A recent study, published by Dr. Robert Hancox, of the University of Otago in Dunedin, New Zealand, finds a strong correlation which indicates that children whom suck their thumbs or bite their nails will have fewer allergies.
According to Hancox, it all relates to the "hygiene hypothesis." The theory is that exposure to bacteria and other microbes early in life programs the immune system to fight infections, and reduces a tendency toward allergic reactions.
The study doesn't prove that either habit will directly lower kids' risk of becoming sensitized to allergens, Hancox acknowledged.
However, his team accounted for a range of things that influence kids' odds of sensitization -- including whether they were breast-fed, exposed to secondhand smoke, lived with pets, or had a family history of allergies.