One report, published in 2008, was carried out by scientists intrigued by anecdotal reports that Jewish children in Israel rarely suffered from peanut allergies. Dr. Gideon Lack, the senior author of the study and a professor of pediatric allergy at King’s College London, compared the allergy rates of Israeli Jewish children with those of Jewish children in Britain, and found that British children were 10 times as likely to have peanut allergies as Israeli children, a disparity that could not be explained by difference in genetic background, socioeconomic class or tendency to develop other allergies.

One of the main differences between the two populations was that starting in infancy, Israeli children ate foods containing peanuts, often in the form of Bamba, a popular peanut-butter puffed corn snack that has the consistency of a cheese puff but is 50 percent peanuts, according to the manufacturer, Osem Group. Was it possible that early exposure to peanuts actually protected the Israeli kids from allergies?

Dr. Lack and fellow scientists tested the hypothesis in a large clinical trial in England. They recruited hundreds of infants aged 4 to 11 months, all of whom were deemed at high risk of developing a peanut allergy because they had eczema or an allergy to eggs. After running skin-prick tests on the babies and excluding those who were already allergic to peanuts, they randomly assigned some babies to be regularly fed peanut products, and others to be denied all peanut-containing foods.

By the time they turned 5, only 1.9 percent of 530 allergy-prone children who had been fed peanuts had developed an allergy, compared with 13.7 percent of the children who were denied peanuts. Among another group of 98 babies who were more sensitive to peanuts at the start of the study, 10 percent of those who were given peanuts developed an allergy, compared with 35 percent of those denied peanuts. The findings, published in The New England Journal of Medicine in 2015, “shook the foundation of the food allergy world,” Dr. Greenhawt said.

The new guidelines divide children by risk. Low-risk infants, who don’t have eczema or an egg allergy and who have started solid foods, can be introduced to peanut-containing foods around 6 months at home by their parents. So can moderate risk children, who have mild eczema.

High-risk infants, who have severe eczema or an egg allergy, should be introduced to peanut-containing foods as early as 4 to 6 months, after they start other solid foods and are evaluated by a doctor for safety.

If your baby is determined to be high-risk, the guidelines recommend an evaluation by an allergy specialist, who may order allergy testing and introduce a peanut food in the doctor’s office, Dr. Greenhawt said. Even if allergy tests show sensitivity to peanuts, the baby isn’t necessarily allergic and may benefit from eating peanut foods, he said. A baby with a stronger reaction to the skin test may already be allergic, however, and the doctor may decide to recommend complete avoidance.