Autistic traits in a population-based ADHD twin sample.

Most diagnostic nomenclatures do not allow for the concurrent diagnosis of autism and attention-deficit/hyperactivity disorder (ADHD). Clinic-based studies suggest autistic symptoms are common in children with ADHD, but such studies are prone to referral bias. This study assesses whether children with ADHD selected from the general twin population have elevated levels of autistic traits. Nine hundred forty-six twins identified by Missouri birth records were assigned to DSM-IV ADHD diagnoses and seven population-derived ADHD subtypes defined through latent class analysis of DSM-IV ADHD symptoms. The Social Responsiveness Scale (SRS) was used as a quantitative measure of autistic traits. Linear regression was used to evaluate whether mean SRS scores differed between ADHD diagnostic groups. Mean SRS scores for DSM-IV predominantly inattentive subtype and combined subtype ADHD groups were significantly higher than for subjects without DSM-IV ADHD (p < .001, both comparisons). Five of the population-derived ADHD subtypes (talkative-impulsive, mild and severe inattentive, mild and severe combined) had significantly higher mean SRS scores compared to the latent class subtype with few ADHD symptoms (p < .001, all comparisons). DSM-IV combined subtype and the population-derived severe combined subtype had the highest mean total SRS scores and the highest mean scores for each of the three autism symptom domains, with a substantial proportion of individuals scoring in the clinically significant range. This study provides population-based evidence for clinically significant elevations of autistic traits in children meeting diagnostic criteria for ADHD. These results have implications for the design and interpretation of studies of both disorders.