Alcohol dependence (AD) in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) describes a complex syndrome characterized by neurobiological, cognitive/motivational and behavioral/social symptoms. Adoption1–4 and twin studies5–11 have consistently shown that genetic factors impact substantially on risk for AD. A number of efforts are now under way to identify risk genes for AD by linkage12 and genome-wide association.13–15 The conceptual basis of these studies is that the DSM-IV syndrome of AD reflects a single dimension of genetic risk. As results from a number of studies suggested that the DSM-IV AD criteria form a single coherent phenotypic factor,16–18 the structure of the genetic risk factors for AD has not been previously examined. Multiple genetic risk factors might be expected because in rodent studies of genetic influences on a wide range of alcohol-related traits suggest that genetic contributions to each are either largely distinct or only weakly correlated.19,20 In this study, using data collected from 7548 adult male and female twins from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (VATSPSUD),21 we perform a multivariate analysis
