Background: Previous twin studies have supported a genetic contribution to the major categories of psychotic disorders, but few of these have employed operational diagnostic criteria, and no such study has been based on a sample that included the full range of functional psychotic disorders.

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.

Cluster analysis of phenomenological variables and the associated symptom profile of self‐mutilation in 74 female prisoners identified a subgroup with a disorder of mood who injured themselves as a symptom‐relieving mechanism and received a diagnosis of borderline personality disorder (BPD). Marked differences were found between the women in this cluster in the form of their behaviour and on a range of different variables compared with a second, more heterogeneous, cluster who had injured themselves as a reaction to life‐events, psychotic illness, or in a suicide attempt. The relevance of a symptom‐relieving mechanism to research into self‐mutilation and BPD is discussed. It is proposed that future research should concentrate on the ‘affective instability’ component of BPD and the affective symptoms reported by subjects with this diagnosis prior to impulsive behaviours such as self‐mutilation.

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