The assumptions and limitations of the method should be kept in mind. One assumption that is often questioned is whether the environment in which DZ twins are reared is really the same as their identical MZ counterparts. While work done thus far has not shown differences in MZ and DZ familial environment to be so significant a factor as to invalidate twin studies (Kendler et al. 1994), the potential must be kept in mind for each particular trait. A second limitation is that a given set of variability estimates can only be considered valid for a specific population and environment. One of the most challenging limitations, however, is the limited power of this model to describe gene-environment interactions. One aspect of this is the limited power to detect GxE and rGE interactions as contributors to variance. The other is a more fundamental challenge to the methodology: whether acting as if genetic and environmental factors can actually be separated and added together as independent factors is inherently problematic, given that genes are linked to phenotypes only through their interaction with environmental factors (Lewontin 1974; Meaney 2010; Tabery 2009; Vreeke 2000).
