Differences in social behaviours are common in young people with neurodevelopmental conditions (NDCs). Recent research challenges the long-standing hypothesis that difficulties in social cognition explain social behaviour differences.

We examined how difficulties regulating one's behaviour, emotions and thoughts to adapt to environmental demands (i.e. dysregulation), alongside social cognition, explain social behaviours across neurodiverse young people.

We analysed cross-sectional behavioural and cognitive data of 646 6- to 18-year-old typically developing young people and those with NDCs from the Province of Ontario Neurodevelopmental Network. Social behaviours and dysregulation were measured by the caregiver-reported Adaptive Behavior Assessment System Social domain and Child Behavior Checklist Dysregulation Profile, respectively. Social cognition was assessed by the Neuropsychological Assessment Affect-Recognition and Theory-of-Mind, Reading the Mind in the Eyes Test, and Sandbox continuous false-belief task scores. We split the sample into training (n = 324) and test (n = 322) sets. We investigated how social cognition and dysregulation explained social behaviours through principal component regression and hierarchical regression in the training set. We tested social cognition-by-dysregulation interactions, and whether dysregulation mediated the social cognition–social behaviours association. We assessed model fits in the test set.

Two social cognition components adequately explained social behaviours (13.88%). Lower dysregulation further explained better social behaviours (β = −0.163, 95% CI −0.191 to −0.134). Social cognition-by-dysregulation interaction was non-significant (β = −0.001, 95% CI −0.023 to 0.021). Dysregulation partially mediated the social cognition–social behaviours association (total effect: 0.544, 95% CI 0.370–0.695). Findings were replicated in the test set.

Self-regulation, beyond social cognition, substantially explains social behaviours across neurodiverse young people.