Background. Somatic and psychiatric morbidity may cluster because of reciprocal effects between them but also as a result of common underlying factors.

Methods. The data come from the 1946 MRC birth cohort (N = 5362). Clustering of 20 chronic medical conditions at the participants' 43rd year was analysed using multi-morbidity coefficients. Associations of somatic and psychiatric morbidity at 43 years, with temperament and behaviour, assessed from adolescence to early adulthood, and background variables like paternal social class, were examined using structural equation modelling. The focus was on whether links applied to both morbidity types or one only, and were direct or were indirect, mediated by prior morbidity.

Results. Reported chronic medical conditions clustered strongly. Somatic multi-morbidity and psychiatric ill-health at 43 years were associated with temperamental and behavioural features assessed between the subjects' 13th and 26th birthday, like neuroticism and aggression, as well as with external variables like parental death before the participants' 16th birthday. However, only neuroticism holds direct links with somatic as well as with psychiatric ill-health, 28% of the former and 52% of the latter association being independent of the simultaneous presence of the other morbidity type and of participants' health status 7 years earlier.

Conclusions. Personality traits like neuroticism not only raise the risk of psychiatric disorder but also, irrespective of whether manifest psychiatric disorders have developed, of a broad spectrum of chronic somatic diseases. This suggests that clinicians and researchers should focus not only on the psychiatric disorders associated with such personality traits but also on their medical consequences.