A second reason for the lower heritability estimated from SNP data compared with pedigree data is that the latter may be biased upwards through confounding with nonadditive effects while, in contrast, the heritability estimates from SNP data are ‘narrow sense' estimates, which reflect purely additive genetic effects. Nonadditive effects have been inconsistently reported for both neuroticism and extraversion with evidence provided from some studies,11, 12, 13, 48, 59, 60, 61 but not others.48, 62 However, nonadditive effects are difficult to distinguish from common-environmental effects in the classical twin design.48, 62, 63 Consequently, common-environmental effects may well be masked by nonadditive effects in twin and family studies resulting in inflated estimates of additive genetic effects and deflated estimates of common-environmental effects. In a sample including both twin pairs reared apart and twin pairs reared together, Pedersen et al.,18 showed that common-environmental factors had a modest contribution on the variance of both neuroticism and extraversion while nonadditive genetic effects were absent in neuroticism but significant in extraversion.
