In mutation-selection balance explanations, there is an optimal adaptive ‘design’ that is the product of selective processes that maximise fitness. Accumulated random mutations are likely to have pleiotropic downstream effects that disrupt this design, deteriorating fitness in various ways (Keller & Miller, 2006; Zhang & Hill, 2005). The deleterious effects of mutation load will be especially apparent in mental functioning, since the brain has such a large mutational target size (over half of the genome is probably expressed in the brain; Sandberg, et al., 2000). This lends itself well to explaining psychiatric disorders, where normal mental functioning is thought to be disrupted by mutation load to the point of drastic dysfunction (Keller & Miller, 2006). It is less clear how mutation load might manifest in traditional personality traits, since they have not generally been considered in the context of good scores (high fitness) or bad scores (low fitness) (Almagor, Tellegen, & Waller, 1995; Zietsch, 2009).
