The current study has several limitations. First, comorbidity between the disorders was not addressed. However, results from this study were based on the odds ratio, which is calculated from presence or absence of the disorder and not severity of the disorder. Therefore, comorbidity would not have skewed these results or affected conclusions. Second there was no method to directly compare rates of disorders between PAE and COA. The permutation tests presented are comparisons of the LOR estimated within each domain, which employed a different control group. Third, this meta-analysis sought to compare rates of the three disorders in PAE and COA literature, and the best effect size measure to compare two binary traits was an odds ratio (Lipsey & Wilson, 2001). Thus, it was impossible to include obvious environmental factors such as education, socio-economic status, parental relationship, stressful life events, and peer relationships which are known to mediate genetic influence in externalizing disorders (Hicks, South, Dirago, Iacono, & McGue, 2009). Parental diagnosis of the externalizing disorder was another factor for which it was impossible to covary. Thus, the inability to
