Common genetic and nonshared environmental factors contribute to the association between socioemotional dispositions and the externalizing factor in children.

Attention-deficit/hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), and conduct disorder (CD) are common and costly childhood disorders (Faraone, Sergeant, Gillberg, & Biederman, 2003; Maughan, Rowe, Messer, Goodman, & Meltzer, 2004) accounting for over half of childhood mental health services referrals (Waschbusch, 2002). Rates of comorbidity of ADHD, ODD, and CD are substantial, ranging from 29 to 71% (Simonoff et al., 1997; Waschbusch, 2002). Further, this comorbidity is associated with increased functional impairment, a greater chance of psychiatric inpatient admissions, and increased risks for later psychiatric problems (Dalsgaard, Mortensesn, Frydenberg, & Thomsen, 2002; Waschbusch, 2002). Recent factor analyses of childhood behavior disorders find a higher-order Externalizing factor consisting of lower-order inattention, hyperactive/impulsive, ODD, and CD factors (Bezdjian et al., 2011; Lahey, Rathouz, et al., 2008), suggesting that these disorders are part of a latent Externalizing factor. The covariation of externalizing disorders is important in terms of both its magnitude and its impact and efforts have been made over the past decade to understand the factors that underlie this covariation, including genetic and environmental factors. Genetically informative studies (e.g., twin studies) can be used to estimate the extent to which individual differences in a behavior are associated with differences in genetic factors (that serve to make biological relatives alike), shared environmental factors (that serve to make individuals who live together more alike), and non-shared environmental factors (that serve to make individuals different from each other). Multiple research groups have used data from twins to disentangle the unique and common genetic and environmental contributions to the comorbidity or covariation among ADHD, ODD, and CD. For instance, Eaves et al. (2000) reported that across multiple informants on a sample of 8- to 16-year-old twins only shared genes significantly contributed to the association between ODD and CD and that genes and non-shared environment accounted for the correlations among inattention, hyperactivity, and impulsivity. Other studies using different measures, methods, and raters consistently support the idea that substantial genetic and small non-shared environmental influences contribute to the overlap among ADHD, ODD, and CD (Dick, Viken, Kaprio, Pulkkinen, & Rose, 2005; Nadder, Rutter, Silberg, Maes, & Eaves, 2002; Rietveld, Hudziak, Bartels, van Beijsterveldt, & Boomsma, 2003; Tuvblad, Zheng, Raine, & Baker, 2009). Notably, one research group has also found significant shared environmental influences on the covariation among these disorders in 11-year-old twins (Burt, Krueger, McGue, & Iacono, 2001, 2003). Whereas these studies clearly show that ADHD, ODD, and CD are influenced by some of the same genetic factors, the reasons for etiological connections are not well understood. Certain theories or models explaining the comorbidity among childhood externalizing disorders stem from the observation that they are reliably related to certain temperament or personality traits that may serve to increase the likelihood of expressing the disorders. An extensive literature provides evidence of the covariation between externalizing behaviors and personality traits involving undercontrol and risk-taking (e.g., Iacono, Malone, & McGue, 2008; Khan, Jacobson, Gardner, Prescott, & Kendler, 2005; see Muris & Ollendick, 2005 for a review). Further, studies show that the externalizing dimension can be extended to include certain personality traits with common genetic and non-shared environmental influences such as novelty seeking (Young, Stallings, Corley, Krauter, & Hewitt, 2000) and lack of constraint (Krueger et al., 2002; Krueger, Markon, Patrick, Benning, & Kramer, 2007; Krueger & Tackett, 2003). Although personality traits involving undercontrol or risk-taking have shown common sources of genes and environment with externalizing behaviors (e.g., Krueger et al., 2002; Young et al.), there are other trait dimensions related to externalizing disorders in children. Lahey and colleagues have posited a developmental propensity model in which the same socioemotional and cognitive dispositions are thought to underlie the various childhood externalizing disorders (Lahey & Waldman, 2003; Waldman, 2005; Waldman, Singh, & Lahey, 2006). Dispositions are proposed to have an indirect role in the manifestation of specific externalizing phenotypes like CD through interaction with the environment. Lahey, Applegate, et al. (2008) created the Child and Adolescent Dispositions Scale (CADS) to test the developmental propensity model (Lahey & Waldman, 2003). The CADS measures three dispositions (Prosociality, Negative Emotionality, and Daring) that were identified based on evidence that they were concurrently and predictively related to conduct problems in children (e.g., Dadds, Fraser, Frost, & Hawes, 2005; Lahey, Rathouz et al., 2008; Leve, Kim, & Pears, 2005; Singh & Waldman, 2010). Lahey and Waldman (2003) define the Prosociality disposition as respect for rules, helping and sharing, guilt over transgressions, and sympathetic concern for others. Negative Emotionality reflects mood lability and stress reactivity. Daring refers to excitement seeking and risk taking. These dispositions are proposed, in part, to underlie the development of CD via common genetic factors coupled with requisite environmental conditions (Lahey and Waldman, 2003). Thus, the developmental propensity model would predict genetic influences on each of the socioemotional dispositions as well as common genetic factors associated with the dispositions and CD and, by extension, externalizing disorders more broadly. In the only published study of its kind, Waldman et al. (2011) tested the developmental propensity model in terms of common genetic influence with an externalizing disorder, CD. Genetic influences unique to Prosociality and Negative Emotionality contributed to 20 and 16% of the variance on CD. Genetic influences unique to Daring accounted for only a small but significant amount of the variance on CD (3%). The overlap between the three socioemotional dispositions and CD was primarily genetic with small effects for non-shared environment. This provided initial evidence that the covariance of socioemotional dispositions with externalizing behavior disorders and perhaps their covariation is likely to be attributable primarily to genetic and non-shared environmental influences.