The contribution of genetics to risk of depression is significantly higher in women than in men, and there may be some genetic factors that operate uniquely in one sex and not in the other, according to an article in the January issue of the American Journal of Psychiatry.

The international team of researchers reported that heritability of depression is higher in women - approximately 42 percent -- than in men, where it is approximately 29 percent.

“Our work, together with colleagues at the Karolinska Institute in Sweden, represents the largest epidemiological study of depression in twins done to date. In addition, it broadly replicates what has been shown by our earlier work using the Virginia Twin Registry. In particular, we have shown that depression is a moderately heritable disorder, suggesting that genetic factors are important, but by no means overwhelming,” said Kenneth S. Kendler, MD, lead author on the study.
The research team employed twin study models to evaluate lifetime major depression of approximately 42,000 twins, including 15,000 complete pairs from the Swedish National Twin Registry.

According to Kendler, the sex-effects are of two kinds - quantitative and qualitative. He said that quantitative sex-effects examine whether heritability is different in males compared with females, and if the overall importance of genetic factors differs between the sexes. In contrast, qualitative sex-effects examine whether the same genes are playing a role in males and females.

For example, Kendler said there may be genes that alter the risk for depression in a woman’s response to cyclic sex hormones, particularly in the postpartum period. Such genes would impact a woman’s risk for major depression, but would not be active in men because men lack the relevant hormonal milieu.