Many people remember their parents pulling their thumbs out of their mouths when they were children for fear they’d be made fun of, have a speech impediment, or wind up with buck teeth. There’s even stuff you can buy to put on kids’ fingers to make them taste bitter.
Now, maybe your aversion to thumb-sucking and nail-biting stems from a fear of germs. I get it; I’m a germaphobe, too.
Well, if your child is a thumb-sucker – or a nail-biter, for that matter – it might actually be a good thing…or at least not a terrible thing. A new study suggests he may go on to develop fewer allergies, so the extra microbes gnawed or sucked off his little hands may be doing him a favor.
Unfortunately, there’s no guarantee your kids won’t chew their nails down to nubs, or you won’t have to spend a few grand on braces and speech therapy.
Researchers looked at a long-term project called the Dunedin Multidisciplinary Health and Development Study, now entering its 5th decade, which has followed more than 1,000 children from Dunedin, New Zealand since birth.
Medical student Stephanie Lynch, who is part of the current study, said that over the years, participants have been asked detailed questions about their lives. Lynch and study lead author Bob Hancox culled data on thumb-sucking and nail-biting from the study.
Children in the study were reported by their parents as being either thumb-suckers or nail-biters at ages 5, 7, and 11.
At ages 13 and 32, the children in the study were given skin-prick tests capable of detecting allergic reactions to 40 different substances.
At age 13, these tests showed that 38% of children who sucked their thumbs or bit their nails had allergies, compared with 49% of children who did neither.
Furthermore, children who had both sucked their thumbs and bit their nails had an even lower risk of allergies – 31%.
These test results remained consistent for the 2nd skin prick test, when the participants were 32 years of age. Gender, parental history, pet ownership, and other factors seemed to have little impact on the outcome.
Hancox said in a statement:
“The findings support the ‘hygiene hypothesis,’ which suggests that being exposed to microbes as a child reduces your risk of developing allergies.” 
The hygiene hypothesis is an idea originally formulated in 1989. It proposes that there may be a link between atopic disease and a lack of exposure to various microbes early in life.
Atopic disease is described by the American Academy of Allergy Asthma & Immunology as “the genetic tendency to develop allergic diseases such as allergic rhinitis, asthma, and atopic dermatitis (eczema). Atopy is typically associated with heightened immune responses to common allergens, especially inhaled allergens and food allergens.”
The hygiene hypothesis argues that some exposure to germs may help program a child’s immune system to fight disease, rather than develop allergies.
Hancox went on to say:
“The hygiene hypothesis is interesting because it suggests that lifestyle factors may be responsible for the rise in allergic diseases in recent decades. Obviously hygiene has very many benefits, but perhaps this is a downside.
The hygiene hypothesis is still unproven and controversial, but this is another piece of evidence that it could be true.”
Fellow study author Malcolm Sears, a professor of medicine at McMaster University in Hamilton, Ontario, who was the original leader for the asthma allergy component of the New Zealand study, said:
“Early exposure in many areas is looking as if it’s more protective than hazardous, and I think we’ve just added one more interesting piece to that information.” 
However, despite the study’s findings, the authors aren’t ready to advise parents to let their kids suck and nibble away at their digits, but it might not be worth obsessing over.
“Many parents discourage these habits, and we do not have enough evidence to [advise they] change this.
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.”