Wouldn’t it be great if there was something we could do to prevent or decrease the development of allergies? Especially if that something was a common behavior that most of our children are already doing? That would make everyone’s life a lot easier.

Well, that’s what a recent study out of New Zealand attempted to identify: is there something that kids who don’t have allergies do that kids who don’t have allergies don’t. The click-bait science headline read that thumb sucking or nail biting can protect children from allergies. As with all studies, the headlines may be misleading and do not truly relay what the study findings identified. Let’s take a deeper dive into this issue to see if thumb sucking, finger sucking and nail biting can truly prevent the development of allergies.

Let’s start with what we know. Sound evidence has clearly demonstrated several important pieces of information:

  1. Environmental allergies and asthma are both highly prevalent among children and adults across the world.
  2. There are many ingredients in the recipe that determines which children may develop allergies, and some are more important than others. Family history and early life exposures are huge contributing factors. We have yet to gain a full understanding of all of the influencing factors.
  3. The hygiene hypothesis (the theory that children that grow up in very clean environments have more allergies) is a prevailing theory as to why allergies are on the rise within many populations. Basically, it has been demonstrated that infants and children who grow up in farming environments, where their immune system is constantly bombarded with bacteria, viruses, endotoxin, animals, dirt (you name it) are less likely to develop allergies. The theory holds that if our immune system practices against these types of exposures, then it is less likely to react to benign parts of our world such as grass pollen or food allergens.
  4. We also know that many infants follow an “allergic march”. They develop eczema around 4-6 months of age, followed by possible food allergies, then environmental allergies in early childhood and finally asthma. For some children, once their allergic “switch” gets turned on, there’s no going back.

With this background, researchers in New Zealand investigated whether common behaviors such as thumb sucking and nail biting (think tons of dirty bacteria constantly challenging the immune system) might prevent allergies and asthma from developing. This was a very large birth cohort of about 1000 infants born in Dunedin between 1972-73. These children were then followed through adulthood, with surveys and testing performed every 2 years until age 15, and then spaced out until they were 38 years old.

So what did the study find? The final conclusion was that children who sucked their thumbs or fingers AND chewed on their nails had about a 30-40% reduction in having positive allergy skin testing to inhalant allergens at 13 and 32 years of age. Sounds pretty good, right? Well, as with any study, there are strengths and limitations.


  • Large cohort of 1013 children providing data.
  • Between 70-90% of participants had skin testing at 13 and 32 years of age.
  • Multiple skin tests to relevant indoor and outdoor allergens were used.
  • Authors analyzed data to see if family history or gender also influenced findings, of which they did not.


  • This study was not randomized or controlled.
  • Findings from an observational study such as this can only be interpreted as an association, and any direct causality from a particular behavior with a certain outcome cannot be determined.
  • Determination of ‘frequent’ thumb sucking or nail biting occurred through parental recall and surveys when their children were 5, 7, 9, and 11 years old. There may have been recall bias from the parent that inaccurately assessed or categorized their child’s behavior.
  • The cutoff used to determine a “positive” skin prick test was a 2 mm wheal diameter. All clinical guidelines for interpretation of skin prick testing utilize a 3 mm wheal diameter larger than a negative saline control as the starting point to determine clinical relevance.
  • A “positive” allergy skin prick test does not correlate very well with true clinical allergy, with false positive results approaching 50% for some testing.
  • Lastly, the study participants did not have any difference in symptoms of allergic rhinitis (hay fever) or asthma at any point in the study. One could argue that this is the truly important outcome variable – was there a difference in actual symptoms or disease between the two groups? There was not.

So, where does this leave us? This is a great study that adds to the growing evidence that there is likely some interplay with early life exposures to microbes and the development of allergies and asthma. Unfortunately, I don’t believe much can be read into the final results of this study given their primary outcome measure (difference in allergy test results) but no differences in actual development of allergy or clinical symptoms.

Of course, we also have to consider the downsides of thumb sucking and nail biting, including poor dentition and increased susceptibility to infections and illness (especially during winter months).

Hopefully this quick dissection of a recent trending headline helps provide some insight into the complicated reality of research studies and challenges in extrapolating findings to the general population. Final verdict – don’t stress if your children do/did or do/did not suck their thumbs or bite their nails.

Categories: Chronic Illnesses + Conditions, Newborns + Infants, Toddlers + Preschoolers