Thumb-sucking, nail-biting have a positive side: Kids less likely to develop allergies

And, if they have both 'bad habits', they are even less likely to be allergic to such things as house dust mites, grass, cats, dogs, horses or airborne fungi.

The research, published in the journal Pediatrics, was completed by researchers of New Zealand's Dunedin School of Medicine, assisted by professor Malcolm Sears of McMaster University's Michael G. DeGroote School of Medicine, and formerly from Dunedin.

"Our findings are consistent with the hygiene theory that early exposure to dirt or germs reduces the risk of developing allergies," said Sears, who is also a researcher for the Firestone Institute for Respiratory Health at McMaster and St. Joseph's Healthcare Hamilton. "While we don't recommend that these habits should be encouraged, there does appear to be a positive side to these habits."

The researchers were testing 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 habits of thumb-sucking and nail-biting were measured in a longitudinal birth cohort of more than 1,000 New Zealand children at ages 5, 7, 9 and 11; and atopic sensitization was measured by skin-prick testing at 13 and 32 years old.

The researchers found 31 per cent of children were frequent thumb suckers or nail biters.

Among all children at 13 years old, 45% showed atopic sensitization, but among those with one oral habit, only 40% had allergies. Among those with both habits, only 31% had allergies. This trend was sustained into adulthood, and showed no difference depending on smoking in the household, ownership of cats or dogs; or exposure to house dust mites.

However, the study did not find associations between the oral habits and development of asthma or hay fever.