Additionally, we ran sensitivity analyses to control for population stratification, which refers to differences in allele frequencies across ancestral or ethnic populations that may create spurious associations between genes and behavior (Cardon & Palmer, 2003). Population stratification was assessed and controlled for in the current study via an admixture mapping method using ancestry informative markers (AIMs) described in Cleveland et al. (2015). Using information from 318,000 SNPs, a single principal coordinate (PC1) that explained 73.4% of the common variance in genetic ancestry was derived; further PCs provided little additional explanatory power. PC1 provides a continuous scale of European ancestry based on genetics. We used PC1 to control for population stratification in two ways: (1) by including the PC1 scale as a continuous covariate in sensitivity analyses and (2) by re-estimating all models keeping only those whose PC1 scores indicated European ancestry (see Cleveland et al., 2015 for details).
