of the paper it seems fairly solid.

The main finding was that the treated group were far more likely to make clinically significant disclosures (CSDs) than the comparison group; CSDs include relevant information about recent activity, sex life, and sexual fantasies.  The offenders tended to make these additional disclosures when taking the polygraph test, often just before the test.  This suggests that the perception of being under scrutiny for deception caused the offenders to disclose; in particular the study design can give no indication of whether the polygraph tests actually detected deception in the offenders.

My limited understanding of the research around polygraph testing (I couldn't find a good scientific review, but if anyone knows of one, let me know see below for a scientific review; here's an interesting historical review [£]) is that they are fairly effective at detecting deception, but that if the test taker has been trained to beat the test they become almost useless.  My main concern (scientific, rather than ethical) about using testing is that it can lull operators into a false sense of security.
Some offender managers stated that ... when polygraph results showed no deception, ... it reassured them of the offender’s honesty ...
This is particularly worrying as the offender mangers (and indeed their fellow criminals) were wont to describe some sex offenders as 'devious', which suggests that they will be particularly likely to try to manipulate the test.  Lastly I note that the study did not consider reoffending rates, which is presumably the outcome we would most like to see reduced.

UPDATE: Tweeter @hjnock pointed me to this review by the British Psychological Society which gives a great summary of the evidence for relating to polygraphs.  I think it justifies my fairly vague comment above!


  1. See also Stephen Fienberg's reports eg In Search of the Magic Lasso: The Truth About the Polygraph
    Stephen E. Fienberg and Paul C. Stern
    Source: Statist. Sci. Volume 20, Number 3 (2005), 249-260.

    1. Thanks - I will check this out!