Medical student Stephanie Lynch, who undertook the study as a summer project, said although thumb-suckers and nail-biters had fewer allergies on skin-testing, they found no difference in their risk for developing allergic diseases, such as asthma or hay fever.
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This is known as the “hygiene hypothesis” – and is also a possible explanation as to why allergies are commoner now than during Victorian era when sanitation was far worse.
“We don’t wish to dismiss these concerns”, Hancox said. “We had hypothesized that we would find a reduced risk of allergies in children with these habits, but really had no idea whether this would turn out to be true”.
There are a number of theories – and myths – about why the number of children with allergies has gone up over recent decades. The prevalence of food allergies jumped from about 3.4% in 1997 to about 5.4% now.
It is based on findings from the long-running Dunedin Multidisciplinary Study, which has tracked the lives of 1037 participants born in 1972 and 1973.
“We certainly don’t recommend encouraging nail-biting or thumb-sucking, but perhaps if a child has one of these habits and [it] is hard [for them] to stop, there is some consolation in the knowledge that it might reduce their risk of allergies”. Just over a third had a positive skin-prick test to at least one common allergen, compared to 49 per cent for those who had not sucked their thumbs or bitten their nails.
When they were again tested at 32, the allergies remained the same. Only 31 percent of those tested positive on the skin prick.
The study findings are based on over 1,000 New Zealand children who entered the study at birth. And there was no link between thumb-sucking or nail-biting and the risk of having those conditions.
Estimates show that around 50 per cent of children suck their thumbs or two fingers and another 30 per cent bite their nails. However, that was not really tested.
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Study lead author professor Bob Hancox said this exposure may alter the way the immune system functioned so that children with these habits became less prone to developing allergy. Asthma also works in a different way than immune sensitization, so whatever effect germ exposure had may have been obscured by those other pathways.
