Mothers may influence the mood and behaviour of their babies through their breast milk, researchers say.

There's growing evidence that mother's milk doesn't just affect the growth of a baby's body "but also areas of their brain that shape their motivations, their emotions, and therefore their behavioural activity," says Katie Hinde, an assistant professor of human evolutionary biology at Harvard University.

In a paper published in the journal Evolution, Medicine and Public Health, Hinde and two other researchers propose a way in which the composition of breast milk could influence a baby's brain and behaviour.

If food is scarce or there are a lot of predators around, it may be better for a mother to have a baby that is calmer and focuses on growing rather than one that is very active and playful, Hinde told CBC Radio's Quirks & Quarks in an interview that airs Saturday.

It may be possible to influence a baby's activity level by changing the composition of the milk to affect the bacteria in the infant's gut, she added.

Breast milk contains a lot of sugars that infants can't digest, but that feed bacteria that live in human intestines. Those bacteria don't just help digest food, said Hinde.

"They can release chemical signals that travel to the infant's brain and shape neurodevelopment."

Studies in mice and rats show that signals released by bacteria in the gut can affect how sociable and anxious a baby is.

A recent study in human children also shows a link between the type of bacteria in their guts and their temperament, the researchers note in the paper.

"Is the mother then manipulating her milk to determine what those bacteria are doing?" asked Quirks & Quarks host Bob McDonald.

"That is exactly our question," Hinde said.

She hopes other researchers who read the paper will start conducting experiments to find out.