Breastfeeding an infant for the first year of life adds four points to their IQ score, the US research suggests.
Children of mothers who sustained breastfeeding for longer had better receptive language at age three and higher verbal and non-verbal intelligence at age seven.
Test scores rose for every extra month of breastfeeding up to 12 months.
Researchers examined data from 1,312 mothers and children, including responses to questionnaires about breastfeeding conducted when the child was aged six and 12 months. Children then underwent cognitive tests at ages three and seven.
Authors said the results 'support a causal relationship of breastfeeding in infancy with receptive language at age three and with verbal and non-verbal IQ at school age'.
They said the findings also support international recommendations to breastfeed a child exclusively up to six months and to continue breastfeeding to one year.