Kids love to put their hands in their mouths. Up to 25% of children suck their thumb or bite their nails, despite warnings not to; dentists and dermatologists caution that thumb sucking can adversely affect gum and mouth health and even lead to skin conditions that increase the risk of infections. And nail biting—not the most hygienic habit, as our hands are teeming with bacteria—often continues into adulthood.
But in a study published in the journal Pediatrics, researchers from New Zealand and Canada found that children who suck their thumbs or bite their nails are less likely to have allergic-type reactions to a variety of things, including pets and mites, than children who don’t. In fact, kids who engage in both habits have the lowest positive tests on skin-prick tests for allergies.
Researchers followed a group of about 1,000 people from birth until age 32 and tested them periodically for allergies using a skin-prick test. While testing positive did indicate they were allergic, it didn’t necessarily mean that the person would have severe reactions to the allergens, like rashes, inflammation, sneezing or wheezing.
About half of the people who didn’t suck their thumb or bite their nails as children tested positive for allergies at age 32. Children who had at least one of the two habits were 40% less likely to test positive as adults, and children who did both showed the lowest rates of reactions to the allergens—31%—as adults.