and Barton 2004; Kopp and Hermisson 2006). However, despite statistical power to detect SNPs of even very small effect size (~0.5% of trait variance), large genome-wide association studies on personality have failed to find strong evidence of association with any SNPs (de Moor et al. 2010; Verweij et al. 2010), suggesting a highly polygenic basis to personality. Nevertheless, modelling suggests that some forms of balancing selection - namely, spatial and temporal environmental heterogeneity, and pleiotropic selection as a side effect of balancing selection on another trait - can maintain variation at a large number of genetic loci, although the requisite conditions are quite restrictive (Burger and Gimelfarb 2002; Turelli and Barton 2004). As such, it remains possible that either of these balancing selection mechanisms could have maintained polymorphisms at many genetic loci underlying personality variation; this would lead to the prediction that the genetic architecture of personality traits consists largely of genetic variants of high frequency. Nonadditive genetic variation in the trait of interest (as opposed to fitness itself) is not a requirement of these latter forms of balancing selection (Turelli and Barton 2004), and since additive genetic variation is maintained (rather than depleted) by balancing selection, a high proportion
