Reliability of psychiatric evidence in serious criminal matters: Fitness to stand trial and the defence of mental illness

Objective: The criminal justice system relies on the opinions of expert witness to assist in decisions about fitness to stand trial (FST) and verdicts of not guilty by reason of mental illness (NGMI). The aim of the present study was to assess the level of agreement between experts about these legal issues using a consecutive series of serious criminal matters in New South Wales. Methods: Pairs of reports from 110 consecutive criminal matters completed by the New South Wales Office of the Director of Public Prosecutions between 2005 and 2007 were examined. The opinions of experts about FST and NGMI were recorded. Results: Agreement about FST was fair-moderate (experts engaged by opposite sides, κ = 0.293; experts engaged by the same side, κ = 0.471), although there was a higher level of agreement in homicide matters. Agreement about NGMI was moderate-good (experts engaged by opposite sides, κ = 0.508; experts engaged by the same side, κ = 0.644) and there was a higher level of agreement when the experts also agreed about the diagnosis of schizophrenia. Further analysis using generalized estimating equations did not find a higher level of agreement about FST or NGMI in pairs of reports containing the opinion of experts from the same side. Conclusions: Little evidence was found for bias in expert opinions about either FST or NGMI, but the comparatively low level of agreement about FST suggests the need for reform in the way that FST is assessed.