Though Hettema et al. meta-analyzed two published twin studies of generalized anxiety disorder (Hettema et al., 2001a), the authors excluded one eligible study because of its complex ascertainment scheme (Roy et al., 1995), and an additional eligible study was published since the meta-analysis (Mackintosh et al., 2006). Table 3 lists the four studies that met our inclusion criteria (Hettema et al., 2001b, Mackintosh et al., 2006, Roy et al., 1995, Scherrer et al., 2000); one study only included males (Scherrer et al., 2000). The weighted mean heritability estimate was 28%.
