The proportion of total trait variation that is accounted for by genetic variation is called broad-sense heritability. This consists of the additive component of heritability (due to the accumulation of the average allelic effects) and may also include nonadditive genetic variation (due to interaction of alleles within (dominance) or between (epistasis) loci). Although it is statistically difficult to distinguish nonadditive from additive genetic variation, there is evidence in humans suggesting that both contribute to personality variation (Eaves et al. 1998; Lake et al. 2000; Keller et al. 2005).
