Children are told repeatedly not to bite their nails or suck their thumbs to avoid exposure to germs, but scientists have found out that these two 'bad' habits may also have a positive health effect. Indeed, they may reduce sensitivity to a number of common allergies.

The study, published in the journal Pediatrics, is based on the notion that nail-biting and thumb-sucking can increase microbial exposure.

Obviously, this can be problematic for the health of children, but the researchers also wanted to test another, less obvious hypothesis. They investigated whether germs could make the immune system more resistant, resulting in a reduced risk of developing allergic reactions.

Atopic sensitization in later life

In the study, the scientists focused on the link between nail-biting and thumb-sucking and the development of allergies to dust mites, grass, cats, dogs, horses or airborne fungi.

baby sucking thumb
Children who suck their thumbs appear less likely to develop allergies. Istock

They collected data from a longitudinal birth cohort of more than 1,000 New Zealand children. They assessed two criteria: how many children sucked their thumbs or bit their nails at 5, 7, 9 and 11 and whether they displayed signs of 'atopic sensitization' at age 13 – a predisposition toward developing allergic hypersensitivity. The researchers found that 31% of children were frequent thumb suckers or nail biters between the age of 5 and 11.

Allergy to dust mites is a common allergy in children Wikimedia Commons/FDA

A common skin-prick test allowed the scientists to assess atopic sensitization for a variety of allergies. They also discovered that among all children at 13 years old, 45% on average showed signs of atopic sensitization. But among children who either bit their nails or sucked their thumbs, this proportion decreased, with only 40% sensitive to allergies.

Having both 'bad habits' was associated to a further reduction in risk, as only 31% of these children had allergies.

This trend was sustained into adulthood, and showed no difference depending on smoking in the household, ownership of cats or dogs, or exposure to house dust mites.

"Our findings are consistent with the hygiene theory that early exposure to dirt or germs reduces the risk of developing allergies," says lead author Malcolm Sears, from McMaster University. "While we don't recommend that these habits should be encouraged, there does appear to be a positive side to these habits."