Genetically informative studies indicate substantial heritability for both CD and ODD behaviors (Eaves et al., 1997) but a mixed picture of etiological similarities between the two disorders. One study suggested that both were manifestations of the same genetic liability (Eaves et al., 2000), whereas others have reported moderate degrees of shared genetic and environmental influences, along with unique effects on both ODD and CD behaviors (Dick, Viken, Kaprio, Pulkkinen, & Rose, 2005; Lahey et al., 2009; Rowe, Rijsdijk, Maughan, Hosang, & Eley, 2008; Tuvblad, Zheng, Raine, & Baker, 2009; Waldman, Rhee, Levy, & Hay, 2001). It is possible that rater effects contribute to differences in findings between twin studies (Burt, McGue, Krueger, & Iacono, 2005). A strong correlation of genetic effects does not, of course, necessarily imply that the phenotypes involved index the same underlying disorder. Strong genetic correlations have been reported for example, between internalizing and externalizing phenotypes (see e.g. Rowe et al., 2008). In addition, models of this kind exclude the gene-environment interplay almost certainly involved in risk for externalizing disorders (e.g., Kim-Cohen et al., 2006). At this
