Infants and toddlers like to put their hands and other objects in their mouths. It’s what they do. The sense of touch is an extremely valuable tool which allows these young humans to explore their world. That goes for the highly innervated oral cavity, where taste buds provide an extra and very useful sensory feature. Most parents worry about where these young hands have been (on the carpet, in the dirt, in the diaper) right before they end up in the mouth. Toddlers, preschoolers, and young school-age children who suck their thumbs and bite their nails also cause concern among parents for many of the same reasons. Parental fear of various microbes gaining entry into their children’s bodies in these ways is enough to keep  companies that make hand sanitizers in business.

Maybe the time has come for parents to get rid of antibacterial soaps and hand sanitizers (and just use regular soap and water). The reason? A new, long-term study from New Zealand, published in this month’s Pediatrics, finds that children who suck their thumbs or bite their nails have lower rates of allergies when they are older. (The protection is greater if they do both!) Pediatrician Perri Klass credits the immune system:

The question of such a connection arose because of the so-called hygiene hypothesis, an idea originally formulated in 1989, that there may be a link between atopic disease — the revved-up action of the immune system responsible for eczema, asthma and allergy — and a lack of exposure to various microbes early in life. Some exposure to germs, the argument goes, may help program a child’s immune system to fight disease, rather than develop allergies…

These differences could not be explained by other factors that are associated with allergic risk. The researchers controlled for pets, parents with allergies, breast-feeding, socioeconomic status and more. But though the former thumb-suckers and nail-biters were less likely to show allergic sensitization, there was no significant difference in their likelihood of having asthma or hay fever.


As kids get older, Dr. Klass says, nail-biting and thumb-sucking present other problems for parents to fret about — teeth alignment, speech (articulation), teasing from peers — in addition to germs:

Thumb sucking, especially in an older child, can still be a problem if it interferes with the teeth, or causes infections on the fingers, or gets a child teased. Lynn Davidson, a developmental pediatrician who is an attending physician at the Children’s Hospital at Montefiore in the Bronx, and the author of a review article on thumb sucking, said she tends to be “very low-key” about thumb sucking, since children often stop on their own as they grow.

With older children, Dr. Davidson suggests that parents, if they are worried, should try to analyze when and why the child resorts to thumb sucking or nail biting, and then try behavioral techniques, like offering a child a foam ball to hold and squeeze at those moments. “In an older child you can use their input, ask, what would you do with your hands instead of putting them in your mouth,” she said.


More PediaBlog on the hygiene hypothesis here and on the microbiome we call “us” here.


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