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.
This is 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.
According to the researchers 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. They 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.
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.
However, despite these findings, the researchers do not suggest parents actively encourage their children to take up thumb sucking and nail biting.
