A hormone in mothers' milk may affect the social behaviour of children long after weaning, new research suggests.
Scientists tested the effects of the stress hormone cortisol in a study of 26 female rhesus monkeys and their infants.
Babies who were suckled on milk containing higher levels of the hormone appeared to be less impulsive at the age of six months.
Higher cortisol concentrations were also associated with less initiation of social behaviours such as grooming and play, but only in male offspring.
Previous studies have linked high cortisol levels in milk to a more nervous temperament in both monkeys and humans.
Results of the research, led by Dr Amanda Dettmer, from the US National Institute of Child Health and Human Development, were presented at the Society of Neuroscience annual meeting in Chicago.
In a summary of the findings, the scientists wrote: "Collectively, our results point to a role for hormones in mother's milk, beginning at birth, in subsequent infant neurological and behavioural development. Future studies will be able to draw upon these results to determine the mechanisms for this type of programming."
Impulsivity in the baby monkeys was tested by tempting them with marshmallows.
Further research on mothers' milk could reveal early biological mechanisms involved in brain development and lead to better infant formula, said the team.