Genes apparently have a larger role in women than in men in the risk of developing major depression, based on the results of a new twin study appearing in the current issue of the American Journal of Psychiatry.

“Previous studies gave us hints of two different kinds of genetic differences between men and women: quantitative - whether the overall role of genes and environment differs - and qualitative - whether the actual liability genes are the same across the sexes,” Dr. Kenneth S. Kendler, of Virginia Commonwealth University’s School of Medicine, told Reuters Health.

In an attempt to replicate their earlier findings in a larger study, Kendler and his colleagues assessed data on lifetime major depression in 42,161 twins, including 15,493 complete pairs, from the national Swedish Twin Registry.

The investigators found that “the heritability of liability to major depression was significantly higher in women (42 percent) than in men (29 percent),” and that the genes that impact depression are correlated, but not identical, between the sexes.

“On average, the effect of genes appears to be substantially more important in women than in men,” Kendler said.

He and his colleagues suggest that there might be genes that alter the risk of depression in women in response to their variable hormonal environment.

“We have pretty good evidence that there is a set of women that are prone to depression, particularly in the postpartum period and during the premenstrual phase of the menstrual cycle,” Kendler pointed out. This type of depression runs in families, he said, but men don’t seem to be affected, probably because they don’t have the hormonal fluctuations that women have.

Kendler expects the new results to have an impact on research looking for genetic vulnerability to depression. “It should emphasize the necessity of studying things separately in men and women.”

American Journal of Psychiatry, January 2006.