To what extent should expert psychiatric witnesses comment on criminal culpability?

Following the Court of Appeal case of R v Edwards in England and Wales, there has been increasing pressure for expert psychiatric witnesses to comment explicitly on how a defendant’s mental disorder affects their culpability. Culpability is the degree to which a person can be held morally or legally responsible for their conduct, but defining culpability has proved difficult. Mental disorder does not translate easily into degrees of legal culpability. Although psychiatric evidence will often be central to such cases, the determination of culpability is a matter for the court, and experts should not comment on it explicitly. Nevertheless, certain areas of psychiatry may have a bearing on culpability, and ways in which experts may comment on these are suggested. Given the pressure on judges to determine culpability, experts need to be honest about the limits of medical science to answer legal questions and the professional necessity to remain within their area of expertise.