Previous studies have reported that the factor structure of AA and AD is best described by a single dimension (Feingold and Rounsaville, 1995; Langenbucher et al., 2004; Saha et al., 2006). This finding was replicated for the current data: the first eigenvalue is 8.407, followed by values well below 1 (the second value is 0.524). A confirmatory factor analysis was run in Mplus 5.2 (Muthen and Muthen, 1998-2007), using only one twin per pair (the first). All symptoms of AA and AD were modeled as binary variables. The standardized factor loadings onto each symptom are reported.
