Findings from behavior genetic modeling of comorbidity suggest that the internalizing spectrum is etiologically coherent, providing further support in favor of a liability-spectrum model. Bivariate biometrical studies of the genetic and environmental influences common to specific combinations of internalizing syndromes (i.e., depression and generalized anxiety, neuroticism and depression) have been universal in concluding that the genetic influences common to depression and anxiety are so great as to make the disorders virtually indistinguishable at the genetic level (see Middledorp, Cath, Van Dyck, & Boomsma, 2005, for a recent review). Research has also shown that the structure of the genetic and environmental influences on psychopathology across the internalizing spectrum parallels the phenotypic structure (Kendler et al., 2003). Hettema et al. (2006) explicitly modeled the comorbidity between neuroticism and seven internalizing disorders using 9,000 twin pairs from the Virginia Twin Registry. They found that similar genetic factors underlie neuroticism and the latent liability to the internalizing spectrum, but environmental factors were largely uncorrelated. Neuroticism largely accounted for the comorbidity among the internalizing disorders, but there were genetic risk factors common to major depression,
