Lick Dirt -- For Good Health
Agata Blaszczak-Boxe reports in Sci Am on a three-decade study that suggests that young kids who thumb-suck or bite their nails may be less likely to develop allergies:

Although the results do not suggest that kids should take up these habits, the findings do suggest the habits help protect against allergies that persists into adulthood, the researchers said.

"Many parents discourage these habits, and we do not have enough evidence to [advise they] change this," said Dr. Robert Hancox, an associate professor of respiratory epidemiology at the University of Otago in New Zealand. "We certainly don't recommend encouraging nail-biting or thumb-sucking, but perhaps if a child has one of these habits and [it] is difficult [for them] to stop, there is some consolation in the knowledge that it might reduce their risk of allergies."

In the study, the researchers pulled data from an ongoing study of more than 1,000 children born in New Zealand in 1972 or 1973. The children's parents were asked about their kids' thumb-sucking and nail-biting habits four times: when the kids were 5, 7, 9 and 11 years old. Researchers also tested the children for allergies using a skin-prick test when they were 13, and then followed up with the kids again when they were 32.

It turned out that 38 percent of the children who had sucked their thumbs or bit their nails had at least one allergy, whereas among kids who did not have these habits, 49 percent had at least one allergy.

Moreover, the link between these childhood habits and a lower risk of allergies was still present among the study participants when they were 32 years old.

...The new findings also lend support the so-called hygiene hypothesis, which holds that environments that have too little dirt and germs may make children more susceptible to certain conditions, including allergies.

I wrote about this here -- Marlene Zuk's talk on how parasites are important to our health at the 2006 Human Behavior and Evolution Society Conference at Penn.