We first fit a standard univariate genetic model to a 5-level AD symptom count measure (0, 1, 2, 3, or 4 or more symptoms) using Mx (38), to confirm the plausibility of the assumption that the liability distribution underlying symptom count has a bivariate normal joint distribution in twin pairs. We then extended this model to the multivariate case by adding (a) the 7-level alcohol consumption factor score (set to missing in individuals with 3 or more AD symptoms), (b) a variable indicating whether the respondent reported clustering of 3 or more AD symptoms in the same 12 month period (defined only in those reporting 3 or more AD symptoms), and (c) DSM-IV alcohol abuse (defined only in those reporting 0–2 AD symptoms), thereby estimating the genetic, shared environmental, and non-shared environmental contributions to, and correlations between, these variables.
