Children who suck their thumbs or bite their nails may be less likely to develop allergies, according to a new study from New Zealand's University of Otago. The finding emerges from the long-running Dunedin Multidisciplinary Study, which has followed the progress of 1,037 participants born in Dunedin, New Zealand in 1972-1973 into adulthood.

The study, which appears in the August issue of the US journal Pediatrics, suggests that childhood exposure to microbial organisms through thumb-sucking and nail-biting reduces the risk of developing allergies.

Study lead author Professor Bob Hancox says that this exposure may alter immune function so that children with these habits become less prone to developing allergy.

Parents of Dunedin Study members reported their children's thumb-sucking and nail-biting habits when their children were ages 5, 7, 9, and 11 years old. 

The members were checked at ages 13 and 32 years old for atopic sensitisation, defined as a positive skin prick test to at least one common allergen. 

At age 13, the prevalence of sensitisation was lower among children who had sucked their thumbs or bit their nails (38 per cent) compared with those who did not (49 per cent). 

Children who both bit their nails and sucked their thumbs had an even lower risk of allergy (31 per cent), Professor Hancox says.