The neuropsychological underpinnings of Attention Deficit Hyperactivity Disorder (ADHD) and Reading Disability (RD) and their comorbidity may be studied usefully with the double dissociation design. The results of studies using the double dissociation method may be linked to the search for an endophenotype of ADHD and RD and their comorbidity. This endophenotype may eventually shed more light on the genetic origins of both ADHD and RD and their comorbidity. Executive functioning appears to be a candidate endophenotype of the comorbidity of ADHD and RD. Future studies, it is argued, should combine both the double dissociation design with the Additive Factor Method in this endeavour. Additionally, more attention needs to be given to the subtypes of ADHD, RD, and their association with Dyscalculia to determine useful endophenotype(s).