A twin study of psychosis and criminality

Lifetime criminal and psychiatric histories were examined in a consecutive series of 280 individuals of twin birth with a diagnosis of major functional psychosis who were seen and followed up at the Maudsley Hospital between 1948 and 1988. Their 210 co-twins, 35% of whom had a similar diagnosis, were ascertained and followed up over the same period. In the absence of reliable general-population estimates for lifetime conviction rates, co-twins were used as case controls. Among the 220 complete pairs, significantly more probands (25.7%) than co-twins (14.0%) were convicted, although there was no evidence for an independent genetic basis for criminal behaviour. Criminal conviction was significantly related to psychiatric diagnosis. There were specific patterns of offending, particularly among the schizophrenic men, who were also significantly more often convicted (48.6%) than the men with affective psychosis (19.4%), and more likely to receive a prison sentence. The schizophrenic patients were younger at their first conviction (mean age 22.6 years v. 30.8 years) and they had committed more violent offences than the affective group. In both diagnostic groups, ages at first psychiatric contact and first conviction were highly correlated.