It might seem like a dirty habit but children who suck their thumbs or bite their nails are 15 percent less likely to have allergy sensitivities in adulthood.
A long-ranging study of more than 1000 children in Dunedin, New Zealand followed their lives from age five to 13 to 32, using skin-prick tests to determine allergy sensitivity.
The results, published in U.S. journal Pediatrics found 49 percent of children who didn't suck their thumbs or bite their nails had developed an allergy by age 13, compared to 38 percent of children who did.
What's more, the kids who both bit their nails and sucked their thumbs had an even lower risk of allergy with 31 percent testing positive.
When they were again tested at 32, the allergies remained the same.
Why would thumb sucking and nail biting reduce allergies?
These two activities involve putting your fingers in your mouth, increasing exposure to germs, or 'microbial organisms'.
Study lead author professor Bob Hancox said this exposure may alter the way the immune system functioned so that children with these habits became less prone to developing allergy.
"The findings support the 'hygiene hypothesis', which suggests that being exposed to microbes as a child reduces your risk of developing allergies," Hancox said.
Dunedin Multidisciplinary Health and Development Research Unit researcher Stephanie Lynch said thumb sucking and nail biting had no effect on more serious allergy related conditions.
"Although thumb-suckers and nail-biters had fewer allergies on skin testing, we found no difference in their risk for developing allergic diseases such as asthma or hay fever," Lynch said.
Allergy and Anaphylaxis Australia president Maria Said told HuffPost Australia, however, that a skin-prick test was not evidence of an allergy disease like asthma or a food allergy but rather a sign of sensitivity.Suggest a correction