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Babies have been seen sucking on their fingers in utero weeks before birth. But the sight of an older child with his fingers constantly in his mouth, sucking her thumb, biting his nails, can drive parents crazy, bringing up fears about everything from social stigma to germs.
A new study suggests that those habits in children ages 5 to 11 may indeed increase exposure to microbes, but that that may not be all bad.
When a pediatrician discusses thumb-sucking, it’s usually because a parent is worried. The thumb is in the mouth so constantly that there’s a worry about speech or about whether the teeth may be affected. It’s gone on too long, and an older child is being teased about it. And in those situations, especially when a child is over 4, we work with parents and children on how to break the habit.
Nail biting worries parents for similar reasons, and we often end up giving similar advice: Don’t make negative comments; look for the situations that bring on the behavior and find alternate strategies; praise and reward the child for not doing it; put a glove or a bandage on the hand to remind the child.
In a study published Monday in the journal Pediatrics, researchers drew evidence from an ongoing study of New Zealand children to show those whose parents described them as thumb-suckers and nail-biters were less likely to have positive allergic skin tests later in life.
The children were in the Dunedin Multidisciplinary Health and Development Study, in which 1,037 children born in 1972-73 in Dunedin, a coastal city in New Zealand, were assessed and tested as they grew up, with the most recent assessment done at age 38. Stephanie Lynch, a student at Dunedin School of Medicine and the first author of the paper, had the idea of using the data to look at a possible relationship between children who tend to have their fingers in their mouths and allergic sensitization.
The question of such a connection arose because of the so-called hygiene hypothesis, an idea originally formulated in 1989, that there may be a link between atopic disease — the revved-up action of the immune system responsible for eczema, asthma and allergy — and a lack of exposure to various microbes early in life. Some exposure to germs, the argument goes, may help program a child’s immune system to fight disease, rather than develop allergies.
In the study, parents were asked about their children’s nail-biting and thumb-sucking habits when the children were 5, 7, 9 and 11 years old. Skin testing for allergic sensitization to a range of common allergens including dust mites, grass, cats, dogs, horses and common molds was done when the children were 13 years old, and then later when they were 32. Thirty-one percent of the children were described as “frequent” nail biters or thumb suckers (or both) at one or more of those ages.
The study found that children who frequently sucked a thumb or bit their nails were significantly less likely to have positive allergic skin tests both at 13 and again at 32. Children with both habits were even less likely to have a positive skin test than those with only one of the habits.
These differences could not be explained by other factors that are associated with allergic risk. The researchers controlled for pets, parents with allergies, breast-feeding, socioeconomic status and more. But though the former thumb-suckers and nail-biters were less likely to show allergic sensitization, there was no significant difference in their likelihood of having asthma or hay fever.
Robert J. Hancox, one of the authors of the study, is an associate professor in the Department of Preventive and Social Medicine at Dunedin School of Medicine, a department that is particularly oriented toward the study of diseases’ causes and risk factors. He said in an email, “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.”
Malcolm Sears, one of the authors of the paper, 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.”
Dr. Hancox pointed out that the study does not show any mechanism to account for the association. “Even if we assume that the protective effect is due to exposure to microbial organisms, we don’t know which organisms are beneficial or how they actually influence immune function in this way.”
Thumb sucking, especially in an older child, can still be a problem if it interferes with the teeth, or causes infections on the fingers, or gets a child teased. Lynn Davidson, a developmental pediatrician who is an attending physician at the Children’s Hospital at Montefiore in the Bronx, and the author of a review article on thumb sucking, said she tends to be “very low-key” about thumb sucking, since children often stop on their own as they grow.
With older children, Dr. Davidson suggests that parents, if they are worried, should try to analyze when and why the child resorts to thumb sucking or nail biting, and then try behavioral techniques, like offering a child a foam ball to hold and squeeze at those moments. “In an older child you can use their input, ask, what would you do with your hands instead of putting them in your mouth,” she said.
Dr. Sears said, “My excitement is not so much that sucking your thumb is good as that it shows the power of a longitudinal study.” (A longitudinal study is one that gathers data from the same subjects repeatedly over a period of time.) And in fact, as researchers tease out the complex ramifications of childhood exposures, it’s intriguing to look at long-term associations between childhood behavior and adult immune function, by watching what happens over decades.
So perhaps the results of this study help us look at these habits with slightly different eyes, as pieces of a complicated lifelong relationship between children and the environments they sample as they grow, which shape their health and their physiology in lasting ways.