The Monell researchers have identified several sensitive periods for taste preference development. One is before three and a half months of age, which makes what the mother eats while pregnant and breast-feeding so important. “It’s our fundamental belief that during evolution, we as humans are exposed to flavors both in utero and via mother’s milk that are signals of things that will be in our diets as we grow up and learn about what flavors are acceptable based on those experiences,” said Gary Beauchamp, the director of the Monell Center. “Infants exposed to a variety of flavors in infancy are more willing to accept a variety of flavors, including flavors that are associated with various vegetables and so forth and that might lead to a more healthy eating style later on.”

There is another reason these exposures have a lifelong impact, he said: “This early exposure leads to an imprinting-like phenomenon such that those flavors are not only preferred but they take on an emotional attachment.”

This puts babies fed formula at a disadvantage because the flavors in packaged formula never change. But according to Ms. Mennella, the opportunity to expose those babies to a range of flavors is not lost. “Just because you’re formula-fed, it’s not hopeless,” she said. “Babies learn through repeated exposure, so the more varied the diet, the more likely they’ll be to accept a novel food.”

Another recent study conducted at the FoodPlus research center at the University of Adelaide in South Australia found that exposure to a maternal junk food diet (defined in the study as any food that was energy dense, highly palatable and had a high fat content) results in children with a preference for these same foods. In a rodent model, the study found that being exposed to too much junk food in utero and through breast milk leads offspring to develop a reward pathway in the brain that is less sensitive than normal. Mothers who were fed foods like Froot Loops, Cheetos and Nutella during pregnancy had offspring that showed increased expression of the gene for an opioid receptor, which resulted in a desensitization to sweet and fatty foods. “The best way to think about how having a desensitized reward pathway would affect you is to use the analogy of somebody who is addicted to drugs,” Jessica R. Gugusheff, a Ph.D. candidate at FoodPlus and the lead author of the study, wrote in an email. “When someone is addicted to drugs they become less sensitive to the effects of that drug, so they have to increase the dose to get the same high,” she wrote. “In a similar way, by having a desensitized reward pathway, offspring exposed to junk food before birth have to eat more junk food to get the same good feelings.”

Ms. Mennella at Monell has also done research on reward pathways for sweetness and has found that sweet flavors have an analgesic effect on babies and children such that babies will cry less and children will leave their hand in a cold water bath for longer periods with sweet flavors in their mouths. Ms. Mennella has also found that in obese children, while the level of sweet they prefer is the same as that of normal-weight children, sweet flavors are not as effective as an analgesic. “I hypothesized maybe it’s because of some disruption in the opioid system, so maybe they need more sweet to get the same effect,” she said.