Previous studies examining the covariation among Attention Deficit Hyperactivity Disorder (ADHD), Oppositional Defiant Disorder (ODD) and Conduct Disorder (CD) have yielded inconsistent results. Some studies have concluded that the covariation among these symptoms is due to common genetic influences, whereas others have found a common environmental overlap. The present study investigated the genetic and environmental correlations among these three childhood disorders, based on a sample of 1,241 twins, age 9-10 years. A latent externalizing behavior factor was found to explain the covariance among ADHD, ODD and CD symptoms. Genetic influences explained more than half of the variance in this externalizing factor in both boys and girls. There were also unique genetic and environmental influences in each set of symptoms, suggesting some etiological independence of the three disorders. Our findings have implications for molecular genetic studies trying to identify susceptibility genes, as well as for possible prevention of externalizing disorders through identification of at-risk children early in life. KeywordsADHD; CD; ODD; covariation; genetic influences; twins Attention Deficit Hyperactivity Disorder (ADHD), Oppositional Defiant Disorder (ODD) and Conduct Disorder (CD) are three of the most prevalent disruptive behavior disorders in children and adolescents. They affect approximately 1-15% of all school age children, and account for a large amount of childhood referrals to mental health clinics (Brown et al., 2001;Maughan et al., 2004). They often lead to stress and frustration in the affected children, their families, teachers and peers. Numerous studies have reported comorbidity among these disruptive behavior disorders in both epidemiological and clinical samples (Angold, Costello & Erkanli, 1999;Biederman, Newcorn & Sprich, 1991;Faraone et al., 1998b;Maughan et al., 2004), often making them difficult to isolate and understand individually. Such high levels of comorbidity among supposedly different domains of externalizing problems, such as inattention, hyperactivity, aggression and defiance make these various problem behaviors complex and particularly hard to treat. To date, the genetic and environmental etiology that underlies the comorbidity among these disruptive behavior disorders is unclear.ADHD is characterized by pervasive and impairing symptoms of inattention, hyperactivity and impulsivity (American Psychiatric Association, 2004). The disorder is associated with academic underachievement, substance use and dependence, psychosocial problems and social NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript maladjustment, including unemployment in adulthood (Biederman & Faraone, 2006;Kuperman et al., 2001).Like ADHD, ODD is also a common psychiatric disorder in children (Maughan et al., 2004). The disorder typically occurs in early childhood and is characterized by a pattern of negativistic, disobedient, hostile and defiant behaviors. In contrast to ODD, CD is a more severe d...

The aim of this study was to examine stability and change in genetic and environmental influences on reactive (impulsive and affective) and proactive (planned and instrumental) aggression from childhood to early adolescence. The sample was drawn from an ongoing longitudinal twin study of risk factors for antisocial behavior at the University of Southern California (USC). The twins were measured on two occasions: ages 9-10 years (N = 1,241) and 11-14 years (N = 874). Reactive and proactive aggressive behaviors were rated by parents. The stability in reactive aggression was due to genetic and nonshared environmental influences, whereas the continuity in proactive aggression was primarily genetically mediated. Change in both reactive and proactive aggression between the two occasions was mainly explained by nonshared environmental influences, although some evidence for new genetic variance at the second occasion was found for both forms of aggression. These results suggest that proactive and reactive aggression differ in their genetic and environmental stability, and provide further evidence for some distinction between reactive and proactive forms of aggression. Keywordsreactive; proactive; aggression; heritability; longitudinal The function of aggressive behavior is related to the motive of the perpetrator, and it has been argued that a distinction can be drawn between reactive and proactive forms of aggression in both children and adults. Reactive aggression refers to angry or frustrated responses to a real or perceived threat. This specific type of aggression has been characterized as involving high emotional arousal, impulsivity, and an inability to regulate or control affect. On the contrary, proactive aggression is conceptualized as a more regulated, instrumental form of aggression, with more positive expectancies about the outcomes of aggression [Crick and Grotpeter, 1996;Dodge, 1991;Dodge and Coie, 1987;Schwartz et al., 1998]. Behavioral genetic studies have provided evidence that heritable factors are important for both reactive and proactive aggression [Baker et al., 2008;Brendgen et al., 2006]. However, the influence of genetic and environmental factors on continuity and change in the development of reactive and proactive aggression remains poorly understood.Theoretical explanations for reactive aggression are derived from the frustration-aggression model, which considers aggression as a hostile, angry reaction to perceived frustration *Correspondence to: Dr. Catherine Tuvblad, Department of Psychology (SGM 501), University of Southern California, 3620 S. McClintock Ave., Los Angeles, CA 90089-1061. Tuvblad@USC.EDU. NIH Public Access Author ManuscriptAggress Behav. Author manuscript; available in PMC 2009 November 1. Published in final edited form as:Aggress Behav. 2009 ; 35(6): 437-452. doi:10.1002/ab.20319. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript [Berkowitz, 1993]. An alternative theoretical perspective for understanding the etiology of proactive aggression is...

The genetic and environmental basis of a well-replicated association between antisocial behavior (ASB) and resting heart rate was investigated in a longitudinal twin study, based on two measurements between the ages of 9 and 14 years. ASB was defined as a broad continuum of externalizing behavior problems, assessed at each occasion through a composite measure based on parent ratings of trait aggression, delinquent behaviors, and psychopathic traits in their children. Parent ratings of ASB significantly decreased across age from childhood to early adolescence, although latent growth models indicated significant variation and twin similarity in the growth patterns, which were explained almost entirely by genetic influences. Resting heart rate at age 9–10 years old was inversely related to levels of ASB but not change patterns of ASB across age or occasions. Biometrical analyses indicated significant genetic influences on heart rate during childhood, as well as ASB throughout development from age 9 to 14. Both level and slope variation were significantly influenced by genetic factors. Of importance, the low resting heart rate and ASB association was significantly and entirely explained by their genetic covariation, although the heritable component of heart rate explained only a small portion (1–4%) of the substantial genetic variance in ASB. Although the effect size is small, children with low resting heart rate appear to be genetically predisposed toward externalizing behavior problems as early as age 9 years old.

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