And on the next page of the same paper: "A second type of gene nonadditivity often mentioned in critiques of twins research, is a gene-environment interaction (Wilson, 1998). According to critics, behavior geneticists relying on twins data ignore possible gene-by-environment interactions in the development of personality. However, studies of MZ and DZ twins, some of whom were raised apart, indicate no such effects (Tellegen et al., 1988). Furthermore, if true, the detection of gene-by-environment interactions would not reduce genetic impact on personality already documented. The variance explained by such interactions would only reduce the residual term, not the genetic effect. In light of the relatively small main effects due to environment in twins research, these interactions would merely add to the role of genetics by documenting that environmental effects are contingent upon genetic predispositions. The independent effect of genetics on personality has already been shown" (p. 17, emphases mine).
The use of twin data to test for the violation of definition B of the additivity assumption is also explained as follows by Hill et al. (2008): "...rMZ>rDZ implies that part of the resemblance is due to genetic factors and rMZ>2rDZ implies the importance of non-additive genetic effects. Conversely, rMZ<2rDZ implies that common environmental factors cause some of the observed twin resemblance." These authors also summarized results from a set of twin studies: "On average the MZ correlation is about twice the DZ correlation across a wide range of phenotypes...the simplest explanation of the results is that additive variance explains most of the observed similarity of twins and non-additive variance is generally of small magnitude and cannot explain a large proportion of the genetic variance."
More recently, Polderman et al. (2015) have outlined two other ways to (supposedly) use twin data to test for the relative importance of additive and non-additive genetic variance: "...there are two simple and parsimonious hypotheses that can be tested across traits from estimated correlation coefficients for monozygotic twin pairs (rMZ) and dizygotic twin pairs (rDZ).
- The first is that the correlations for the monozygotic and dizygotic twin populations (ρMZ and ρDZ) are the same, consistent with twin resemblance being solely due to non-genetic factors.
- The second hypothesis involves a twofold ratio of ρMZ to ρDZ, consistent with twin resemblance being solely due to additive genetic variation."\cite{Polderman_2015}
There is a clear pattern here: if rMZ = 2rDZ (or very close to that), then this is interpreted as evidence that most genetic variance--and indeed most variance overall--is additive in nature. And indeed as noted above this was found by Hill et al., and more recently Polderman et al. reached the same conclusion, at least most of the time: "...across all traits 69% of studies showed a pattern of monozygotic and dizygotic twin correlations consistent with an rMZ that was exactly twice the rDZ".
The underlying reasoning seems to be based on the fact that the genetic similarity is twice as high between two MZ twins (100%) compared to between two DZ twins (50%). Hill et al. note that this method is dependent on the validity of the equal environment assumption. Unsurprisingly, they dismiss concerns that this assumption may be false, writing, "Attempts to test this hypothesis have not found any evidence to reject it."