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Bias in expert testimony
When forensic psychologists or psychiatrists provide expert testimony in court they’re supposed to be objective, but a new study shows that their judgement tends to be biased towards the side that recruited them – the prosecution or defence (Psychological Science).
Daniel Murrie at the University of Virginia and his colleagues recruited just over a hundred forensic experts (most were psychologists) to take part in what they were told was a contract to review sex-offender cases for either a public defender service or a specialised prosecution service.
Following the cover story, the participants completed a two-day training workshop on use of two established risk instruments – Hare’s Psychopathy Checklist (PCL-R) and the Static-99R that’s used to predict sexual recidivism. Three weeks later the participants met with either a defence or prosecution attorney (played by the same actor) and used these tools to review objectively four real-life sex-offender case files.
Normally, inter-rater agreement for the PCL-R and Static-99R is very high, but in this context the participants’ ratings were significantly different depending on whether they thought they were acting for the prosecution or the defence. The finding raises concerns, Murrie and his team said, because it shows that ‘some experts who score ostensibly objective assessment instruments assign scores that are biased toward the side that retained them’.
In fact, the researchers believe such biasing effects are likely to be larger in real life. The study involved just 15 minutes spent with the retaining lawyer, yet in real cases this contact could last weeks or months. Moreover, the current research required that the participants review the exact same files and interview transcripts whereas in real-life cases there could be the potential for seeking different sources or interviewing offenders in different ways. The researchers also noted the scope for bias could be greater for more subjective judgements, such as when assessing emotional harm.
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