Nature, nurture and depression: a twin study.

We studied a series of twins systematically ascertained through 214 probands (84 monozygotic, 130 dizygotic) who had had one or more episodes of hospital-treated major depression. A variety of definitions of depression were applied to the co-twins all of which resulted in (a) markedly higher rates of disorder than are found in the general population, (b) significantly higher monozygotic than dizygotic concordance. The results of applying a simple additive model in which depression is considered as a threshold trait suggested that both genetic factors and shared family environment make substantial and significant contributions to the familiality of depression.