Personality and major depression: a Swedish longitudinal, population-based twin study.

CONTEXT Prior studies suggest that the personality traits of neuroticism and extroversion may be related to the liability to major depression (MD). OBJECTIVE To clarify the magnitude and nature of the association between neuroticism and extroversion and the risk for MD. DESIGN Longitudinal population-based twin cohort. SETTING General community. PARTICIPANTS A total of 20 692 members of same-sex twin pairs from the population-based Swedish Twin Registry who completed a self-report questionnaire assessing neuroticism and extroversion in 1972 and 1973 and were personally interviewed for lifetime history of MD more than 25 years later.Main Outcome Measure Lifetime history of modified DSM-IV MD. RESULTS Levels of neuroticism strongly predicted the risks for both lifetime and new-onset MD. Twin modeling indicated that the association between neuroticism and MD resulted largely from shared genetic risk factors, with a genetic correlation of +0.46 to +0.47. Levels of extroversion were weakly and inversely related to the risks for lifetime and new-onset MD. This effect disappeared when we controlled for the level of neuroticism. Twin modeling produced similar results. CONCLUSIONS Results from both longitudinal and genetic analyses support the hypothesis that neuroticism strongly reflects the liability to MD. This association arises largely because neuroticism indexes the genetic risk for depressive illness. However, substantial proportions of the genetic vulnerability to MD are not reflected in neuroticism. By contrast, extroversion is only weakly related to risk for MD.