Nail Biting and IQ: Is Biting Your Nails a Sign of Intelligence?

Nail biting is not a known sign of intelligence, and there is no evidence that it raises or lowers IQ. No published study has compared the IQ scores of nail biters and non-biters, and the one study that gave nail biters formal cognitive tests found no difference from non-biters in impulsivity or mental flexibility.

This article covers how common nail biting is, what studies of nail biters' minds have found, where the idea that nail biters are smart comes from, and when the habit is worth raising with a professional.

"Onychophagia," the medical term for chronic nail biting, is very common. A 2022 review by Debra Lee and Shari Lipner estimates that it affects 20% to 30% of the general population and up to 45% of children between age 10 and puberty, and that it tends to fade with age. Two studies give a sense of the range:

• Children: in a random sample of 743 primary school children in Shiraz, Iran, parents reported that 22.3% had bitten their nails in the previous three months (24.4% of boys and 20.1% of girls).

• Young adults: in a survey of 339 Polish medical students, 46.9% had bitten their nails at some point (19.2% currently and 27.7% in the past). Almost all of them (92.2%) described it as automatic, something they did without noticing.

Most nail biting is a harmless habit. When it is frequent, hard to stop and causes distress or real problems, the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) places it under "other specified obsessive-compulsive and related disorders" as a "body-focused repetitive behaviour" (BFRB), alongside lip biting and cheek chewing. The links between nail biting and OCD itself are weak, as discussed below. Our page on OCD and IQ covers the intelligence research for that condition.

The habit runs in families, partly through genes. In 1,131 pairs of 12-year-old Japanese twins, Syuichi Ooki found that identical twins were more alike in nail biting than fraternal twins, and estimated that about 50% of the variation in nail biting was due to genetic influences in both boys and girls.

The closest thing to a test of the intelligence claim comes from Austin Blum, Sarah Redden and Jon Grant. They recruited 87 young adults aged 18 to 29, of whom 34 bit their nails, and gave everyone computerised tests of motor impulsivity (how well you can stop an action already under way) and cognitive flexibility (how well you can switch between rules). The nail biters did no better and no worse than the others on either measure. Neither task is an IQ test and the sample was small, so the only direct evidence shows no difference, with wide uncertainty.

Studies of nail biting and mental health give a similar picture of weak or inconsistent links:

• Anxiety and OCD: in the Polish student survey, 22.5% of lifetime nail biters met criteria for an anxiety disorder and 3.1% for OCD, compared with 26.2% and 5.0% of those who had never bitten their nails. The authors found no correlation between nail biting and either condition and concluded that no single condition explained the habit.

• Children in the community: in the Shiraz sample, nail biters scored lower on the "prosocial" scale of a parent questionnaire, a measure of helpful and cooperative behaviour.

• Children in a clinic: among 63 nail biters seen at a child mental health clinic in Iran, 74.6% had ADHD, 36% oppositional defiant disorder and 11.1% OCD, and 9.5% had an intellectual disability.

That last study describes children already referred for psychiatric help, with no comparison group of non-biters, so it is no evidence that nail biting lowers IQ.

The popular claim has a plausible origin in two small laboratory studies about when people bite their nails.

In the first, Tim Williams, Rebecca Rose and Sarah Chisholm observed 40 undergraduates who reported biting their nails under four conditions in random order. People bit their nails most when they were left alone with nothing to do (the boredom condition) and when they were solving maths problems (the frustration condition). They bit least during conversation and when they were told off for it. Nail biting, in other words, shows up when people are working on something hard, which makes it easy to associate with thinking. The same misreading attaches to fidgeting.

The second study, by Sarah Roberts, Kieron O'Connor, Frederick Aardema and Claude Bélanger in Montreal, compared 24 people with a body-focused repetitive behaviour and 23 controls. The BFRB group felt a stronger urge to pick, pull or bite in a boredom and frustration condition than in a relaxation condition, and they scored higher on a "maladaptive planning style" marked by high standards and difficulty relaxing. Coverage ran under headlines such as "Why constant nail biting means you're probably a perfectionist." Perfectionism has its own small and complicated relationship with IQ, covered in our article on perfectionism and IQ.

Neither study measured intelligence. Moving from "people bite their nails while doing maths problems" to "nail biters are better at maths problems" is a leap the data do not support. Some samples also flatter the habit by accident: medical students are a highly selected group, so a high rate of nail biting among them says nothing about whether nail biting goes with high ability in the general population.

The confounds that matter in this research are mostly about who gets studied. Nail biting peaks in childhood and adolescence and declines with age, so any comparison of biters and non-biters has to account for age. Clinic samples are loaded with ADHD, which has its own research literature on IQ, summarised in our article on ADHD and IQ. Family patterns could reflect shared genes, shared stress or simple imitation. None of the studies above adjusted for parental education or socioeconomic status when looking at cognition, because none of them were designed to study cognition.

