A longitudinal study of personality and major depression in a population-based sample of male twins

Background. The relationship between personality and psychiatric illness is complex. It is not clear whether one directly causes the other. Method. In a population-based sample of male twins (n=3030), we attempted to predict major depression (MD) from neuroticism (N) and extraversion (E) and vice versa, to evaluate the causal, scar, state, and prodromal hypotheses. In a longitudinal, structural equation twin model, we decomposed the covariation between N and MD into (a) genetic and environmental factors that are common to both traits, as well as specific to each one and (b) direct causal effects of N at time I on subsequent MD, as well as between MD and subsequent N. Results. E was negatively correlated with lifetime and one-year prevalence of MD. N predicted the new onset of MD, and was predicted by both current and past MD. It did not predict the time to onset of MD. All of the covariation between N and MD was due to additive genetic and individual-specific environmental factors shared by both traits and a direct causal path between MD and N assessed later. No genetic factors were unique to either trait. Conclusions. In men, N may be a vulnerability factor for MD but does not cause it directly. However, MD may have a direct causal effect on N. The genetic overlap between N and MD in men may be greater than in women.