Evolutionary genetic modelling on all forms of balancing selection reveals that it only maintains polymorphisms at high frequencies (i.e. both alleles are common), because at low allele frequencies the balancing mechanisms become unstable and the rare allele is lost (Mani et al. 1990; Curtsinger et al. 1994; Turelli and Barton 2004; Kopp and Hermisson 2006; Penke et al. 2007). Thus, alleles responsible for personality trait variation should be at a higher frequency than expected under neutrality if they have been maintained by balancing selection (Johnson and Barton 2005). Most models in which balancing selection acts directly on a trait (e.g. negative frequency-dependent selection, sex-dependent selection, overdominance resulting from antagonistic pleiotropy) make the additional prediction that variation can only be maintained at a small number of genetic loci per trait (Curtsinger et al. 1994; Burger 2000; Barton and Keightley 2002; Turelli 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
