Twin studies examine the etiology of a trait by comparing MZ twins who share 100% of their alleles identical by descent and DZ twins who share, on average, 50% of their alleles identical by descent. Using this information, correlations between MZ and DZ twins on a measured trait are compared to estimate (1) heritability/genetic influences on the trait (a2 or A), i.e., effects due to cumulative effects of genetic variants that are shared by family members to differing extents, (2) shared environmental influences (c2 or C), i.e., effects due to environments that family members share in common, and (3) non-shared environmental/residual error influences (e2 or E), i.e., effects due to environments uniquely experienced by family members. The classical twin study is limited in its ability to estimate non-additive genetic (d2 or D) and shared environmental effects simultaneously because both parameters rely on the difference between MZ and DZ twin correlations, making the model unidentified. Given the absence of sibling or adoption data, separate models including shared environmental and non-additive effects were compared using the Akaike Information Criterion (AIC); however, given
