News that the next edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM) may classify nail biting as a form of obsessive-compulsive disorder has confirmed the worst fears of the psychiatric handbook's critics. The threshold of what is deemed a disorder is lowered with each successive edition, with nearly all forms of human behaviour now becoming pathologised. Of the 180 or so disorders one could have suffered from in the mid 1980s, there are now approaching 400.

Academics, GPs and psychiatrists have all spoken out against this trend. Petitions have been raised expressing grave doubts about DSM, and a stream of academic books and articles ridicule its scientific claims. Yet these critiques seem to be ignored. Most clinicians working in the NHS use the DSM, and a fitness-to-practise case has even been brought against a clinician who challenged the DSM categories she was supposed to be applying.

The proposed inclusion of nail biting in lists of OCD symptoms is a good example of the manual's failures. Pre-DSM psychiatry emphasised the difference between symptom and underlying structure. Someone could bite their nails as a way of redirecting anger when they felt cross, or even because they had the delusional belief that their nail embodied some evil that had to be excised from the body. The symptom – nail biting – was simply the clue to what lay beneath it.

OCD itself is another case in point. The DSM treats it as a disorder, defined by symptoms such as compulsions, rituals and intrusive thoughts. Yet the actual category of OCD is suspect for a simple reason: the same surface symptoms can appear in two distinct underlying clinical groups – the neuroses and the psychoses.

In neurosis, obsessive symptoms can be a way of warding off anxiety, particularly about the proximity of love and hate. One of Freud's patients worried that a stone in the road might cause an accident when his loved one's carriage travelled along it later that day. He put it by the side of the road, only to then worry that this was absurd and then return it to its original place. Behind the repetitive ritual was a conflict of affection and aggression.

In psychosis, although the person may complain of the cleaning, checking or counting rituals they have to carry out, these activities may protect them from more acute terrors. It is well-known, for example, that the appearance of OCD-style phenomena in schizophrenia or manic-depressive psychosis is generally a good prognostic sign. By introducing an order, they can be less a problem than a way of treating a problem.

In the DSM approach, this distinction is all too often lost. The piece of behaviour becomes in itself transparent, simply one more item on a checklist of symptoms. You don't need to know what the nail biting means to that individual patient, just whether they do it or not. Meaning has been stripped from the diagnostic enterprise, in favour of pure external classification.

Clinicians who want to pursue a dialogue here find that they are allocated less and less time with their patients by a bureaucratic and managerial healthcare system. The tragedy is that this deprives us of having any authentic understanding of the symptom, and it introduces a rigid, normative vision of human behaviour. We can know what is a disorder, and what isn't, without listening to what the person has to say.

Yet nail biting might be a totally irrelevant detail for one person, a terrible curse or a pleasurable habit for another. Classifying such behaviour externally as a symptom, without taking into account what it means to that person, is profoundly inhuman. It is yet another vehicle for imperatives telling us how we should live and how we shouldn't.