Does Thumb-Sucking Prevent Allergies?

My four-month-old started sucking his thumb more regularly and vigorously (he doesn’t use a pacifier). When I tried to find out if this is okay, I came across an article that stated thumb-sucking reduces the likelihood of him getting hay fever or a dust allergy. As I have bad hay fever, I would welcome sparing my son from this. How good is the data to support that thumb-sucking reduces hay fever?

—Mum wishing to eradicate hay fever

This is such an interesting question. At face value, it may seem totally implausible, but in fact there are some reasons that such a link might exist. In particular, there is a theory — sometimes called the “hygiene hypothesis” — that exposure to more microorganisms in youth lowers your risk of developing allergies.

Since sucking thumbs and biting nails introduces microorganisms, this link seems like it could be there.

There is one study that tests this. The authors use a longitudinal dataset covering about 1,000 people to look at the link between thumb-sucking and nail-biting as a young child and allergies at age 13 and then age 32. They find that at both ages, kids who suck thumbs or bite nails have a lower risk of reacting to an allergy test and a lower risk of having allergies to pets and grass.

Overall, the reduction in risk is 30% to 40%, with the lowest risk for the kids who both bite their nails and suck their thumbs.

This is only one study, and I would not want to overstate the conclusions. And of course, there are some downsides to thumb-sucking (like the introduction of germs and the impacts on teeth). But if you’re looking for a silver lining, it’s there.