Background Psychiatrists are often asked to answer legal questions. The extent to which they answer strictly legal rather than medical matters is not known.

Aim To investigate how strongly psychiatrists in England and Wales express opinions on one legal question – that of diminished responsibility in respect of a murder charge, and how this is related to outcome in court.

Method Our data were extracted from psychiatric reports and case files supplied by the then Department of Constitutional Affairs (now the Ministry of Justice) on cases heard in the Crown Courts between 1 January 1997 and 31 December 2001 in which the defence of diminished responsibility had been raised. The cases had been selected by the Law Commission in their earlier review of partial defences to murder. We devised a reliable system of rating the presence/absence and strength of expression of a legal opinion in the medical reports. We tested the data for relationship between nature and strength of opinion and progression to trial and verdict.

Results Psychiatric reports were available on 143 of 156 cases in which diminished responsibility was considered. They yielded 338 opinions on at least one aspect of diminished responsibility. In 110 (93%) of the 118 cases in which there was a diminished verdict, this was made without trial and, therefore, without reference to a jury. In only eight (27%) out of the 30 cases that went to trial, was a diminished responsibility verdict made. Half of the reports (169) gave a clear opinion on diminished responsibility, a third (121) invited the court to draw a particular conclusion and only 11% (36) provided relevant evidence without answering the legal questions. When there was an opinion or an invitation to make a finding on the legal question, a trial was less likely. A trial was also less likely if reports agreed on what the verdict should be.

Conclusions Psychiatrists frequently answer the legal question of diminished responsibility. The judiciary and medical experts should join in research to examine the consequences of different styles or approaches in presentation of essentially similar evidence in court. Copyright © 2008 John Wiley & Sons, Ltd.