Executive functioning (EF) is impaired in autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD), and EF challenges are exacerbated in youth with ASD and ADHD (ASD+ADHD), which may impact diagnostic timing. We hypothesized youth with ASD+ADHD would be more impaired in EF (as opposed to other functional domains) compared to autistic youth without ADHD (ASD-only), with particular deficits in metacognition and inhibition. We also predicted youth with ASD+ADHD would be at significant risk for delayed ASD diagnosis and that greater EF challenges would predict earlier age of ASD diagnosis across groups.

Data from a clinical database was extracted for 400 youth who received a new diagnosis of ASD after age 5, either with a co-occurring diagnosis of ADHD (ASD+ADHD group: n=297; Mage of ASD diagnosis=10.49; 25.9% female; 48.1% white) or without a co-occurring ADHD diagnosis (ASD-only group: n=100; Mage of ASD diagnosis=12.02; 34.0% female; 44.7% white). EF was measured with the BRIEF-2 parent-report, and ASD symptom strength was measured with the SRS-2 School Age form. Independent samples t-tests investigated whether a) the ASD+ADHD group was uniquely impaired in EF compared to the ASD-only group, b) parents of ASD+ADHD report elevated EF problems, and c) the ASD+ADHD group was at significant risk for delayed ASD diagnosis. Pearson correlations examined the association between age of ASD diagnosis and EF for each diagnostic group. Hierarchical linear regressions further analyzed whether specific EF domains concurrently predicted age of ASD diagnosis, after controlling for the known predictors of assigned sex at birth, FSIQ, and ASD symptom strength.

The ASD+ADHD group had greater challenges in overall EF (t=-6.42, p<.001), metacognitive skills (t=-6.47, p <.001), and inhibition skills (t=-7.06, p<.001). There was no significant difference in parent-reported autism symptoms between the ASD and ASD+ADHD groups (t=0.973, p=.331). The ASD+ADHD group received ASD diagnoses earlier than the ASD-only group (t=4.194, p<.001). In the ASD-only group, age of ASD diagnosis was not significantly correlated to overall EF, metacognitive skills, nor inhibition skills (ps>.05). In the ASD+ADHD group, ASD diagnosis was significantly related to overall EF (r(297)=.128, p=.027) and metacognitive skills (r(297)=.329, p<.001) but not inhibition skills (r(297)=.078, p=.180). Hierarchical linear regressions controlling for assigned sex at birth, FSIQ, and SRS-2 T-scores were used to determine whether these EF components significantly predicted age of ASD diagnosis. Overall EF did not predict age of ASD diagnosis in the ASD+ADHD group (ß=.034, t=1.417, p=.157), but metacognitive skills did (ß=.123, t=5.582, p<.001).

Our findings suggest youth with ASD+ADHD have greater impairment in overall EF, metacognition, and inhibition compared to ASD-only youth, despite similar levels of ASD traits, consistent with hypotheses. Contrary to our hypothesis, youth with ASD+ADHD in this sample were diagnosed with ASD earlier. However, results also suggest EF problems, specifically metacognitive deficits, predict later age of ASD diagnosis. Future research is needed to replicate findings and better understand how EF and other functional domains predict ASD diagnostic timing.