The rate of co-occurring ADHD in the ALSPAC TS sample is substantially lower than those reported in other population-based studies (36%–100%),4,14,17,19,20,30 although it is higher than the 8% ADHD rate reported in the Israeli TS study.15 This finding may be attributable to the instrument used to diagnose ADHD in ALSPAC, as previous studies have demonstrated that the DAWBA parent-form alone underestimates the true rate of ADHD in the population.25,31 Thus, our estimate of co-occurring ADHD should be considered a minimum prevalence in this sample.
