In the first series of twin analyses, we applied a standard univariate twin model where the variance in LTH-AD is accounted for by A, C, and E, to the index of caseness. In these models, the phenotype was modified as a function of the index of caseness. If an individual had an index of caseness of 0.30, he would be regarded as having LTH-AD if the cutoff (i.e., point at the continuous score where cases above the point is regarded as having AD) was below 0.20, and not having LTH-AD if the cutoff was set at 0.40. We ran 10 consecutive univariate twin models across 10 increasing levels on the index of caseness (equally sized groups with cut-points at: 0.13, 0.14, 0.15, 0.26, 0.27, 0.29, 0.40, 0.48, and 0.64).
