In order to determine the extent of genetic and environmental influences on risk of INATT, HYP/IMP, CDP, and AlcProb, genetic structural equation models were fitted to the twin data using the Mx statistical modeling package (Neale et al., 1992). In genetic twin analyses, models are tested that partition variance in a variable of interest into genetic [additive (A) and non-additive (D)] and environmental [shared (C) and nonshared (E)] components. Additive genetic influences (A) describe the effect of multiple genes that exert influence in a linear or additive fashion. In general, non-additive genetic effects describe interactive effects of different alleles and include genetic dominance (within locus interaction) and epistasis (across locus interaction). However, in most twin studies, non-additive effects are modeled as genetic dominance (Rettew et al., 2008). Shared or common environmental effects (C) are those influences that make members of a family more similar to one another. Nonshared or unique environmental effects (E) make members of twin pairs different. Important to note, E also includes measurement error. Considering proportions of variance, we denote the following: a2 for the proportion of
