Objective: To assess the genetic and environmental etiology of the co-occurence of ADHD symptoms and reading difficulties using the largest sample to date, distinguishing two dimensions of ADHD and two of reading.

Methods: Data were obtained from 6428 12-year old twin pairs drawn from the population-based Twins Early Development Study. ADHD symptoms (combined ADHD, inattentiveness and hyperactivity-impulsivity) were assessed using parent ratings. Reading was assessed using a battery of reading comprehension and word decoding tests.

Results: Heritabilities were high, around 70% for ADHD symptoms and 45-65% for the reading measures. Some of the same genes affected combined ADHD and reading with a genetic correlation of -.31. The most notable finding was that the genetic correlation with reading was significantly greater for inattentiveness (-.31) than for hyperactivity-impulsivity (-.16), suggesting that genetic overlap between combined ADHD and reading is largely driven by inattentiveness. Moreover results showed that it is word decoding rather than reading comprehension that is differentially related to the ADHD dimensions (lower genetic correlation to hyperactivity-impulsivity than to inattentiveness).

Conclusions: Genetic overlap between ADHD and reading is largely driven by inattentiveness rather than hyperactivity-impulsivity.