Stress hormone cortisol found in mother’s milk could affect social behaviour of children after weaning

Scientists studied the effects of the hormone on female rhesus monkeys and their infants

'Our results point to a role for hormones in mother’s milk, beginning at birth, in subsequent infant neurological and behavioural development' (PA)

Cortisol, the stress hormone found in mother’s milk, may affect the social behaviour of children long after they are weaned, new research suggests.

Research carried out by the US National Institute of Child Health and Human Development tested the effects of cortisol levels on the 26 female rhesus monkey infants.

Scientists found that the babies suckled on milk containing higher levels of cortisol appeared to be less impulsive at the age of six months.

The baby monkeys were tempted with marshmellows by the scientists to test their impulsivity.

Scientists also found an association between higher levels of cortisol and less initiation of social behaviours in the male babies, such as grooming and play.

Last year it was discovered that the levels of cortisol in breast milk can vary between mothers and that the hormone affects sons and daughters differently, while previous studies have linked high levels of the hormone to a more nervous temperament in both monkeys and humans.

“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 of this type of programming,” the researchers wrote.

The scientists believe it could be possible to discover the early biological mechanisms involved in brain development through further research into mother’s milk, and also to the development of better instant formula.

Additional reporting by PA