In their letter, Freitag et al raise several questions as to the interpretation of the results of our study. In response to the concerns expressed, we offer the following comments.
First, as may become evident from the title, the focus of our article was on behavioral symptoms and not on the diagnosis of ADHD, indicating that we are following a dimensional approach to define ADHD. Such an approach describes the different types of ADHD on dimensions of psychopathology, primarily by grading of their severity and phenotypic features.1 Continuous measures of inattention and hyperactivity-impulsivity have been shown to have high heritability and are strongly correlated with the categorical diagnosis of ADHD.2,3 Evidence from these studies indicates that “genetic methods based on continuously varying traits . . . are well suited for investigating the genetics of ADHD symptomatology.”4(p117) Thus, in our study, the question is not whether genetic or environmental factors increase risk for the disease category of ADHD but whether these factors influence levels of psychopathological dimensions underlying this disorder (and probably shared with other disorders). Our findings suggest that symptoms of inattention and hyperactivity-impulsivity are the result of a complex gene × environment interaction between the DAT1 gene and psychosocial adversity.
Laucht M, Skowronek MH, Becker K, Schmidt MH, Esser G, Schulze TG, Rietschel M. Environmental Risk Factors and Attention-Deficit/Hyperactivity Disorder Symptoms—Reply. Arch Gen Psychiatry. 2008;65(3):357-358. doi:10.1001/archpsyc.65.3.357