Heterogeneity in children with Attention Deficit Hyperactivity Disorder (ADHD) symptoms is a well-known phenomenon. Empirically, this heterogeneity is evident in at least three different respects: expression of the two ADHD symptom domains (hyperactivity/impulsivity and inattention), neuropsychological impairments, and comorbid behavior problems. The major aim of the present thesis was to examine the heterogeneity characterizing children with ADHD symptoms to enhance our understanding by examining neuropsychological factors with regard to common and independent contributions, and specificity of the two ADHD symptom domains in relation to neuropsychological factors and comorbid behavioral problems. Particular emphasis is placed on prominent neuropsychological processes such as executive functions, state regulation and delay aversion. The present thesis is based on findings from four studies on community-based samples of children – studies involving concurrent and longitudinal designs as well as both categorical and dimensional approaches.

Results provide support for the notion that executive function and state regulation, but not delay aversion, constitute independent pathways to ADHD, primarily to symptoms of inattention. However, delay aversion was shown to have an effect in combination with state regulation on both hyperactivity/impulsivity and inattention. Additionally, symptoms of hyperactivity/impulsivity and inattention have different primary correlates concerning neuropsychological factors and comorbidity. More specifically, executive function, state regulation, internalizing problems and academic achievement were specifically related to inattention but not to hyperactivity/impulsivity. Oppositional Defiant Disorder (ODD) was specifically related to hyperactivity/impulsivity, but not to inattention.

The present thesis has contributed with important and new knowledge about the heterogeneity of children with ADHD symptoms concerning neuropsychological pathways, and specificity of the two ADHD symptom domains in relation to neuropsychological factors and comorbid behavioral problems. Knowledge such as this can help us understand how to identify more homogeneous ADHD subgroups, and contribute to the further development of multiple pathway models within this area of research.