Thumb-sucking and nail-biting are common behaviors in children that likely increase exposure to microbes. The hygiene hypothesis suggests that children with early exposure to a more diverse array of microbes may have a decreased risk of developing atopic diseases. The authors of this study examine the relationship between thumb-sucking and nail-biting habits in children and the development of atopy.

This was a population-based birth cohort with 1037 subjects born in Dunedin, New Zealand. Subjects were managed to age 38 years.

Parents were asked to report their children’s thumb-sucking and nail-biting behaviors at ages 5, 7, 9, and 11 years. Skin prick testing for common aeroallergens was completed at age 13 years and again at age 32 years. Participants were regarded as having atopic sensitization if they had 1 or more positive skin prick test results. Asthma status was based on reported diagnosis and symptoms. The presence of hay fever was based on self-report at age 13 and 32 years. Logistic regression analysis of thumb-sucking and nail-biting behaviors and atopic sensitization, asthma, and hay fever was performed for subjects at ages 13 and 32 years.

Thumb-sucking and/or nail-biting was reported in 317 of 1013 children (31%). Skin prick testing was completed in 724 children at age 13 years. Atopic sensitization was significantly less prevalent in those with either or both thumb-sucking or nail-biting behaviors compared with those without those behaviors at both 13 (odds ratio 0.67; 95% confidence interval 0.48–0.92) and 32 years of age (odds ratio 0.61; 95% confidence interval 0.46–0.81). The prevalence of atopic sensitization was 49% in children with neither behavior, 40% in those with 1 behavior, and 31% in those with both behaviors. Children with a history of either behavior had fewer total positive skin prick test results and smaller wheal sizes at age 13 and 32 years. However, there was no significant difference in the prevalence of asthma or hay fever among those with thumb-sucking or nail-biting behaviors compared with participants without those habits.

The authors concluded that thumb-sucking and nail-biting behaviors in early childhood were associated with a decreased risk of atopic sensitization at age 13 years, with continuation of this effect through age 32 years. There was no protective effect of these behaviors on the development of asthma or hay fever. The authors suggested that this could be due to factors besides atopy that contribute to the development of asthma and hay fever. They also attributed this to the subjective nature of the assessment of asthma and hay fever in this study. The authors concluded that their study supports the notion of the hygiene hypothesis in the development of allergies. Although they suggest a beneficial effect of thumb-sucking and nail-biting habits, they do not recommend encouraging these behaviors in children.

Perhaps parents everywhere don’t need to yell, “Get your hands out of your mouth!” quite so often. The authors of this fascinating article study the association of common childhood behaviors that likely increase oral exposure to bacteria in early childhood with the development of atopy. Although a long-lasting association of less atopic sensitization was identified, there was no similar effect on the actual development of atopic diseases, such as asthma and hay fever. This limits the clinical applicability of these study results, similar to multiple other studies identifying lower allergic sensitization associated with prenatal and/or early life exposures but not necessarily less allergic disease. Adverse effects of thumb-sucking and/or nail-biting, such as infection and dental malocclusion, must also be considered before we advise parents to encourage these behaviors in the hopes of preventing allergies later in life. Although these data further support the hygiene hypothesis, the determination of onset of allergic disease remains a complicated amalgam of genetic and environmental contributions that have yet to be fully understood.