For decades, scientists have been trying to figure out how to explain ballooning rates of food allergies and asthma. Peanut allergies, which now seem to plague elementary schools, have been particularly confounding.

The incidence of peanut and tree nut allergies in the United States quadrupled in the past 13 years, according to The New England Journal of Medicine. Young children in the United States are now allergic at what scientists believe are unprecedented levels—that's food allergies generally, and peanut allergies specifically. Peanut allergy is the country's leading food-allergy cause of anaphylaxis and death.

But what long seemed to be an inexplicable and growing threat now seems "extremely simple," said Scott Sicherer, a pediatrician and a researcher at the Jaffe Food Allergy Institute at Mount Sinai. "We might have caused this increase by telling people not to eat these things."

Sicherer was part of a team of doctors who helped shape a startling American Academy of Pediatrics recommendation in 2008 that said parents didn't have to wait to introduce foods like peanut butter to otherwise healthy babies. The new guideline represented a reversal from an earlier recommendation, established in 2000, that parents should wait until a child turned 3 years old to try foods associated with high risk of allergies. While doctors once believed that infant immune systems "weren't ready" for foods like nuts, eggs, and fish, Sicherer and other doctors began hypothesizing that avoiding such foods might prompt a more dramatic immune response later. "Maybe waiting longer and longer for the baby to eat that food, that was counterproductive, maybe their immune system was better off seeing it earlier," Sicherer told me. "We erased the previous recommendations because we looked at the literature and said, 'Wait a minute, there's nothing supporting it and maybe it's counterproductive.'"

There's now mounting evidence that Sicherer and his colleagues were right.

The New England Journal of Medicine on Monday published the results of a long-awaited landmark study on peanut allergies that finds the introduction of peanuts to children in infancy "significantly decreased" a child's likelihood of developing a peanut allergy. The study concludes:

More than 500 infants at high risk for peanut allergy were randomly assigned to receive peanut products (consumption group) or to avoid them (avoidance group) ... At 5 years of age, the children were given a peanut challenge to determine the prevalence of peanut allergy. The results are striking—overall, the prevalence of peanut allergy in the peanut-avoidance group was 17.2 percent as compared with 3.2 percent in the consumption group.

But the results, while striking, don't offer simple choices for parents faced with the question of when to introduce notorious foods to their babies.

"It does leave people with questions like, 'When is the best time to do this?' And no one has a perfect answer," Sicherer said. "Every single person doesn't behave similarly. Maybe for some babies it's better that they wait, and maybe for some babies it's better for them to get it earlier, and maybe for lots of other babies it's somewhere in between."

For parents trying to decide how best to protect their children, the latest findings about peanut allergies evoke a similarly stunning reversal in pediatric medicine—the 1994 recommendation to put babies to sleep on their backs, which remains in place and is widely credited with a dramatic decline in sudden infant death syndrome. But the question of when to introduce foods is more complex. Even if ideal timing could be discerned, it would depend on an individual's medical history and risk factors. It should be noted, too, that the latest peanut study focused just on babies who were already at high risk for developing a peanut allergy—either because immediate family members were allergic, the babies had severe eczema, or other risk factors. And though the findings may eventually prompt new guidelines about the best timing for specific food introduction, the American Academy of Pediatrics is not advocating feeding peanut butter to infants. Its position since 2008 has been that there isn't convincing evidence to support waiting to introduce peanut butter to an otherwise healthy baby. (As always, parents should check with their doctors on this kind of stuff.)

The latest findings about early introduction to peanuts are only a small—if crucial—piece of a complicated and evolving global allergy complexion. That was true when doctors concluded peanuts should be avoided and it's true today. "I'm not going to say they were stupid [to suggest avoidance] because it might turn out we were stupid," Sicherer said. "It's a changing landscape."

As The New England Journal of Medicine puts it:

Given the results of this prospective, randomized trial, which clearly indicates that the early introduction of peanut dramatically decreases the risk of development of peanut allergy (approximately 70 to 80 percent), should the guidelines be changed? Should we recommend introducing peanuts to all infants before they reach 11 months of age? Unfortunately, the answer is not that simple, and many questions remain unanswered: Do infants need to ingest two grams of peanut protein (approximately eight peanuts) three times a week on a regular basis for five years, or will it suffice to consume lesser amounts on a more intermittent basis for a shorter period of time? If regular peanut consumption is discontinued for a prolonged period, will tolerance persist? Can the findings of the LEAP study be applied to other foods, such as milk, eggs, and tree nuts?

Researchers have looked with interest to countries outside the United States, especially Israel, where babies routinely snack on Cheez Doodle-esque peanut puffs called Bamba. A 2008 study in the Journal of Allergy and Clinical Immunology found that babies in the United Kingdom—where peanut avoidance was encouraged—had 10 times higher prevalence of peanut allergies than kids in Israel, many of whom had been eating Bamba since infancy.

There are likely other factors at play. American babies haven't just been avoiding peanuts, they've been avoiding all kinds of germs and environments, for better and for worse. Which helps explain the theory that some exposure to different people and materials may help ward off allergies.

"A larger family is more protective. Farm living is more protective. Being around more people might be more protective," Sicherer said. "We still have germs but we don't have the variety of germs and illnesses we had before. You wouldn't want to have all the illnesses we used to get. But ultimately, there may have been a tradeoff there."