Background: Parents' expression of positive emotion towards children who are typically developing (TD) is generally associated with better social development. However, the association between parents' negative emotion expression and social development can be positive or negative depending upon a number of factors, including the child's emotion regulation abilities. Given the lower emotion regulation capabilities of children with intellectual disability (ID), we hypothesised that parents' negative emotion expression would be associated with lower social development in children with ID compared to those with TD. Methods: Participants were 180 families of children with or without ID enrolled in a longitudinal study. Parents' positive and negative affect were coded live from naturalistic home interactions at child ages 5-8 years, and child's social skills were measured by using mother report at child ages 6-9 years. We examined mothers' and fathers' emotion expression as a time-varying predictor of social skills across ages 5-9 years. Results: Mothers, but not fathers, expressed less positive affect and more negative affect with ID group children. Parents' positive affect expression was related to social skills only for TD children, with mothers' positive affect predicting higher social skills. Contrary to expectations, fathers' positive affect predicted lower social skills. Parents' negative affect predicted significantly lower social skills for children with ID than for children with TD. Conclusions: Findings support the theory that low to moderate levels of negative expression may be less beneficial or detrimental for children with ID compared to children with TD. Implications for further research and intervention are discussed.