this could be used in, e.g., GWAS to identify genetic factors that contribute to the endophenotype and the behavioral components of ADHD. Without data from twins or other genetically informative studies of relatives, it is not possible to know in advance whether there is any evidence for genetic factors influencing the common factor F1. The addition of twin data enables examination of the impact of genes on variation in each of the common factors, as shown in Figure 2. Here, variation in factors F1, F2, and F3 has been partitioned into genetic and environmental components, and the same separation is applied to the residual VCs specific to each measure. The parameters of this model can be freely estimated with data from a study of MZ and DZ twins. Possibly, the common factor F1 would have substantial genetic variation (a1 is large relative to c1 and e1). If the covariance between the endophenotypes and the behavioral measurements were entirely due to genetic factors, then c1 = e1 = 0 (which can be empirically tested with twin data), and maximum likelihood estimates of the individual F1 factor scores would be an especially promising candidate for GWAS.
