The implementation of population-based twin studies, inclusion of measured environments into twin studies, and advances in biometrical modeling techniques for twin data made it possible to study gene-environment interaction within the framework of the classic twin study. Traditional twin studies involve comparisons of monozygotic (MZ) and dizy-gotic (DZ) twins reared together. MZ twins share all of their genetic variation, whereas DZ twins share on average 50% of their genetic make-up; however, both types of twins are age-matched siblings sharing their family environments. This allows heritability, or the proportion of variance attributed to additive genetic effects, to be estimated by (a) doubling the difference between the correlation found between MZ twins and the correlation found between DZ twins, for quantitative traits, or ( b ) comparing concordance rates between MZs and DZs, for qualitative disorders (McGue & Bouchard 1998). Biometrical model-fitting made it possible for researchers to address increasingly sophisticated research questions by allowing one to statistically specify predictions made by various hypotheses and to compare models testing competing hypotheses. By modeling data from subjects who vary on exposure to a specified environment, one could test whether there is differential expression of genetic influences in different environments.
