A second possibility for explaining the maintenance of genetic variation in personality traits is mutation-selection balance (Lande 1975; Zhang and Hill 2005; Keller and Miller 2006). In this view, deviations from an optimal personality trait level (averaged across environments) are selected against, eliminating alleles that do not predispose to this optimum, and thus reducing genetic variation. In the meantime though, new mutations affecting the trait arise in the population. The vast majority of mutations that affect fitness are deleterious (Eyre-Walker and Keightley 2007), since they randomly disrupt finely-tuned systems. Mutations with strong and dominant effects are purged quickly by selection; mutations with recessive and/or weak effects, which are less visible to selection, may persist for many generations before being eliminated, but are unlikely to become common in the population because of the selection against them (Eyre-Walker 2010). As a result of this and the constant influx of new mutations, individuals each carry an accumulated `mutation load' consisting of alleles that tend to be rare, (partially) recessive, and mildly deleterious. Individuals' mutation loads can vary in many ways, such as their
