Background: The potential role of social reinforcement in the development of self-injury has not yet been subjected to empirical analysis. In this 2-year prospective study, the pattern of social interactions related to the early presentation of self-injury were examined to identify a potential association with an increase in self-injury. Methods: The self-injurious behaviour and social contact with adults of 16 children with intellectual disability (ID) with self-injury of recent onset were observed at 3-month intervals over 2years. Results: Increase in self-injury over a 2-year period was positively correlated with a distribution of social contact relative to episodes of self-injury that is consistent with a mutual social reinforcement paradigm. When this paradigm was operative, self-injury was evoked under stable antecedent conditions over time but tended to evoke increasing levels of social interaction. Conclusions: These results support the hypothesis that increases in the frequency of early self-injury in children with ID may be determined by social reinforcement with implications for early intervention and proactive identification of children at risk for increases in self-injury.