["Journal of Intellectual Disability Research, EarlyView. ", "\nABSTRACT\n\nBackground\nChildren with intellectual disabilities display fewer prosocial behaviours and increased behavioural and emotional problems compared to children without intellectual disabilities. Mother–child closeness may be an important factor in improving behavioural and emotional outcomes in children with intellectual disabilities over time. We aimed to examine the covarying relationship between mother–child closeness and child externalising and internalising behaviour problems, and child prosocial behaviour, respectively, over time.\n\n\nMethods\nParallel process growth modelling was conducted using data from 353 maternal primary caregivers who took part in three waves of the 1000 Families Study. Mother–child closeness was measured at each wave using the Child–Parent Relationship Scale. Child behavioural and emotional outcomes were measured using the Strengths and Difficulties Questionnaire. We controlled for time‐varying and time‐invariant covariates including the child's level of communication skills, autism diagnosis, maternal psychological distress and family economic adversity.\n\n\nResults\nThe trajectory of mother–child closeness remained relatively stable across the three waves. After controlling for covariates, the trajectories of mother–child closeness and child prosocial behaviour significantly covaried. However, the trajectory of mother–child closeness did not significantly covary with the trajectory of either child internalising or externalising behaviour problems.\n\n\nConclusion\nInterventions aiming to improve child prosocial behaviour and/or mother–child closeness in families of children with intellectual disabilities may benefit from considering the relationship between mother–child closeness and child prosocial behaviour. Findings highlight the need for additional research to understand the relationship between these factors and further examine underlying individual differences in these families.\n\n"]