The secret to mother-baby bonding might be breast milk, according to new research that determines that breast-feeding mothers are more likely than formula-feeding moms to bond with their infants in the months after they’re born. They also demonstrate stronger brain responses when they hear their baby cry, according to a study published in the May issue of the Journal of Child Psychology and Psychiatry.
Researchers at the Child Study Center at Yale University divided moms into two groups — nine breast-feeders and eight formula-feeders — and performed functional MRIs (fMRI) on them about a month after their babies were born.
(More on Time.com: Milk Banks vs. Milk Swaps: Breast Milk’s Latest Controversy)
While participants lay in a scanner and listened to clips of their own baby and an unknown child crying, researchers tracked what areas of their brains lit up. All mothers’ brains were more active when listening to their own baby’s cry, but the changes in the breast-feeding mothers’ relevant brain regions were far more significant.
The results are far from the last word: the sample was admittedly small, meaning the results may not necessarily apply to larger groups. But researchers took care to match the women in both groups as closely as possible in terms of age and education and income levels since other studies have showed that formula-feeding moms may fall into a lower socioeconomic bracket. As a result, the women in the fMRI study were a fairly homogeneous bunch: white, middle- to upper-class women with at least a college degree.
The study, which the researchers say is the first to link brain activity with maternal behavior, is likely to reinforce the convictions of breast-feeding mothers that they’re doing right by their babies and make formula-feeding moms squirm a little. But lead researcher Pilyoung Kim, a developmental psychologist at the National Institutes of Mental Health who worked at Yale when the research was conducted, cautions against feeling smug — or depressed.
“We want to suggest to people that a number of different factors may be related to breast-feeding mothers’ greater sensitivity to infant cries and infant cues, not just because they’re breast-feeding but because of hormonal levels or other personal experiences,” says Kim.
(More on Time.com: Babies on the Bottle: How Long Is Too Long?)
Hormonal levels, for example, vary between mothers who breast-feed and those who don’t. Oxytocin — the “love hormone” that helps nurture emotional bonding between infants and mothers and is involved in breast milk let-down — is higher in breast-feeding moms. But psychological aspects are also likely at play.
“Moms who decide to breast-feed might be reflecting a general tendency of mothers to be more empathetic to their infants or perhaps they were more able to bond easily with their fetus when they were making the decision about wanting to breast-feed,” says Kim.
(More on Time.com: Pumping at Work: The Government Asks Working Moms How They Do It)
Kim breast-fed, so she understands the unpleasantly competitive undertone that accompanies infant feeding choices. “I know how challenging this is,” she says. “We didn’t have a particular agenda in our minds when we decided to conduct this research.”