Equivocal Rulings on Expert Psychological and Psychiatric Evidence: Turning a Muddle into a Nonsense

Although expert opinion, especially on medical and scientific matters, has been admitted as evidence in court increasingly often since the nineteenth century, psychological or psychiatric evidence has always been treated as a special case and has not been welcomed so readily by the courts. For more than 20 years, the extremely influential Court of Appeal decision in R. v. Turner' had the effect of excluding psychological and psychiatric evidence from numerous cases. The effect of the Turner rule has been to exclude evidence that does not deal with mental disorders, mental handicap, or automatism on the assumption that other psychological functions and phenomena are matters of "common knowledge and experience" that a jury can understand without the help of experts, although the underlying assumption about the transparency of non-clinical psychological processes is certainly open to challenge. The Turner rule has been used to exclude psychological and psychiatric evidence on issues of, inter alia, duress, provocation, and mens rea, but the courts have generally adopted a more indulgent attitude towards evidence regarding diminished responsibility,3 except in cases involving the voluntary consumption of alcohol or other drugs. The purpose of this article is to summarise and comment on recent developments in this area. The decision in R. v. Sally Lorraine Emery' appeared at first to have