Abstract/Summary

Background: Autism and attention-deficit/hyperactivity disorder (ADHD) overlap to a considerable degree and have been associated with mental health difficulties, yet there is limited research on this relationship. Young adulthood is a time of heightened risk for mental health problems in general. The risk may be greater for individuals with these conditions, for whom societal demands tied to this transitional time may heighten the impact of internalising behaviours. Elucidating the relationships between neurodevelopmental differences and vulnerability to psychopathology may inform future adaptations for specialised support. Methods: This study explored whether autistic and ADHD traits and their interaction were associated with symptoms of depression and anxiety as well as meeting diagnostic criteria for internalising disorders in a sample of 556 young-adult twins (mean age 22 years 5 months, 52% female), controlling for sex, age, cognitive ability, and parental socioeconomic status. Four participant groups were created based on traits assessed in young adulthood: high autistic traits, high ADHD traits, high autistic and ADHD traits, and low ADHD and autistic traits. Results: High autistic and ADHD traits were independently associated with higher self-reported depression and anxiety symptoms and likelihood of meeting diagnostic thresholds for an anxiety or low mood disorder. While co-occurrence of autism and ADHD exhibited the greatest risk for mental health challenges, no evidence was found for interaction effects between these traits at stringent corrected thresholds. Females with high levels of autistic traits exhibited particularly high risk for concomitant psychopathology. Conclusions: Our findings suggest that those with high levels of autistic and/or ADHD traits may require individualised care strategies, in light of the complex interplay between traits of neurodivergence and mental health outcomes. Future research may explore the efficacy of psychoeducation and specific adaptations to established therapeutic interventions needed to optimise outcomes for adults with these conditions.