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 numerousness, recessiveness, and which trait(s) they affect. Traits that are affected by a large number of loci and that therefore have a large “mutational target size” will tend to be disrupted to a larger extent by mutations (Houle 1998). Given that over half the genome is expressed in the brain (Sandberg et al. 2000), it is possible that personality traits have a large mutational target size and that much of their genetic variation is mutational.
