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.