The study found people who had sucked their thumbs or bit their nails in childhood were less likely to show a positive reaction in a skin-prick test for common allergies.
So concludes a new study from the University of Otago in Dunedin, New Zealand, that is due to be published in the August issue of the journal Pediatrics.
Study leader Bob Hancox, an associate professor with research interests in respiratory health, suggests that thumb sucking and nail biting probably exposes children to more microbes, which in turn alters their immune function and makes them less prone to developing allergies.
He and his colleagues used data from the long-running Dunedin Multidisciplinary Study, which has followed 1,037 participants from birth to adulthood for over 40 years.
The participants' parents were surveyed about their children's thumb-sucking and nail-biting habits at the age of 5, 7, 9, and 11 years.
The participants underwent skin prick tests when they were 13 and 32 years old. The test is positive if the skin is sensitive to at least one common allergen.
A positive result suggests the person is at higher risk of developing an allergy to the source of the particular allergen, such as cats, dogs, horses, grass, house dust mites, or airborne fungi.
'Findings support the hygiene hypothesis'
The results at age 13 showed only 38 percent of the participants who had sucked their thumbs or bit their nails tested positive for at least one allergen, compared with 49 percent who had neither of the habits in childhood.
The results were even more striking for participants who had both sucked their thumbs and bitten their nails in childhood. Only 31 percent of those tested positive on the skin prick.
These links were still evident at age 32 and were just as strong when the researchers took into account factors that might influence them, such as gender, history of allergy in the parents, pet ownership in childhood, parents' smoking status, and breastfeeding.
However, despite these findings, the researchers do not suggest parents actively encourage their children to take up thumb sucking and nail biting. It is not clear, says, Prof. Hancox, whether there is a true health benefit.
Perhaps the message is, if your children tend to bite their nails or suck their thumbs, then there is no need to get too stressed about it.
"The findings support the 'hygiene hypothesis,' which suggests that being exposed to microbes as a child reduces your risk of developing allergies."
Prof. Bob Hancox
The researchers note that while the study found a link between these childhood habits and higher chance of allergen reaction in the skin prick test later in life, they found no such link with allergic diseases such as asthma and hay fever.