Children who suck their thumbs or bite their nails are less likely than others to have allergies later, according to a new study.

According to the “hygiene hypothesis,” early life exposure to microbes may decrease the risk of developing allergies, which may explain why kids with oral habits have fewer allergies as adults, although this idea is still controversial, the authors write.

In this observational study it was not possible to prove that thumb sucking or nail biting causes a reduction in allergies, said senior author Dr. Robert J. Hancox of the Dunedin School of Medicine at the University of Otago in Dunedin, New Zealand.

“It is always possible that there is another explanation for this association, although it is difficult to suggest what this might be,” he told Reuters Health by email. The study followed more than 1,000 children born in 1972-1973 whose parents reported their nail biting and thumb-sucking tendencies at ages five, seven, nine and 11.

The same kids were tested for allergies with pin-prick tests at ages 13 and 32. At some point in childhood, almost a third of the kids had frequently bitten their nails or sucked their thumbs. This group was less likely to have reactions to common allergens at ages 13 and 32 compared to the group that never engaged in the habits.

Almost 40 percent of girls and 50 percent of boys had an allergy at age 13, which increased to about 60 percent of both men and women at age 32. Among 13 year olds, allergy rates were 38 percent in kids who bit their nails or sucked a thumb, versus 50 percent in kids who did not. The relationship persisted to age 32.

The researchers accounted for parental allergies, breastfeeding, pet ownership, household crowding, socioeconomic status, and parental smoking. The risk of asthma was not tied to childhood oral habits, however, according to the report in Pediatrics.

“We used skin tests to test for allergies,” Hancox said. “Although a lot of asthma is due to allergies, sometimes it is not, so maybe this is why we didn’t find the association.”

The suggestion that thumb sucking and nail biting may reduce the risk of allergies later needs to be supported by different studies in different geographical regions around the globe, said Emmanuel Prokopakis of the University of Crete School of Medicine in Greece, who was not part of the new study.

“I don't think that we know enough to make a recommendation yet,” Hancox said. “Many parents worry about the dental effects of thumb sucking in particular and we don't wish to dismiss these concerns, but if a child has a habit that is difficult to break, maybe there is some consolation in the fact that there may be a reduction in the risk of allergies.”