Thumb sucking and nail biting not key to preventing ...

"Children who suck their thumbs and bite their nails suffer fewer allergies, study finds," The Daily Telegraph reports.

Researchers have reported a link between these common childhood habits and a lower rate of positive allergy tests; with the important exceptions of hay fever and asthma.

The researchers were interested in what is known as the "hygiene hypothesis". The idea that a level of exposure to germs during early childhood may actually be a good thing as it helps "train" the immune system. And a trained immune system may be less likely to mistake harmless substances, such as pollen, as a threat and trigger an allergic reaction.

Thumb sucking and nail biting are plausible candidates for exposing young children to germs in their immediate environment.

This study involved asking the parents of young children about thumb sucking and nail biting behaviours, and then giving the child allergy skin tests from the age of 13 to 32.

Despite the headlines, the results were not that impressive. Overall, the study found 38% of children who sucked their thumb or bit their nails had a skin reaction compared with 49% who didn't have these habits.

The results were quite mixed, with no clear links to the habits individually, to individual allergic substances – and importantly no links at all with asthma or hay fever.

There's no known way to "train" your child's immune system. Probably the best thing is just to encourage regular play as normal – with other children, indoors and outdoors – while making sure they wash their hands regularly.

Where did the story come from?

The study was carried out by three researchers from University of Otago in New Zealand, and McMaster University and St Joseph's Healthcare, in Canada. Funding was provided by the New Zealand Health Research Council, and one author was also supported by an Otago Medical Research Foundation-Kellier Charitable Trust Summer Scholarship.

The study was published in the peer-reviewed medical journal Pediatrics on an open-access basis so you can download a PDF of the study for free.

The Daily Telegraph and Daily Mail report the findings at face value – that these habits do reduce a child's risk of allergies – without considering the many limitations or the paucity of the reported benefits.

What kind of research was this?

This was a cohort study which aimed to see whether parental reports of their child's thumb sucking and nail biting were linked with allergies in adulthood.

The "hygiene hypothesis" is the theory that it's a good thing for children to be exposed to varied microbes because this may reduce their risk of developing allergies. Thumb sucking and nail biting – habits of up to a quarter of young children – could transfer more germs on the hands into the mouth, so the researchers' theory was that these habits could reduce risk of asthma, hay fever and other allergies.

The problem with cohort studies is that they can't prove cause and effect between one exposure and an outcome – especially with subjective reports such as how often a parent reports their child puts their fingers in their mouth.

What did the research involve?

This study used data collected as part of The Dunedin Multidisciplinary Health and Development Study – a population-based birth cohort study involving 1,037 children born in the New Zealand city of Dunedin in 1972–73.

Parents were asked about their child's thumb sucking and nail biting habits when they were 5, 7, 9 and 11 years of age. They were asked if the statements "frequently sucks their finger/thumb" or "frequently bites their nails" applied to their child "not at all", "somewhat" or "certainly". Children were considered to suck their thumbs or bite their nails if their parents reported "certainly" at least once.

Sensitivity to allergens was tested by skin prick tests of various allergic substances (including dust mites, grass, animal fur, wool) carried out at 13 and then 32 years of age. Allergic sensitivity was defined as having a reaction to one or more of the tested substances.

Children were considered to have asthma if they "reported a diagnosis of asthma and had compatible symptoms or treatment in the previous 12 months" when aged nine. They were considered to have hay fever if this was reported at age 13 or 32 years.

When looking at the relationship between thumb sucking and nail biting and these various allergies they took account of potential confounders, including:

gender
whether they were breast fed
parental allergies and smoking history
socioeconomic status
cat or dog ownership
how many other children were in the house

What were the basic results?

Just under a third of the children (317, 31%) were reported by their parents to "certainly" either suck their thumb or bite their nails. Overall 45% of the children showed a reaction to at least one of the allergic substances aged 13.

However, the prevalence of allergic sensitivity was significantly lower among children with reported thumb sucking or nail biting (38%) compared to those without these habits (49%). The lowest prevalence was among those with both these habits (31%).

Overall, when adjusted for confounders, thumb sucking or nail biting were linked with over a third reduced odds of having an allergic sensitivity at age 13 (odds ratio (OR) 0.64, 95% confidence interval (CI) 0.45 to 0.91) and age 32 (OR 0.62, 95% CI 0.45 to 0.86).

However, while links were significant for either habit, when looking at each habit alone they remained significant for thumb sucking, but not for nail biting, at age 13. At age 32, there was no link with either habit individually.

When looking at specific allergic substances, rather than all of them together, links were only significant for house dust mite age 32, not for any specific substances at 13, or any others age 32.

There were no links between thumb sucking or nail biting and asthma or hay fever at any age.

How did the researchers interpret the results?

The researchers conclude: "Children who suck their thumbs or bite their nails are less likely to have atopic sensitization in childhood and adulthood."

Conclusion

This study does not provide good evidence that thumb sucking or nail biting have any effect on a child's likelihood of developing allergies.

Overall the results give a mixed picture. Although children who sucked their thumb or bit their nails were slightly less likely to have a reaction to the skin tests, when the habits were looked at individually only thumb sucking was linked to a skin test reaction at 13 – and neither habit individually for skin tests at 32.

There were also no clear links for any specific allergic reaction – and no links at all with reported asthma or hay fever. So this doesn't give a clear answer of whether these habits are linked with allergy risk or not.

Further important limitations include:

The subjective nature of the parental reports. Parents were asked whether their child "not at all", "somewhat" or "certainly" sucked their thumb or bit their nails. The researchers then compared children for whom the parents replied "certainly" with the other children. However, there is likely to be a wide range and frequency of habits among children for whom parents give the different responses. For example, a child who sucked their thumb every now and again – some parents could call this "somewhat" while others could say "certainly" because they see them do it.
The skin tests may indicate sensitivity but it's difficult to tell from this how much the individual child would be affected by allergies. Links with actual diagnosis of asthma or hay fever would have been more notable findings – though even then, it could be questioned whether the children meeting the study definition of asthma at age nine actually had a medically-confirmed diagnosis. Eczema is another notable exception of an allergy that was not examined in this study.
Even though the researchers tried to take account of various potential confounders, it is difficult to prove direct cause and effect between the habit and allergy because other health, lifestyle and environmental factors could still be having an influence.
The study was conducted in a population of children born more than 40 years ago. Health, lifestyle, environmental factors and medical care could also have changed considerably over this time meaning these results can't be applied to children today.
Also when considering generalisability – this was a sample from a single New Zealand city. Environmental factors and allergy prevalence may also be considerably different there compared to the UK.

Thumb sucking or nail biting are common childhood habits. Most children grow out of them and they are only usually considered a problem requiring treatment if they persist once a child has started school.