The equal environments assumption (EEA) refers to the assumption that MZ and DZ twins are equally correlated in their exposure to trait-relevant environments. A violation of this assumption could result in inflated estimates of heritability and deflated estimates of the effect of shared environment. Furthermore, a violation of this assumption among one sex but not the other could account for sex differences (or the lack thereof) in the magnitudes of A, C, and E. For example, a violation of this assumption among females could artificially inflate heritability estimates for females. This could result in either a finding of higher heritability among females than males, or it could increase the heritability of females to be equal to that of males.
