The phenotypic correlations among the variables are presented in Table 2. Lack of independence due to the fact that the sample consisted of twin pairs was accounted for in assessing the significance of correlations. For males, all the correlations were significant. For females, all but the P3-EXT correlation were significant. As might be expected from an epidemiological sample, the correlations between the endophenotypes and EXT were modest. Therefore, to further highlight their importance and to link our findings to the literature which often examines clinical samples or offspring of clinical samples compared to healthy controls, we conducted a quintile split on each of the endophenotypes, separately for males and females, and compared EXT scores between those high (top 20%) and low (bottom 20%) on each of the endophenotypes. Differences between groups corresponded to effect sizes (Cohen’s d; the mean difference between EXT scores) in the small to moderate range (P3: males d=.31, females d=.20; TF-PC1: males d=.34, females d=.29; Beta: males d=.37, females d=.18). These effect sizes, considered together with the modest phenotypic correlations, suggest that the correlations may underestimate the endophenotypes’ abilities to differentiate between those high and low in EXT symptoms.
