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, where personality differences make virtually no difference at all to fitness in any environments. Penke et al. argue that this is implausible, given the pervasive importance of personality differences in social and romantic relationships among other things, but it ultimately depends on the correlation between the net effect of a specific genetic variant (across potentially multiple pleiotropic functions) and total fitness. The other possible mechanism for maintaining genetic variation is mutation-selection balance. In mutation-selection balance, the appearance of new mutations is balanced by purifying selection, which eliminates deleterious mutations. The time lag of purifying selection means all individuals carry a load of mildly deleterious mutations that have yet to be eliminated by selection2. Trait variation corresponds to variation in individuals’ mutation load. Traits under mutation-selection balance are expected to be influenced by very many rare genetic variants of small effect (Keller & Miller, 2006; Penke, et al., 2007; Zhang & Hill, 2005). This is not inconsistent with the present results,
