Our multivariate twin model was designed to examine the degree to which genetic influences on AD are shared with the genetic influences on our four measures of AC. This was tested in a series of nested models. The first or full model is depicted in Fig. 1A. The model contains seven separable sources of genetic risk, two of which (A1 and A2) are common factors, and five of which are specific to AD (A3) and the 4 forms of AC (A4 to A7). The first common factor—A1—influences both AD and the forms of AC. Critically, the second common factor—A2—influences only the 4 forms of AC. When the full model is fit, A2 represents common genetic risk shared by the AC measures after removing the genetic influences these measures share with AD.
