Relationship between IQ and Internalizing and Externalizing Symptoms in Children with Autism and Children with ADHD

ABSTRACT Introduction Findings regarding the relationship between IQ and comorbidity in autism and ADHD are inconsistent. Methods Mothers rated 1,436 children with autism and 1,056 with ADHD without autism (IQs 9–149, ages 2–17) on the Pediatric Behavior Scale. Results Children with IQs < 70 (vs. ≥ 70) had less oppositional behavior, irritability, tantrums, conduct problems, generalized anxiety, and depression and more separation anxiety. Children with ADHD-Inattentive had less comorbidity than children with autism and children with ADHD-Combined. For autism, 80% had ADHD-Combined symptoms and 9% had ADHD-Inattentive symptoms. Most children with autism and with ADHD-Combined with IQs ≥ 70 had oppositional defiant disorder, as did almost half with IQs < 70. For autism, 45% with IQs ≥ 70 had generalized anxiety and 47% with IQs < 70 had separation anxiety. Conclusions Given high rates of comorbid symptoms in autism and ADHD, clinicians should be alert to and assess for psychiatric comorbidity.