Because there are no set criteria for establishing a threshold to create maximally informative scores (Evans, Visscher, & Wray, 2009), we conducted preliminary analyses using a series of p-value thresholds ranging from 0.05 to 0.50 to evaluate what threshold maximized the variance accounted for (R2) in the externalizing disorder composite in the adolescent subsample. We maximized R2 in this subsample because it was the one in which we subsequently evaluated gene-by-environment effects, and previous work suggests that SNPs with a nominal main effect may be enriched for gene-by-environment interaction (Thomas, 2010). There was little variability in the percent variance accounted for (range 5.5% to 5.9%); p < 0.30 (176,562 SNPs) maximized R2 and was used to create polygenic scores for adolescents and young adults.
