broad heritability, non-additive, when they play a role) genetic factors. Several assumptions and potential limitations underlying the analysis of twin data that need to be considered25, including: a) whether the equal environment assumption (which posits that the correlation for C is 1.0 in both MZ and DZ twin pairs) is justified. Despite concerns, general support for this assumption has been reported across a range of psychiatric phenotypes.26 b) The assumption of random mating, the violation of which can inflate estimates of shared environment; (c) If some of the genetic variation in outcome is due to non-additive genetic effects (dominance or epistasis), the importance of shared family environmental influences may be underestimated. d) Genetic effects and gene by shared environment effects are confounded. Nonetheless, it is possible to explicitly model this gene-environment interplay by obtaining estimates of heritability that are conditioned on, or vary as a continuous function of, environmental exposure.
