Neuroticism is substantially heritable, with estimates of its heritability varying somewhat by sex and age. The heritability of neuroticism peaks in early adolescence and early adulthood, with no sex differences in heritability at those ages (Gillespie, Evans, Wright, & Martin, 2004; Lake, Eaves, Maes, Heath, & Martin, 2000; Rettew et al., 2006; Viken, Rose, Kaprio, & Koskenvuo, 1994). In this age range, 50–60% of the variance in neuroticism scores is estimated to be attributable to genetic factors in both sexes. After early adulthood, the heritability of neuroticism declines gradually with age in both sexes, but slightly more so in males, resulting in somewhat greater heritability in females during later adulthood (Eaves, Eysenck, & Martin, 1989; Lake et al., 2000). Thus, it is possible that there are sex differences in some of the genes that influence neuroticism, or sex differences in their expression, at older ages that result in these small sex differences in heritability in later adulthood (Macaskill, Hopper, White, & Hill, 1994; Rettew et al., 2006).
