Behavioral Genetic Approaches to Understand the Etiology of Comorbidity

Abstract

Comorbidity, the co-occurrence of two or more disorders in the same individual, is the rule rather than the exception for most developmental psychopathologies. However, the etiology of this ubiquitous phenomenon is not well understood. This chapter provides an overview of behavioral genetic methods that can be used to test the primary competing explanations for comorbidity between complex disorders, illustrating each approach with examples from studies of reading disability (RD) and attention-deficit/hyperactivity disorder (ADHD). Family studies indicate that RD and ADHD are both significantly familial and suggest that shared familial risk factors contribute to comorbidity between RD and ADHD. Twin analyses can then be used to disentangle these shared familial influences to assess the extent to which genetic and shared environmental influences contribute to each disorder and their co-occurrence. Shared genetic influences accounted for nearly all of the phenotypic covariance between RD and ADHD and also contributed to comorbidity between both RD and ADHD and a range of internalizing and externalizing symptoms. However, genetic influences on RD were only associated with oppositional defiant disorder and conduct disorder if the selected proband also met criteria for ADHD. Promising future directions for behavioral genetic research on comorbidity include longitudinal twin studies of the development of comorbidity, analyses that incorporate neuropsychological and neuroimaging phenotypes, and studies that integrate molecular genetic methods.