In the current study, a range of twin models were utilized to estimate the heritability of LTH-AD in men (Neale and Maes, 2004). In all models, individual differences in the common liability underlying the reporting of AD are assumed to have 3 different etiological sources: additive genetic (A) comprising all genetic effects that contribute additively, shared environment (C) comprising all environmental exposures that contribute to similarity, and individual-specific or nonshared environmental factors (E) comprising all environmental factors contributing to differences between the twins, including measurement error. As MZ twins share all their genes, they are assumed to share all of their A factors. As DZ twins on average share half of their genes identical by descent, the expectation is that additive genetic factors will contribute about half as much to phenotypic resemblance compared with MZ twins. Both MZ and DZ twins are assumed to share none of their E factors and all of their C factors. The Mx statistical program, which uses a raw data maximum likelihood estimation approach, was used to fit the models to the data (Neale et
