Why Is It Hard to Find Genes that are Associated with Social Science Traits ? Theoretical and Empirical Considerations

Though most behavioral traits are moderately to highly heritable, the genes that influence them are elusive: many published genetic associations fail to replicate. With physical traits like eye color and skin pigmentation, in contrast, several genes with large effects have been discovered and replicated. We draw on R.A. Fisher’s geometric model of adaptation to explain why traits of interest to behavioral scientists may have a genetic architecture featuring hundreds or thousands of alleles with tiny individual effects, rather than a few with large effects, and why such an architecture makes it difficult to find robust associations between traits and genes. In the absence of strong directional selection on a trait, alleles with large effect sizes will probably remain rare, and such a lack of strong directional selection is likely to characterize most traits currently of interest in social science. We evaluate these predictions via a genome-wide association study (GWAS) that carefully measured over 100 physical and behavioral traits with a sample size typical of candidate gene studies. While we replicated several known genetic associations with physical traits, we found only two associations with behavioral traits that met the nominal genome-wide significance threshold. We use the theory and findings to discuss (1) the challenges for social science genomics, particularly the likelihood that genes are connected to behavioral variation by lengthy, nonlinear, interactive causal chains; (2) the prospects for dealing with these challenges; and (3) the inherent tradeoff between two ways of meeting these challenges: increasing sample size and improving phenotype measurement. “Why Is It Hard to Find Genes ...” – p. 3