This randomized clinical trial of methylphenidate in children with intellectual disability (ID) by Simonoff et al. (2013) advances the field in several ways useful to clinicians. The three-figure widely representative sample more definitively confirms findings previously reported from smaller studies and studies with a differently selected sample. The medium placebo-controlled effect size found is in line with previous more tentative suggestions for ID, such as those summarized by Aman et al. This sample, selected for ID but coincidentally including some children with autism (a third of the sample), nicely complements the RUPP Autism Network (2005) study of 72 children with autism, most of whom also had ID (mean IQ 62.6, range 16-135). Similar effect was found in both studies, suggesting that one might expect a medium effect widely in the intellectual & developmental disability (IDD) population, with a 40-50% response rate.