我们为何会咬指甲？

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What do ex-British prime minister Gordon Brown, Jackie Onassis, Britney Spears and I all have in common? We all are (or were) nail biters.

It's not a habit I'm proud of. It's pretty disgusting for other people to watch, ruins the appearance of my hands, is probably unhygienic and sometimes hurts if I take it too far. I've tried to quit many times, but have never managed to keep it up.

Lately I've been wondering what makes someone an inveterate nail-biter like me. Are we weaker willed? More neurotic? Hungrier? Perhaps, somewhere in the annals of psychological research there could be an answer to my question, and maybe even hints about how to cure myself of this unsavoury habit.

My first dip into the literature shows up the medical name for excessive nail biting: 'onychophagia'. Psychiatrists classify it as an impulse control problem, alongside things like obsessive compulsive disorder. But this is for extreme cases, where psychiatric help is beneficial, as with other excessive grooming habits like skin picking or hair pulling. I'm not at that stage, falling instead among the majority of nail biters who carry on the habit without serious side effects. Up to 45% of teenagers bite their nails, for example; teenagers may be a handful but you wouldn't argue that nearly half of them need medical intervention. I want to understand the 'subclinical' side of the phenomenon – nail biting that isn't a major problem, but still enough of an issue for me to want to be rid of it.

It’s mother’s fault

Psychotherapists have had some theories about nail biting, of course. Sigmund Freud blamed it on arrested psycho-sexual development, at the oral stage (of course). Typical to Freudian theories, oral fixation is linked to myriad causes, such as under-feeding or over-feeding, breast-feeding too long, or problematic relationship with your mother. It also has a grab-bag of resulting symptoms: nail biting, of course, but also a sarcastic personality, smoking, alcoholism and love of oral sex. Other therapists have suggested nail-biting may be due to inward hostility – it is a form of self-mutilation after all – or nervous anxiety.

Like most psychodynamic theories these explanations could be true, but there's no particular reason to believe they should be true. Most importantly for me, they don't have any strong suggestions on how to cure myself of the habit. I've kind of missed the boat as far as extent of breast-feeding goes, and I bite my nails even when I'm at my most relaxed, so there doesn't seem to be an easy fix there either. Needless to say, there's no evidence that treatments based on these theories have any special success.

Unfortunately, after these speculations, the trail goes cold. A search of a scientific literature reveals only a handful of studies on treatment of nail-biting. One reports that any treatment which made people more aware of the habit seemed to help, but beyond that there is little evidence to report on the habit. Indeed, several of the few articles on nail-biting open by commenting on the surprising lack of literature on the topic.

Creature of habit

Given this lack of prior scientific treatment, I feel free to speculate for myself. So, here is my theory on why people bite their nails, and how to treat it.

Let's call it the ‘anti-theory’ theory. I propose that there is no special cause of nail biting – not breastfeeding, chronic anxiety or a lack of motherly love. The advantage of this move is that we don't need to find a particular connection between me, Gordon, Jackie and Britney. Rather, I suggest, nail biting is just the result of a number of factors which – due to random variation – combine in some people to create a bad habit.

First off, there is the fact that putting your fingers in your mouth is an easy thing to do. It is one of the basic functions for feeding and grooming, and so it is controlled by some pretty fundamental brain circuitry, meaning it can quickly develop into an automatic reaction. Added to this, there is a ‘tidying up’ element to nail biting – keeping them short – which means in the short term at least it can be pleasurable, even if the bigger picture is that you end up tearing your fingers to shreds. This reward element, combined with the ease with which the behaviour can be carried out, means that it is easy for a habit to develop; apart from touching yourself in the genitals it is hard to think of a more immediate way to give yourself a small moment of pleasure, and biting your nails has the advantage of being OK at school. Once established, the habit can become routine – there are many situations in everyone's daily life where you have both your hands and your mouth available to use.

Understanding nail-biting as a habit has a bleak message for a cure, unfortunately, since we know how hard bad habits can be to break. Most people, at least once per day, will lose concentration on not biting their nails.

Nail-biting, in my view, isn't some revealing personality characteristic, nor a maladaptive echo of some useful evolutionary behaviour. It is the product of the shape of our bodies, how hand-to-mouth behaviour is built into (and rewarded in) our brains and the psychology of habit.

And, yes, I did bite my nails while writing this column. Sometimes even a good theory doesn't help.