There are numerous examples of twin studies that examined sex differences in the heritability of mental disorders including major depression, alcoholism, schizophrenia and ASD (eg, Taylor et al. 100 ). Sex differences have been of particular interest for depression. In contrast to the lack of sex differences observed in familial transmission of depression, there is moderate evidence for greater heritability of major depression in women than in men. For example, in a sample of nearly 40 000 twins, Kendler and colleagues 101 found a moderate correlation in genetic risk factors for depression in male and female twins, but that heritability was greater in women than in men (42% vs 29%), replicating his earlier evidence for a slightly greater estimate of heritability for depression in female than male twins. 101 , 102 Developmental twin studies have shown that the heritability of depression and anxiety emerges in early adolescence, with prepubertal depression more likely to reflect environmental influences. 103
