Personality and major depression: a Swedish longitudinal, population-based twin study
Kendler KS, Gatz M, Gardner CO, Pedersen NL;
Commented by , 26 Oct 2006
Background
Prior studies suggest that the personality traits of neuroticism is associated with increased risk of developing depression. However, it is still unclear whether neuroticism is part of prodromal symptoms of depression or whether, alternatively, neuroticism is an independent long-term predictor of depression.
Method
The study included a total of 20,692 members of same-sex twin pairs from the population-based Swedish Twin Registry. The twins completed in 1972 and 1973 18 items of the Eysenck Personality Inventory, which is a self-report questionnaire assessing neuroticism and extroversion. More than 25 years later the twins were personally telephone interviewed for lifetime history of Major Depression using the Composite International Diagnostic Interview Short Form (CIDI).
Results
Levels of neuroticism strongly predicted the risk for both lifetime (OR=1.49, 95%CI: 1.44-1.55) and new-onset MD (RR=1.31, 95%CI: 1.26-1.36). The correlations for neuroticism were substantially higher in the monozygotic twins than in the dizygotic twins for women (0.56 versus 0.27) and men (0.48 versus 0.22).
Twin modeling indicated that the association between neuroticism and Major Depression resulted largely from shared genetic risk factors, with a genetic correlation of +0.46 to +0.47. The corresponding environmental correlations were +0.05 for women and +0.10 for men.
Professor Kessing's comments
This is an elegant study investigating the association between neuroticism and depression from an epidemiologic, longitudinal, and genetic perspective. As pointed out by the authors, the study has 3 methodological strengths:
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it is large and representative
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the personality measures were obtained at least 25 years before the assessment of lifetime MD
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the sample is genetically informative.
This is the first study that clearly shows that neuroticism is an independent long-term predictor of incident depression. Further, the study clearly indicates that the association between neouroticism and depression is genetically based. As noted by the authors, the environmental correlations is strikingly low in the present study compared to the findings in the Virginia twin study.
The discrepancy probably arise from the difference in the timing of the assessments of neuroticism and MD: contemporaneous in the Virginia sample and separated by 25 years or more in the Swedish sample.
It should, however, be noted that the prevalence of neuroticism increased the risk of developing depression with approximately 30%, only. Clearly, important aspects of the liability to major depression is poorly reflected by neuroticism. It is possible that neuroticism is associated with what has been called community depression and not with melancholic or somatic depression.
There is a need for further elucidating the association between neuroticism and melancholic depression in the future.