The Scientific Defense of Nail-Biting That Nail-Biters Have Been Waiting For

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Photo: Nina Shannon

There comes a point, it seems fair to say, when the methods for breaking a given habit become more unpleasant than the habit itself. A quick internet search of how to stop kids from sucking their thumbs, for example, turns up a host of tricks that range from creatively gross (coating the fingernails in nasty, bitter-tasting polish) to just plain mean (taping gloves to their hands). They may work more quickly than some other, gentler methods, but you kind of have to wonder if the trade-off is really worth it.

And besides, thumb-suckers — and nail-biters, for that matter — have a leg to stand on when they say that their habits aren’t all that bad, especially if they start early in life (which, for thumb-sucking in particular, you kind of hope that would be the case). According to a new study in the journal Pediatrics, these two behaviors may have a small protective effect against common allergies.

The study authors followed around 1,000 kids from age 5 through age 11, asking their parents to report back on their thumb-sucking and nail-biting habits every two years, and administered allergy tests when they turned 13 and 32. Over the course of the study, 31 percent of the kids had at least one point where the parents said that they regularly sucked their thumb or bit their nails (or both). At both points down the road, the same 31 percent were more likely to pass their allergy tests with a clean bill of health, and the kids who had both habits were the likeliest of all, even after the authors controlled for other factors like having pets, parents’ smoking, and socioeconomic status. (They also tested for asthma and hay fever, but didn’t find the same patterns.)

The researchers emphasized that their findings shouldn’t be spun into any kind of homegrown allergy-prevention method — “We don’t recommend that these habits should be encouraged,” study co-author Malcolm Sears, a professor of medicine at Canada’s McMaster University, said in a statement — especially because both habits can also have long-lasting downsides. In some cases, too much nail-biting can permanently damage the nail tissue; thumb-sucking, meanwhile, can lead to dental problems, with the pressure from the mouth’s sucking motion shifting the teeth and jaw.

But the study is a piece of evidence for what’s known as the “hygiene hypothesis”: the idea that our clean, germ-free modern lifestyle may actually be harming us in the long run. “As we move from a developing to a developed environment, we have less exposure to microbes in general. We treat every symptom with antibiotics; we’ve changed our gut microflora with the diets that we eat,” the immunologist Kathleen Barnes explained in a 2011 Scientific American interview. “The hypothesis is that as we make the shift from dirt to sterile that you’re changing the direction of your immune response,” a phenomenon that’s been used to explain the rise in atopic diseases, or conditions characterized by some sort of hypersensitivity (like allergies, asthma, and eczema, among other things).

The key here, as in so many things, seems to be moderation. They say a little dirt is good for kids, for example, but no one’s advocating for the end of baths (no adults, anyway). In the same way, it’s probably a bad idea to start shoving your kid’s fingers in their mouth in an attempt to recalibrate their immune system — but it’s also probably not great to panic about the germs they ingest each time they lift their thumb to their lips. A better approach, it seems, might be to put a cap back on the bitter nail polish, wait to see whether they grow out of it, and in the meantime take comfort in the fact that it just might be conferring a little bit of benefit.

There’s Now a Scientific Defense of Nail-Biting