Purpose: This study investigated the multiple components of flexibility in autistic children with and without co-occurring ADHD, and their associations with social functioning. We aimed to determine ADHD co-occurence influence on flexibility profiles and the links between flexibility and social outcomes.

Method: 124 children aged 6-12 years were drawn from the Geneva Autism Cohort. Flexibility was assessed across three domains: restricted and repetitive behaviors (RRBs; EC2R), daily behavioral flexibility (BRIEF-2), and cognitive flexibility (MTT and IED: CANTAB, and CST: Early Years Toolbox). Socialization skills were measured using the Vineland-II. Group comparisons and correlation analyses were conducted across three groups: typically developing (TD), autism (ASD), and autism with co-occurring ADHD (ASD + ADHD).

Results: Both ASD and ASD + ADHD groups showed higher levels of RRBs compared to TD, with no difference between ASD and ASD + ADHD. Daily and cognitive flexibility difficulties were more pronounced in ASD + ADHD, particularly on tasks involving time pressure or implicit learning. BRIEF scores correlated strongly with RRBs in both ASD groups, while links with task-based flexibility were weak. Socialization difficulties were observed in both ASD groups and were strongly associated with caregiver-reported flexibility. Notably, the inverse correlation between RRBs and socialization was significant only in ASD, suggesting co-occuring ADHD may alter the regulatory function of RRBs.

Conclusion: ADHD co-occurrence contributes to a distinct flexibility profile in autistic children, especially under implicit instructions or timed conditions. Flexibility difficulties were strongly related lower socialization scores, underscoring the need for tailored supports that address both executive and social domains across neurodevelopmental profiles.