Causation, correlation, or confound? What the comorbidity of language impairment and ADHD can tell us about the etiology of these disorders

Language impairment (LI) and ADHD are two relatively common developmental disorders that frequently co-occur and have thus been said to be comorbid. The overall aim of this research is to investigate the nature of comorbidity between LI and ADHD in a large population-based sample. The project comprises two parts. The first study aims to quantify the extent of comorbidity between LI and ADHD, and asks whether there is any evidence for a shared liability between the two disorders on the basis of family history data. The second study hypothesizes that comorbidity between LI and ADHD arises because the two disorders share a common genetic etiology. Genetic variants previously associated with ADHD are tested for association to LI. Association is found with the dopamine receptor D4 (DRD4) and the serotonin transporter gene (SLC6A4). Implications for this in relation to language are discussed with respect to reinforcement and associative learning.