What do ex-British prime minster 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 inveterate¹ nail-biter like me. Are we weaker willed? More neurotic²? 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'. Psychiatrists³ classify it as an impulse control problem, alongside things like obsessive⁴ compulsive disorder⁵. But this is for extreme cases, where psychiatric help is beneficial, as with other excessive grooming⁶ 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 intervention⁷. 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 myriad⁸ 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 sarcastic⁹ personality, smoking, alcoholism and love of oral sex. Other therapists have suggested nail-biting may be due to inward hostility¹⁰ – 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 speculations¹¹, 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, chronic¹² 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 random¹³ 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 shreds¹⁴. 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 touching¹⁵ yourself in the genitals it is hard to think of a more immediate¹⁶ 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 bleak¹⁷ 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 evolutionary¹⁸ 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 psychology¹⁹ of habit.

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

英国前首相戈登·布朗（Gordon Brown）、杰奎琳·肯尼迪（Jackie Onassis）、布兰妮·斯皮尔斯（Britney Spears）之间有什么共同点呢？他们都爱（或曾经都爱）咬指甲。

这嗜好我可不喜欢。被别人看到多恶心呀，把双手的形象全毁了，很可能不卫生，如果咬得太深还会受伤。我也几次试图想要戒掉，但从未持之以恒。

最近，我总在想是什么让像我这样爱咬指甲的人恶习难改呢？难道和其他人相比我们意志更薄弱？更神经质？更有欲望？也许心理学研究的文献能够给我答案，也许还能给我一些暗示，教我如何自我摆脱这个令人讨厌的习惯。

第一次浏览文献就发现了过度咬指甲的医学术语：“咬甲癖”（onychophagia）。心理学家把它归于一种冲脉控制问题，即类似强迫症问题。但这仅指极端案例，精神病学对此的帮助颇有成效，对其他过度怪癖也是如此，如：皮肤搔抓症，拔毛癖。我可没到那种程度，只是和大多数爱咬指甲的人一样，一直有这样的癖好，但没什么严重的不良反应。45%以上的青少年爱咬指甲，比如：青少年可能只占一小部分，但你不会说他们中一半人需要接受医学治疗。我想要了解这种现象临床症状不明显的一面——咬指甲没什么大惊小怪，但它对我来说依然是件大事，得去克服。

都是妈妈的错

当然，心理学家在咬指甲方面有许多理论。西格蒙德•弗洛伊德（Sigmund Freud）把它归结于性心理发育不良，当然是在口腔期。典型的弗洛伊德理论认为，口欲滞留的原因有很多，诸如：喂食不足或喂食过盛、哺乳时间过长、或与母亲关系不睦。当然，各原因交错也导致了诸多症状的发生：当然，咬指甲是其一，还有为人尖酸刻薄、吸烟、酗酒、喜欢口交。其他理疗师建议将咬指甲归于“内在敌意”——毕竟是某种形式上的自残——或是紧张焦虑。

像大多心理动力论这样解释也许是说的通的，但没有特别的理由说服我们去相信这些解释是对的。对我来说最重要的是：他们并没有任何强有力的说法，如何自己戒掉这个习惯。就母乳喂养的程度而言，我早已过了那时期。甚至在我最放松的情况下我也咬指甲，所以似乎也没有什么简单的解决办法。更不用说，现今没有证据证明基于这些理论上的治疗有任何特别的成效。

可惜在种种猜测之后，也慢慢无从所知了。检索相关科学文献只发现一些基于如何治疗咬指甲癖的研究。其中一项研究指出：任何让人们愈发认识到咬指甲这个癖好的治疗似乎是有所帮助的，但除此之外并没有什么有关该癖好的报道。事实上，几篇为数不多关于咬指甲的文章公开评述该问题上的文献严重不足。

习惯的奴隶

鉴于之前没有接受过科学治疗，我可以轻松地自我省视。所以以下就是我自己的理论，解释为什么人们会咬指甲，以及如何治疗这个毛病。

让我们把这个理论称为“反理论”之理论。我假设没有特殊的原因引发咬指甲——不是因为什么母乳喂养、长期焦虑、缺乏母爱。把这些统统都排除的好处是我们不需要在我和戈登、杰奎琳、布兰妮之间找出一个特别的连接点。我认为：咬指甲仅仅是由于诸多因素引发的——由于随机变异——一些人形成坏习惯的结合。

首先，有这样一个事实：把你的手指放进嘴里是件很容易的事。这是喂养最基本的功能之一。所以它是受一些最基本脑神经元回路控制的，也就是说它能迅速发展成为一种自动条件反射。此外，对咬指甲有一种“打理”因素——让指甲始终是短的——这就意味着就短期而言至少它能让人心情舒畅，即使其主要是让你的手指苦不堪言。这种奖励因素同行为者采取的愉悦相结合，意味着这种癖好能够轻松养成；除了手淫之外，真的很难想象有什么更为直接的方式让自己愉悦片刻了，而且咬指甲的好处就是在学校这么做完全没问题。这种癖好一旦建立，就会变成常规——每个人的日常生活都有许多你都得使用你的双手和嘴的情况。

把咬指甲当作是一种癖好去理解，对治疗的作用并不大，因为遗憾的是我知道摆脱坏习惯有多难。大多数人一天中至少一次会无意识地咬指甲。

在我看来，咬指甲并不是揭示了什么人格特征，也不是一些对人类有益进化行为的不良反映。它是我们身体构造的产物，是手对嘴行为如何在人类大脑中建立以及习惯心理的产物。

是的，我在写这篇专栏的时候确实咬指甲了。有时就算是一个好理论也帮不上忙。