Stephanie Lynch, a medical student who is part of the current study, said that through the years, participants were asked detailed questions about their lives. From these answers, Lynch and study lead author Bob Hancox were able to collect the data on thumb-sucking and nail-biting.

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Children in the study were reported by their parents as being thumb-suckers or nail-biters at ages 5, 7, 9 and 11.

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These study participants were given skin-prick tests at ages 13 and 32 to detect allergies. These tests can reveal allergic reactions to 40 substances, according to the Mayo Clinic.

The results of the first skin-prick tests showed that 38 percent of children who were either thumb-suckers or nail-biters had allergies, compared with 49 percent of children who had neither habit.

Furthermore, children who both sucked their thumbs and bit their nails had an even lower risk of allergies — 31 percent.

Researchers found that results of skin-prick tests remained consistent when the second test was given at age 32. Gender, parental history, pet ownership and other factors seemed to have altered the outcomes very little.

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“The findings support the ‘hygiene hypothesis,’ which suggests that being exposed to microbes as a child reduces your risk of developing allergies,” Hancox said in a statement.

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Hirsh Komarow, a staff clinician at the National Institute of Allergy and Infectious Diseases, isn’t entirely convinced about the study’s conclusions. “It’s an interesting observation, but it needs more analysis,” Komarow said.

Some of the questions that he says are raised by the study include what kind of bacteria are on participants’ hands and in their stomachs.

Komarow said each person has a certain variety of species that are unique to that individual. He suggested that looking at flora on hands and in the gut could help make a better connection among thumb-sucking, nail-biting and allergies.

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“Guts and hands are two different areas,” he said. “You could compare hands of thumb-suckers to control people. There’s different flora on skin. You could compare flora of the gut. Is that different [between participants]? It would help you better analyze those groups.”

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Komarow also suggested that thumb-sucking and nail-biting could be indicative behaviors that either thwart or encourage allergic reactions. He said being part of a large family and being exposed to microbes from many siblings may affect a child’s allergic sensitization.

Still, Komarow and the New Zealand researchers agree that modern living has affected who gets allergies and who doesn’t. Less exposure to animals and vegetation seems to promote more allergies, Komarow says.

Lynch said she hopes the study encourages parents to think about letting their kids get a little dirty.