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Kids Who Bite Their Nails and Suck Their Thumbs Develop Fewer Allergies

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Parents, you can stop fretting about your child’s disgusting habits. An analysis of more than 1,000 kids between the ages of 5 and 11 reveals that nail-biters and thumb-suckers are less likely to develop allergic sensitivities later on in life.

New research published in the journal Pediatrics shows that children with these habits are less likely to develop allergies to house dust mites, grass, cats, dogs, or airborne fungi. It’s further evidence in support of the so-called “hygiene hypothesis,” which suggests that childhood exposure to microbial organisms and other nasties alters immune function, resulting in a decreased risk of developing allergies.


“While we don’t recommend that these habits should be encouraged, there does appear to be a positive side to these habits,” study co-author Malcolm Sears of McMaster University said in a statement.

For the study, the researchers took data from the ongoing Dunedin Multidisciplinary Study, which is following the progress of 1,037 participants born in Dunedin, New Zealand from 1972 to 1973. Back when this study got started, parents reported their children’s thumb-sucking and nail-biting habits when they were 5, 7, 9, and 11 years old. When these children turned 13, and again at 32, they were checked for atopic sensitization—a skin prick test that detects a sensitivity to at least one common allergen.


Slightly less than a third of all children enrolled in the study were frequent nail biters or thumb suckers. By the time these kids reached their teenage years, nearly half showed an atopic sensitization of some sort—so oral habit or not, half of these kids still developed an allergy. But for those with one oral habit, their chance of developing an allergy dropped to 40 percent. And for those with both habits, this figure plummeted to 31 percent. That’s significant.

This trend held true into adulthood, and was not altered by factors such as smoking in the household, owning cats or dogs, exposure to house dust mites, or breastfeeding. And this study did not find associations between nail-biting and thumb-sucking with the onset of asthma or hay fever.

Oral habits such as thumb-sucking and nail-biting are likely providing a channel through which microbial organisms can enter the body, particularly at a young age. But it would be a mistake to deliberately expose our children to these pathogens, which could lead to sickness. Moreover, the authors of the new study aren’t suggesting that children be encouraged to take up these habits, saying there’s no clear-cut evidence of a true health benefit.


Relatedly, previous studies have shown that most peanut allergies can be staved off by not withholding peanuts from children before the age of three. This makes sense when you think about it. An allergic reaction is basically the body’s immune system freaking out when it detects a potentially dangerous foreign agent. By “training” the immune system at a young age to recognize certain substances as being harmless—whether they be benign microbes or peanut particles—we may be able to reduce our chances of developing an allergy in the future.