Scientists know much less about cortisol’s effects on human babies, because it is not possible to run carefully controlled experiments on them the way Dr. Hinde and her colleagues do on monkeys. Still, what little they do know is intriguing.
In a 2013 study, for example, researchers found that babies who drank high-cortisol breast milk tended to be more fearful and harder to soothe. But scientists can’t say whether human babies are using the same strategy as baby monkeys are.
Deciphering the signals that babies detect in milk might lead someday to changes in formula. Right now, manufacturers try to replicate the nutrition, and even the microbes, in natural breast milk. But they may also need to consider the messages the formula is — or isn’t — sending.