A new study suggests thumb-sucking and nail-biting as a young child may reduce their risk of allergies.
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The Otago University study found children who sucked their thumb or bit their nails were much less likely to develop allergies in later life.
The study used participants who are part of the Multidisciplinary Study which follows a thousand children born in Dunedin in the early seventies and has been looking at, among other things, the risk of developing asthma and allergies.
Associate Professor Bob Hancox said one of the interesting hypotheses that has come out was that asthma and allergies may develop because we are too clean - the so-called hygiene hypothesis.
"Because we are now much cleaner, we have many fewer infections and much less bacteria than we used to have, that maybe that's caused our immune system to have nothing much else to do, but maybe to develop allergies. That's a very simplistic explanation but that's the basic idea."
Fifth year medical student Stephanie Lynch came up with the idea that thumb-sucking might protect children against allergies.
Professor Hancox said he realised they had the data in the Dunedin study to look at this, and widened the study to look at nail-biting and thumb-sucking.
They tested whether the children who were reported to bite their nails or suck their thumbs when they were between the ages of 5 and 11, were at greater risk of developing allergies by the age of 13. The group were tested for skin allergies at the age of 13 and again at 32.
About a third of the group sucked their thumb between the ages of 5 and 11, another third bit their nails and a smaller group did both.
"What we found was that the children who bit their nails or sucked their thumb had about a 30 to 40 percent lower risk of having an allergy at the age of 13, and what's more this persisted right up until the age of 32."
He said the children who did both had the lowest risk to allergies of all.
"So that very strongly supports the hygiene hypothesis that perhaps introducing more bugs into the system means you are less likely to develop an allergy. We didn't however, find an impact on asthma, so we can't say that it prevents children from developing asthma.
"And we can't really say it prevents people from developing allergies, because it's always possible there's some other explanation. Although it's quite hard to imagine what that other explanation might be," he said.
He said he was not knocking hygiene, which was very important, but there was a risk of over doing it.
Nor was he encouraging people to suck their thumbs or bite their nails, which leads other problems including a heightened risk of skin and nail infections.
Professor Hancox said it wasn't possible to repeat the study, but he was hopeful other researchers would consider a similar study.
"It would be a fantastic if somebody else with a different cohort, perhaps from a different part of the world, perhaps with different ages, could look at the same idea and see whether they support our idea or not.
"For the moment I think we're the only group that have looked at this. It still is important to find other people who will replicate the findings or find something different," he said.
The findings have just been published in the American journal Paediatrics.
"It's fantastic for a student project, so Steph did very well with a student project. She came up with an amazing idea and I'm really glad that it worked out the way it did," he said.