Kids who constantly shove their fingers into their mouths may actually be doing themselves – and more specifically, their microbiomes – a favor, by exposing themselves to as many germs as possible. Though this is precisely what so many parents fear when they see their ill-mannered offspring sucking their thumbs or biting their nails, new research suggests that this is actually good for their health, and reduces their chance of developing allergies.

The authors of the study – which appears in the journal Pediatrics – were interested in testing out the long-standing but as yet unproven hygiene hypothesis. Introduced several decades ago, this theory states that children from larger families are likely to come into contact with more microbes at a young age, causing their bodies to develop a strong defense mechanism known as a T-helper immune response. As a result, they are said to be less likely to develop atopy, which refers to a hyperallergic condition that often leads to asthma or eczema.

Since kids aren’t exactly famous for having clean hands, nail-biting and thumb-sucking tend to introduce them to a wide range of microorganisms, including Enterobacteriaceae – the family of bacteria that includes E. coli – and intestinal parasites. This, the authors suspect, may increase the diversity of children’s microbiomes, ultimately reducing their susceptibility to atopy.

They therefore recruited a cohort of more than 1,000 participants, all of whom were born in 1972 to 1973. Parents of these participants were originally asked about their child's thumb-sucking and nail-biting habits at age 5, 7, 9, and 11.

At the age of 13, each person was then given a skin test for atopy, before being tested again at age 32. During these examinations, they were evaluated for a number of allergies, including dust mites, grass, several animals, and a range of common molds.

During the first round of testing, 45 percent of 13-year-olds tested positive for atopy, although only 40 percent of those who either bit their nails or sucked their thumbs were found to have the condition. For those with both of these habits, this figure dropped to just 31 percent. The same trend was found 19 years later when participants were tested again, indicating that this susceptibility to allergies persists into adulthood.

In a statement, study co-author Malcolm Sears explained that the findings “are consistent with the hygiene theory that early exposure to dirt or germs reduces the risk of developing allergies.” Parents may therefore want to think twice before scorning their kids for sticking their fingers in their mouths. "While we don't recommend that these habits should be encouraged, there does appear to be a positive side to these habits," insists Sears.

^{Image: Atopy often manifests itself as asthma or eczema. bubutu/Shutterstock}