Germs protect from allergies, biting nails helps

(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at:, or follow me on Twitter.

A very interesting theory:

When they tested at 13 for allergies to common things such as dust, grass, cats, dogs and molds, they found that 38% of those who had an “oral habit” tested positive — whereas 49% of those who didn’t suck their thumbs or bite their nails tested positive. This “protection” was still there at 32.

This fits with the “hygiene hypothesis,” which says that when children are exposed to germs early in life, their immune system gets trained to attack germs, rather than attacking itself as we see in allergies, asthma, and eczema (of note, the researchers didn’t find protection against asthma or hay fever, and didn’t report a measure of eczema). This hypothesis doesn’t explain everything we see about allergies and other examples of the body attacking itself, but it certainly may play a role — and when kids suck their thumbs or bite their nails, they do put all sorts of new germs into their mouths and therefore their bodies.

There can be downsides to thumb sucking, as it can lead to problems from pushing secondary teeth as they come in. But those don’t come in until later, when children are in elementary school. Most children stop sucking their thumb by themselves before they get to elementary school. The American Dental Association recommends that parents start encouraging their children to stop sucking their thumb after age 4 — but even then, they advocate a gentle, positive approach. Many children suck to relieve anxiety, and making them anxious about sucking can backfire! Instead, praise children for not doing it — and help them find other ways of soothing themselves. Putting a Band-Aid on the thumb can help them remember; at night, using a sock or something similar can help with the child who sucks their thumb without realizing it. But again, this is only an issue for older children.

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