GENETIC AND ENVIRONMENTAL FACTORS IN THE DEVELOPMENT OF EXTERNALIZING SYMPTOMS FROM CHILDHOOD TO ADOLESCENCE

Externalizing problems, such as hyperactivity, impulsivity, aggression, violation of legal or social norms, and delinquent behavior, tend to persist from childhood to adolescence. There are also high levels of comorbidity among supposedly different domains of externalizing problems. Previous research also suggests that persistence over time and comorbidity may be two of the strongest predictors of adult outcome, especially risk for antisocial behavior, delinquency and substance abuse. This thesis has therefore applied multivariate twin methods to longitudinal data to clarify the action of genetic and environmental factors in ADHD, antisocial behavior, and psychopathic personality during the development from childhood to adolescence. The data used in this thesis comes from the Twin study of CHild and Adolescent Development (TCHAD), an ongoing prospective longitudinal study with data collected from parents when the twins were 8-9 years old and from both parents and twins at age 13-14 and 16-17. Paper I in this thesis shows that the relative high stability in ADHD-symptoms over a 5year period was to a large extent explained by genetic factors, whereas both genetic and nonshared environmental factors contributed to changes in ADHD symptoms during the period from childhood to early adolescence. Paper II shows that the associations between and within symptoms of hyperactivity-impulsivity and inattention from childhood to adolescence was influenced by persistent cross-subtype, persistent subtype-specific, age-limited cross-subtype, and age-limited subtype-specific genetic influences. Paper III demonstrates that the three psychopathic personality dimensions were significantly linked to a highly heritable “psychopathic personality” factor, even though non-shared environmental factors also contributed significantly. The callous/unemotional and impulsive/irresponsible dimensions were also influenced by unique genetic factors. The results from Paper IV suggest that the association between psychopathic personality and persistent adolescent antisocial behavior was primarily explained by a common genetic factor. Genetic and environmental influences that were unique to persistent adolescent antisocial behavior were mainly explained by shared environmental influences. The data in this thesis shows that a common genetic factor (i.e., persistent cross-subtype genetic influences) influences the development of ADHD from childhood to adolescence. Comorbidity between supposedly different domains of externalizing problems (i.e., psychopathic personality and persistent adolescent antisocial behavior) is also primarily explained by a common genetic factor. Nevertheless, this thesis also shows etiologic specificity in externalizing problems during the development from childhood to adolescence. The callous/unemotional and impulsive/irresponsible dimensions of the psychopathic personality are influenced by unique genetic variance and the developments of ADHD subtypes have independent genetic etiologies (i.e., persistent subtype-specific genetic influences). Future research needs to consider the existence of general predisposing factors that influence a broad range of externalizing problems, but also specific predisposing factors that differentiate between different domains of externalizing problems. Identification of early emerging general and specific predisposing factors (e.g., endophenotypic markers) should be of high priority. Knowledge from such studies may not only facilitate identification of susceptibility genes, but also provide tools needed to identify effective intervention targets.