To investigate the predictive power of comorbid attention-deficit/hyperactivity disorder (ADHD) symptoms on adaptive behavior skills in children who have an autism specrum disorder (ASD) diagnosis.

This case-control study recruited 347 children from specialty clinics, primary care, and the community. Linear regression was used to test whether ADHD Rating Scale, Fourth Edition, scores of autistic children associated with poorer adaptive behavior scores, after controlling for the effects of age, intelligence, sex, and ASD symptom severity. Adaptive behaviors were measured with the Vineland Adaptive Behavior Scales, Second Edition. Subsequent analyses tested this relation in a subset of the ASD sample with subclinical ADHD symptoms (n = 179) and another with teacher ratings (n = 153). Prior relations between age with adaptive behaviors and ADHD symptoms were replicated and age was explored as a moderator.

ADHD symptoms predicted poor adaptive behavior scores in the full ASD sample (caregiver ratings, ΔR² = 0.033–0.119; teacher ratings, ΔR² = 0.113–0.163) and in the subset with subclinical ADHD symptoms (caregiver ratings, ΔR²= 0.023–0.030; teacher ratings, ΔR² = 0.097–0.159) after controlling for confounds. Age correlated negatively with ADHD symptoms (r = −0.21) and adaptive behaviors (−0.17 < r < −0.39) in the full ASD sample. Age did not moderate the effect of ADHD symptoms on adaptive behaviors.

ADHD symptoms predict poorer adaptive behavior for autistic children across settings, even for children with subclinical co-occurring ADHD symptoms. Findings support a Research Domain Criteria framework that behavioral impairments and functional outcome measures exist along a continuum.