Several twin studies of various phenotypes have examined the EEA. One way to test the validity of the EEA is to examine whether a trait of interest is influenced by perceived versus assigned zygosity. The effect of perceived zygosity can be added as a “specified” familial environment in a univariate ACE twin model (Kendler et al., 1993) and if this parameter can be omitted without any significant loss in data fit, it can be assumed that the EEA holds for the phenotype under study. These studies generally report that the EEA assumption holds for numerous phenotypes such as physical activity, eating behavior, psychiatric disorders (e.g., major depression, generalized anxiety disorder, phobia, alcohol, and drug abuse; Eriksson et al., 2006; Hettema et al., 1995; Kendler et al., 1993; Klump et al., 2000; Xian et al., 2000), including child and adolescent psychopathology such as anxiety disorder, ADHD, oppositional defiant disorder, conduct disorder, antisocial behavior (Cronk et al., 2002; Jacobson et al., 2002; Tuvblad et al., 2011) as well as aggressive behavior (Derks et al., 2006).
