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How objective are forensic experts when they are retained by one of the opposing sides in an adversarial legal proceeding? Despite long-standing concerns from within the legal system, little is known about whether experts can provide opinions unbiased by the side that retained them. In this experiment, we paid 108 forensic psychologists and psychiatrists to review the same offender case files, but deceived some to believe that they were consulting for the defense and some to believe that they were consulting for the prosecution. Participants scored each offender on two commonly used, well-researched risk-assessment instruments. Those who believed they were working for the prosecution tended to assign higher risk scores to offenders, whereas those who believed they were working for the defense tended to assign lower risk scores to the same offenders; the effect sizes (d) ranged up to 0.85. The results provide strong evidence of an allegiance effect among some forensic experts in adversarial legal proceedings.

Many studies reveal strong interrater agreement for Hare's Psychopathy Checklist-Revised (PCL-R) when used by trained raters in research contexts. However, no systematic research has examined agreement between PCL-R scores from independent clinicians who are retained by opposing sides in adversarial legal proceedings. We reviewed all 43 sexual-offender civil-commitment trials in one state and identified 23 cases in which opposing evaluators reported PCL-R total scores for the same individual. Differences between scores from opposing evaluators were usually in a direction that supported the party who retained their services. These score differences were greater in size than would be expected based on the instrument's standard error of measurement or the rater agreement values reported in previous PCL-R research. The intraclass correlation for absolute agreement for the PCL-R Total score from a single rater (ICC 1,A = .39) was well below levels of agreement observed for the PCL-R in research contexts, and below published test-retest values for the PCL-R. Results raise concerns about the potential for a forensic evaluator's "partisan allegiance" to influence PCL-R scores in adversarial proceedings.

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