We know of no previous quantitative summary of the heritability of bipolar disorder employing twin studies that meet modern methological standards. Table 2 lists the three studies that met our inclusion criteria (Kendler et al., 1993, Kendler et al., 1995, Kieseppa et al., 2004, McGuffin et al., 2003). Note that the affected subjects in the Maudsley study by Cardno et al (Cardno et al., 1999) were included in the later study by McGuffin et al. (McGuffin et al., 2003). The weighted mean heritability estimate was 85%.
