A small-scale twin study of FA yielded mixed results, with evidence for heritability obtained only under specific conditions (Allen et al., 1997; Coan et al., 2003). A subsequent twin study of FA has shown modest but significant heritability (27%) for asymmetry measured using F3 and F4 scalp sites but not more lateral (F7 and F8) sites, with no evidence for the contribution of shared familial environment (Anokhin et al., 2006a). Consistent with previous studies, heritability of alpha power at F3 and F4 locations was very high, 87% and 86%, respectively. Another twin study yielded slightly higher heritability estimates for FA in young adults (32% in males and 37% in females), however, genetic influences were non-significant in middle-aged adults. Smit et al. also found modest but significant correlations between FA and a “risk factor score” for anxiety and depression, a composite measure derived from several self-report assessments, with the relationship being mediated by genetic factors (Smit et al., 2007b). Finally, a recent study using a large sample of 9-10-year-old twins yielded low (11-28%) but significant heritability estimates for FA (Gao et
