Attention Deficit Hyperactivity Disorder (ADHD) is a common, yet complex and heterogeneous developmental disorder. Our understanding of co-morbidity in ADHD lacks nuance, despite the fact that it is frequently co-diagnosed with other developmental disorders, including reading disabilities. Furthermore, the available interventions for ADHD, such as stimulant medication and behavioral therapy, are not universally effective, necessitating the development of alternative or supplementary treatment options. This dissertation draws from the Dual Pathway model of ADHD, which suggests that ADHD is the result of underlying executive dysfunction and/or motivational problems associated with delay aversion, in order to investigate co-morbidity in and novel treatments for ADHD.

In Study 1, I use hierarchical linear modeling to develop a clearer understanding of the associations between ADHD symptoms, reading challenges, and executive functioning. I show that ADHD symptoms are more strongly related to reading comprehension than to word reading, and that executive functioning partially explains the associations between ADHD symptoms and reading ability. In Studies 2 and 3, I test the effectiveness of randomized control trials for children with ADHD: the first is an executive function training intervention and the second is a motivation intervention. In study 2, I demonstrate that executive function training has the potential to improve ADHD symptoms, working memory and inhibitory control, in particular. In Study 3 I show that it is feasible to teach concepts from motivation theory, such as growth mindset, to children with ADHD and their parents. Findings from this dissertation underscore the importance of acknowledging the complex etiological heterogeneity in ADHD, which has implications for treatment. They demonstrate that executive functioning alone cannot explain co-morbidity and that the development and implementation of multiple methods of intervention (e.g., executive function training and motivation interventions) is likely necessary, as children may need more than one type of treatment or a tailored treatment plan to target the complexity of their ADHD symptoms.