That New Study Linking Nail Biting To Fewer Allergies Doesn't Mean What You Think It Does

A new study finds that biting one’s nails in childhood, while gross, may lower one’s risk of allergies in adulthood.

The study’s authors say that the findings support the unproven “hygiene hypothesis,” which suggests if a child’s environment is too clean, their immune system isn’t properly trained to recognize and fight off germs. Proponents of this hypothesis say an underdeveloped immune system could then put the child at a higher risk of developing hay fever, asthma, eczema and other medical conditions later on in life.

However, it’s important to note that this study didn’t find evidence of exactly the same thing. While researchers did find that kids with certain hand-mouth fixations had lower risk of testing positive for certain allergies on skin prick tests, they weren’t less likely to develop actual allergic diseases. This is because the skin prick test measures only sensitization to an allergen, and it can’t actually predict whether or not the substance you’re allergic to will actually cause a reaction in the nose (resulting in hay fever) or in the lungs (resulting in asthma). In other words, just because you test positive for a cat allergy on a skin prick test, that doesn’t mean you’re going to start sneezing and coughing in the presence of a cat.

“Although thumb-suckers and nail-biters had fewer allergies on skin testing, we found no difference in their risk for developing allergic diseases such as asthma or hay fever,” said Stephanie Lynch, a medical student at the Dunedin School of Medicine in New Zealand, in a statement about her research.

Because of the lack of any real health benefit, Lynch and co-author Dr. Robert Hancox, also of The Dunedin School of Medicine, say that kids shouldn’t be encouraged to suck their thumbs or bite their nails over these findings.

How the study worked

Researchers followed about 1,000 people who had been born in 1972 and 1973 and checked in on their thumb-sucking and nail-biting habits at ages 5,7, 9 and 11 years old.

They found that kids who had either sucked their thumbs or bitten their nails had a lower rate of testing positive for an allergy with the skin prick test at age 13. Specifically, only 38 percent of kids who had either sucked their thumbs or bit their nails tested allergic for at least one common allergen (like dog or cat dander, dust mites and grass), as opposed to 49 percent of kids who did not suck their thumbs or bite their nails. And among kids who both bit their nails and sucked their thumb, only 31 percent tested positive for an allergen.

Except for the dose-responsive relationship that found kids who both sucked their thumbs and bit their nails were even less likely to test positive on an allergy skin prick, these associations generally held true 19 years later when the participants were pricked with common allergens for a second test.

And they remained even after controlling for potentially confounding factors like sex, family history of allergies, pet ownership, breastfeeding and parental smoking. The theory is that more exposure to microbes under the nails or on the thumb may have prepared their immune systems to handle allergens better than kids who didn’t bite their nails or suck their thumbs.

What we should take away from this study

This study aligns with past research that finds, for instance, that babies whose parents suck their pacifiers to “clean” them (passing oral microbes from the adult to the infant) are less likely to develop eczema and asthma in toddlerhood.

Malcolm Sears, a professor of medicine at McMaster University in Canada and a co-author on the paper, wants to emphasize that this paper wasn’t designed to recommend whether or not a parent should encourage nail biting or thumb sucking. Instead, he simply described it as an observation that adds to scientific knowledge of the potential causal connection between exposure to microbes in childhood and a healthier immune system later in life.

”The bottom line, I would say, is that what the paper tells us is that a little exposure to dirt is not necessarily a bad thing.” ― Malcolm Sears

“We don’t go out and tell people to eat a handful of dirt, even though we know a little dirt exposure, whether through the microbes or other substances, does influence the immune system in making it more tolerant to allergens,” he said in a phone call with HuffPost. In the same way, Sears explained, parents shouldn’t take this study as license to encourage nail biting or thumb sucking in their children.

In fact, Sears himself is conducting another long-term study in Canada that follows pregnant moms and their children in order to examine the potential contributors to asthma and allergy risk. He doesn’t plan on asking about thumb sucking and nail biting because there’s no practical application to the finding.

This is because, as the paper also points out, there are documented health risks to sucking thumbs and biting nails, and they include teeth misalignment, gum injury and finger infections.

”The bottom line, I would say, is that what the paper tells us is that a little exposure to dirt is not necessarily a bad thing,” Sears concluded.

The research was published in the August issue of the journal Pediatrics.