Your nail-biting, thumb-sucking child might have fingertips that look like they’ve been left in a swimming pool for days, and then run over a cheese grater, but there may be a benefit to their unhygienic habits. Kids who constantly jam their hands into their mouths may suffer fewer allergies when they get older.
Plenty of people say that you should let kids shove any old thing into their mouths in order to "build up their immune systems," but is there any truth to this piece of folk wisdom? A new study published in Pediatrics found out the long way. The study followed 1,037 participants from childhood to adulthood. These subjects were born in Dunedin, New Zealand, between 1972 and 1973, and were followed up on every odd year (age 3, 5, 7, etc.) until age 21, then at 26, 32, and 38 years.
As children, the parents were asked about the the nail-biting and thumb-sucking habits of their offspring, and as adults, those kids had their allergies assessed by skin-prick testing. Tested allergens included dust mites, dogs, cats, horses, and wool, and participants were also assessed for hay fever throughout the study. The researchers also took into account factors such as the allergic, asthmatic and hay fever histories of the parents and whether the families had pets.
The results show that those participants who couldn’t keep their fingers out of their mouths as children did indeed benefit from their dirty habits. Around a third of the children in the study sucked or bit their fingers habitually. When the kids were skin-prick tested at age 13, the suckers and biters were less likely to have allergies. "This apparent protective effect persisted to age 32 years," says the study.
Further, kids who enjoyed both habits were more "protected" than those who indulged in just one. Interestingly, there was no effect on hay fever or asthma one way or the other. In those cases, the results were neutral.
The report also mentions another study which found that children were less likely to develop asthma and eczema if their mothers "cleaned" their pacifiers by sucking them.
The conclusion, then, is that eating all that extra dirt and filth may indeed increase microbial exposure, and therefore increase the diversity of the child’s microbiome, which is to say that a 30-odd-year-long study just confirmed a piece of folk wisdom.
It also means that parents can stop fighting their kids, trying to stop them from chewing on their fingers the whole time. And if you’re thinking, "Okay, but I should stop my kids from biting their nails because it’ll gross out other people," then don’t worry about it. Everybody on Earth except you finds your kid’s snotty nose, drool-covered chin, and clammy hands disgusting, so there’s not much you can do to fix the situation. Just let them be, and maybe they’ll grow up not hating you.
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