TOPLINE: Early breastfeeding, particularly during the first 4 months of life, was associated with fewer emotional and behavioural problems in children at the age of 4 years. Children breastfed for this duration had reduced internalising and externalising problems, aggressive behaviour, and symptoms related to autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD). No additional benefit was observed beyond 8 months of breastfeeding.