Based on six cohorts with twin data, the meta-analysis broad-sense heritability was 48 % for Neuroticism and 49 % for Extraversion (total N = 29,496 and 29,501 twin pairs, respectively). There was clear evidence of non-additive genetic variance for both traits. Although this finding could be partly due to a scale effect (the test information curves are slightly skewed, so therefore the distributions of sum scores and IRT score estimates are skewed as well, see (van den Berg et al. 2007; van den Berg and Service 2012), the relatively large size of the dominance genetic variance component suggests there is truly non-additive gene action. Sex differences in the kind of, and the relative size of, genetic factors on Neuroticism and Extraversion were suggested in only a subset of cohorts. The meta-analysis showed that qualitative sex effects were only significant for Extraversion. Proportions of additive and non-additive genetic variance were not significantly different across sexes.
