The Curious Evolutionary Basis For Thumb-Sucking

Children from all over the world innately put their hands in their mouths as babies toddlers, whether it be thumb-sucking or nail-biting. Pediatric researchers have long assumed that this was some sort of evolutionary mechanism for building the immune system, called the “hygiene hypothesis.” Now, a thirty-eight year study by pediatricians in New Zealand has finally confirmed this long-standing theory.

A recent paper in the journal Pediatrics explains the data behind this finding. This study was part of a larger research project, the Dunedin Multidisciplinary Health and Development Study, which sought to gather long-term data on individuals’ health throughout their lives.

This study sought to find any causation between thumb-sucking and immune system health indicators.

For this project, medical researchers followed nearly a thousand individuals from birth until their thirty-eighth birthday, gathering and analyzing health data such as allergies, asthma, hay fever, and various immune system deficiencies. Observations at birth and early childhood were made to monitor which individuals sucked their thumbs or bit their nails, among other behaviors and indicators.

The results showed that children who bit their nails or sucked their thumbs had much lower rates of immune system-based allergies, called atopic sensitivity, even into adulthood:

The findings from this study support our hypothesis that children who frequently suck their thumbs or bite their nails have a lower risk of developing atopic sensitization. Children who were reported to have either of these habits were less likely to have positive skin-prick tests at age 13 years and this apparent protective effect persisted to age 32 years.

Children who both bit their nails and sucked their thumbs had the lowest overall rates of allergies and other immune system sensitivities. The asthma and hay fever data was slightly more complicated and more difficult to analyze due to the difficulty in diagnosing and self-reporting these maladies.

Before you start encouraging your baby to keep her hands in her mouth to ward off allergies, note that the researchers do not necessarily recommend such behaviors:

Although we do not suggest that children should be encouraged to take up these oral habits, the findings suggest that thumb-sucking and nail-biting reduce the risk for developing sensitization to common aeroallergens.

While they might provide positive effects for the immune system, thumb-sucking and nail-biting do come with their own sets of negative consequences, such as misalignment of teeth and jaws or anxiety over being teased for these behaviors. At the very least, this data should reassure parents of thumb-sucking children that nothing is wrong with their babies - they’re merely building their bodies’ defense systems.