Parents often reprimand their children over sucking their thumbs or biting their nails, but recent research indicates they might want to lighten up. A long-running New Zealand study found that children who suck their thumbs or bite their nails are less likely to have allergies later in life

If kids have both habits, they are even less likely to be allergic to house dust mites, cats, dogs, grass, and other common allergens.

"Our findings are consistent with the hygiene theory that early exposure to dirt or germs reduces the risk of developing allergies," said Malcolm Sears of McMaster University.

The researchers wanted to test the idea that the common childhood habits of thumb-sucking and nail-biting would increase microbial exposures, affecting the immune system and reducing the development of allergic reactions, also known as atopic sensitization.

The researchers found 31 per cent of New Zealand children were frequent thumb suckers or nail biters. Their thumb-sucking and nail-biting habits were measured at ages 5,7,9, and 11, and their atopic sensitization — the tendency to be "hyperallergenic" — was measured by skin-prick testing at 13 and 32 years old.

Among all children at 13 years old, 45 percent showed atopic sensitization, but among those with one oral habit, only 40 percent had allergies. Among those with both habits, only 31 percent had allergies.

The trend extended into adulthood, and remained the same regardless of smoking in the household, ownership of cats or dogs; or exposure to house dust mites.

However, the study did not find associations between thumb-sucking and nail-biting and  the development of asthma or hay fever.

"While we don't recommend that these habits should be encouraged, there does appear to be a positive side to these habits," said Sears.

The research was published in the journal Pediatrics.