Kiwi researchers have discovered children who suck their thumbs and bite their nails are less likely to develop allergies later in life.
The findings come from the long-running Dunedin Multidisciplinary Health and Development Study (DMHDS), which has tracked the lives of more than 1000 people born in the early 1970s.
Study lead author Professor Bob Hancox of the University of Otago say it's yet more proof of "the 'hygiene hypothesis', which suggests that being exposed to microbes as a child reduces your risk of developing allergies".
Allergies are on the rise in the developed world, but not in poorer countries. There's a growing consensus among scientists it's because kids in wealthier countries aren't exposed to a wide range of microbes, leaving their immune systems without a training ground to learn how to fight pathogens properly.
The DMHDS looked at children's thumb-sucking and nail-biting habits when they were five, seven, nine and 11. At ages 13 and 32 they were checked for 'atopic sensitisation' - testing positive to a common allergen, delivered via a skin prick.
At 13, fewer kids who sucked or bit tested positive - 38 percent - compared to those who didn't - 49 percent. Kids who did both had fewer positive results - 31 percent.
Tested again in their 30s, the results stood, even after other factors - such as parental allergies, pet ownership and breastfeeding - were taken into account.
"Thumb-sucking and nail-biting are often seen as undesirable habits and are discouraged by many parents," the study, published in US journal Pediatrics, notes.
"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 sensitisation to common aeroallergens."
That's because sucking and biting have been linked to hand infections, misaligned teeth and injuries to the gums.
Sucking and biting also don't appear to reduce the likelihood of all allergies.
"Although thumb-suckers and nail-biters had fewer allergies on skin testing, we found no difference in their risk for developing allergic diseases such as asthma or hay fever," says medical student Stephanie Lynch, who came up with the idea for the study.
The New Zealand Allergy Clinic says previous studies have shown greater rates of allergy in children growing up in smaller families. Kids with three or more older siblings are less likely to have allergies than only-children, as are only children who attend daycare.