Although the twin study design has proven highly useful in partitioning contributors to variance of complex traits, it relies on some key assumptions that have been questioned. The “equal environments assumption” is that that the environment for MZ and DZ twins can be treated as identical. Some researchers have objected to this based on the observation that identical twins may elicit different social responses than nonidentical twins. Several studies have attempted to determine to what extent the equal environments assumption is valid, finding that, in general, the discrepancy between social environments does not seem to play a significant role, although its potential impact may need to be looked at individually for specific traits (Derks, Dolan, & Boomsma, 2006; Kendler & Gardner, 1998; Plomin, Willerman, & Loehlin, 1976). Chorionic differences can be thought of as another type of environmental factor, and there has been some evidence that chorionic type may influence variance within MZ twins for some phenotypes (Jacobs et al., 2001), although not all (Hur & Shin, 2008; Loos, Beunen, Fagard, Derom, & Vlietinck, 2001).
