A robust theoretical framework could also help to gain a fuller understanding of the genetic basis of complex traits. In this vein, Penke, Denisson and Miller (2007) provided an evolutionary framework for relating the genetic architecture of personality traits to the selective pressures they have been under. Penke et al. argued that personality traits are most likely to have been under balancing selection by environmental heterogeneity (i.e. different selective pressures in different environments), often mediated by negative frequency-dependent selection (another form of balancing selection, where a phenotype is advantageous only when it is rare in the population). According to evolutionary genetic theory, traits under balancing selection should be influenced by a relatively limited number of common genetic variants with medium effect sizes (Barton & Keightley, 2002; Penke, et al., 2007; Roff, 1997). However, our findings falsify this prediction, since no individual common genetic variants account for more than half a percent of personality trait variation in our data. This suggests that personality variation is likely to be maintained by a mechanism other than balancing selection. One possibility is selective neutrality,
