CHILDREN who suck their thumb and bite their nails are less likely to develop some allergies, according to new research.
The findings support the 'hygiene hypothesis' that allergies are partly caused by children not being exposed to germs at an early age as they would be in the natural world.
There are a number of theories - and myths - about why the number of children with allergies has gone up in recent decades.
The researchers found 45 per cent of 13-year-old children had at least a mild reaction to an allergen, such as cats, dogs, dust mites, grass, horses and airborne fungi.
But, among those who had sucked their thumbs and bitten their nails, this dropped to just 31 per cent. If they had one of the habits, the figure was 40 per cent.
However, neither habit appeared to have an impact on whether the children developed asthma or hay fever.
One of the researchers, Professor Malcolm Sears, of McMaster University in Canada, said: "Our findings are consistent with the hygiene theory that early exposure to dirt or germs reduces the risk of developing allergies.
"While we don't recommend that these habits should be encouraged, there does appear to be a positive side to these habits."
The idea is that by putting fingers in their mouths children are increasing their exposure to microbes which affects the development of the immune system.
The study, by academics of Dunedin School of Medicine in New Zealand and Professor Sears, found the trend was sustained into adulthood and was not affected by having a smoker in the home, ownership of cats or dogs, or exposure to house dust mites.
The research was published in the journal Pediatrics.
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