There is also now a robust literature implicating genetic factors in the etiology of depression and other psychiatric disorders. Depression is known to run in families; people with major depressive disorder are three times more likely than those without the disorder to have a first degree relative who also has depression.23 Twin studies, which allow for simultaneous quantification of genetic and environmental influences, suggest that depression is moderately heritable. Specifically, twin studies have estimated that approximately 40% of the variation in the population risk of depression is attributable to genetic variation.24
