Abstract
Autism and Attention Deficit Hyperactivity Disorder (ADHD) are both linked to internalising problems (e.g., anxiety, depression) but their frequent co-occurrence makes these relationships difficult to disentangle. We, therefore, adopted a trait-based approach to examine the unique associations of autism and ADHD with internalising problem diagnoses in a large general population sample of adults from the United Kingdom and United States (N = 4996). We then assessed whether these associations would conceptually replicate in a clinical sub-sample (n = 292) using a case-control design. We found that, across the whole sample, ADHD traits were an overall stronger predictor of internalising problem diagnoses than autistic traits. The case-control analyses revealed that both autistic adults and adults with ADHD had greater odds of being diagnosed with anxiety, depression and an overall internalising problem (i.e., depression and/or anxiety) than neurotypical adults. Although the clinical groups did not significantly differ from each other in their associations with mental health diagnoses, having and ADHD diagnosis was more closely linked with depression while having an autism diagnosis was more closely linked with anxiety. We discuss potential psychological mechanisms underlying these findings and the implications for research and clinical practice concerning neurodevelopmental conditions.