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Reuters/Fabrizio Bensch
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Allergy study: Nail-biting and thumb-sucking are great for kids in one way

By Kelsey Kennedy

Good news for youngsters who stick their dirty fingers in their mouths, and their parents. Researchers in New Zealand have found that kids who bite their nails or suck their thumbs are less likely to develop allergies to common allergens like pollen and dust mites.

The study, published July 11 in the journal Pediatrics, analyzes data from a New Zealand study that tracked the health of over 1,000 people from birth to age 38.

When the group turned 13 and 32, all were tested for common allergies. Kids who sucked their thumbs or bit their nails as children (between 5 and 11 years old) were 33% less likely to react to common allergens like dust mites, cats, and grass at age 13. By age 32, they were 39% less likely to react.

This finding fits with the widely-held hygiene hypothesis, which holds that early exposure to microbes helps prevent the development of allergies. When kids stick their fingers in their mouths, they’re exposed to whatever microbes they picked up throughout the day. Kids who grow up around animals, or with a lot of siblings (pdf), are also more exposed to microbes during their early years, and get similar benefits. This time period is critical for the immune system, because that’s when it learns to recognize which common allergens are harmful and which are harmless.

The New Zealand study only showed that thumb-sucking and nail-biting can fend off allergies—not other common immune system reactions like asthma or hay fever. In fact, kids with those hand habits grew up to have the same rates of asthma and hay fever as everyone else. And since constantly putting your hands in your mouth can cause other health problems, like misaligned teeth or infections, it’s not something most parents will want to encourage. But at least now we know there’s also a benefit.