Perhaps we did not identify strong evidence for association of DA genes with the general disinhibitory factor because our design assessed disinhibitory phenotypes in subjects with AD. Other investigators who have been successful in finding associations with a factor score (of internalizing behavior) included a broader range of subjects (Boomsma et al., 2000; Hettema et al., 2008). Another explanation is that variation in DA genes may contribute more to risk for specific disorders than the liability to a general disinhibitory spectrum of disorders.