One well-designed study has found a possible upside, though not an intellectual one. Stephanie Lynch, Malcolm Sears and Robert Hancox used the Dunedin birth cohort in New Zealand, whose parents reported children's thumb-sucking and nail-biting at ages 5, 7, 9 and 11. The 31% of children who were frequent thumb-suckers or nail-biters at one or more of those ages had lower odds of a positive allergy skin-prick test at age 13 (odds ratio 0.67) and at age 32 (odds ratio 0.61). The associations held after adjusting for sex, parental allergy, breastfeeding, pets, household crowding, socioeconomic status and parental smoking, and there was no link with asthma or hay fever. The authors read it as support for the "hygiene hypothesis," the idea that early exposure to microbes protects against allergy. It is one cohort.

For most people nail biting is a cosmetic nuisance. Lee and Lipner list possible harms from heavy biting, including bacterial infection around the nail, permanent shortening of the nail and wear or damage to the front teeth. If the habit causes pain, infection or visible damage, a family doctor or a dermatologist is the right first stop, and a dentist for problems with teeth or gums. If it comes with significant distress, other repetitive behaviours such as skin picking or hair pulling, or signs of anxiety or ADHD, a licensed psychologist or psychiatrist can assess what is going on. Lee and Lipner's review describes several small trials of "habit reversal training," a behavioural therapy that teaches people to notice the urge and replace it with a competing action, with encouraging results.

There is no evidence that they are. No study has compared IQ scores of nail biters and non-biters, and the only study using formal cognitive tests found no difference in impulsivity or mental flexibility.

One small study of 47 adults found that people with body-focused repetitive behaviours scored higher on a planning style marked by high standards and difficulty relaxing. That is suggestive but far from proof, and it says nothing about intelligence.

Not exactly. DSM-5 files severe nail biting under "other specified obsessive-compulsive and related disorders," but in a survey of 339 students, nail biters were no more likely than non-biters to have OCD.

Laboratory work suggests nail biting is most frequent during boredom and during frustrating tasks such as hard maths problems. It seems to be a response to a state of mind, and not a marker of ability.

Partly. A study of 1,131 Japanese twin pairs estimated that about 50% of the variation in nail biting is due to genetic influences.

Nail biting is common, partly heritable and mostly harmless, and it has no demonstrated link with IQ in either direction. The idea that nail biters are smart probably comes from the fact that people bite more while bored or stuck on a hard problem, plus a small study on perfectionism that became a headline. If you want to know where your reasoning ability actually stands, take a full-length online IQ test instead of reading meaning into your fingernails.

1. Lee, D. K., & Lipner, S. R. (2022). Update on diagnosis and management of onychophagia and onychotillomania. International Journal of Environmental Research and Public Health, 19(6), 3392. [doi.org/10.3390/ijerph19063392](. doi.org

2. Ghanizadeh, A., & Shekoohi, H. (2011). Prevalence of nail biting and its association with mental health in a community sample of children. BMC Research Notes, 4, 116. [doi.org/10.1186/1756-0500-4-116](. doi.org

3. Pacan, P., Grzesiak, M., Reich, A., Kantorska-Janiec, M., & Szepietowski, J. C. (2014). Onychophagia and onychotillomania: Prevalence, clinical picture and comorbidities. Acta Dermato-Venereologica, 94(1), 67-71. [doi.org/10.2340/00015555-1616](. doi.org

4. Ooki, S. (2005). Genetic and environmental influences on finger-sucking and nail-biting in Japanese twin children. Twin Research and Human Genetics, 8(4), 320-327. [doi.org/10.1375/twin.8.4.320](. doi.org

5. Blum, A. W., Redden, S. A., & Grant, J. E. (2017). Neurocognitive findings in onychophagia (pathological nail biting). Psychiatric Quarterly, 88(4), 747-753. [doi.org/10.1007/s11126-017-9496-9](. doi.org

6. Ghanizadeh, A. (2008). Association of nail biting and psychiatric disorders in children and their parents in a psychiatrically referred sample of children. Child and Adolescent Psychiatry and Mental Health, 2, 13. [doi.org/10.1186/1753-2000-2-13](. doi.org

7. Williams, T. I., Rose, R., & Chisholm, S. (2007). What is the function of nail biting: An analog assessment study. Behaviour Research and Therapy, 45(5), 989-995. [doi.org/10.1016/j.brat.2006.07.013](. doi.org

8. Roberts, S., O'Connor, K., Aardema, F., & Bélanger, C. (2015). The impact of emotions on body-focused repetitive behaviors: Evidence from a non-treatment-seeking sample. Journal of Behavior Therapy and Experimental Psychiatry, 46, 189-197. [doi.org/10.1016/j.jbtep.2014.10.007](. doi.org

9. Lynch, S. J., Sears, M. R., & Hancox, R. J. (2016). Thumb-sucking, nail-biting, and atopic sensitization, asthma, and hay fever. Pediatrics, 138(2), e20160443. [doi.org/10.1542/peds.2016-0443](. doi.org

Hero image: Nail clippers (compound lever), by TheArchivistMan, CC0. Via Wikimedia Commons.