Background: The Social Information Processing (SIP) model has helped to identify specificities in the stages preceding the execution of social behaviour in people with mild intellectual developmental disorder or borderline intellectual functioning. However, uncertainties remain about the involvement of the underlying processes and their developmental expression. Method: The aim of this research was to explore the stages of SIP from a comparative and developmental perspective. A total of 175 participants, from childhood to adulthood, with or without mild intellectual developmental disorder or borderline intellectual functioning, watched videos before answering a structured questionnaire relating to the SIP model. Results: The results showed developmental specificities in encoding, a hostile intention attribution bias and the absence of specific behavioural patterns in a population with mild intellectual developmental disorder or borderline intellectual functioning compared with a neurotypical population. Conclusion: These advances are discussed in relation to current clinical remediation therapies and new exploration methods.