Genes play an important role in risk for BD. The rate of concordance for monozygotic twins is 40%, compared with a 5% rate in dizygotic twins (Kendler et al., 1995, Kieseppä et al., 2004 and McGuffin et al., 2003), and risk among the first-degree relatives of individuals with BD is ten-fold greater than risk among the general population (Barnett and Smoller, 2009). However, as with other psychiatric disorders, the genetics of BD is complex, probably due to a high degree of genetic heterogeneity and considerable phenotypic heterogeneity of clinical populations (Potash et al., 2007).
