The following correction was printed in the Guardian's Corrections and clarifications column, Monday March 3 2008

We exaggerated the precociousness of John Stuart Mill in saying he wrote his famous work Utilitarianism in 1861 at the age of 19 in the article below. He was 55 at the time.

Discuss the positive aspects of mental disorder, and the British public seems to feel threatened. When actor Stephen Fry gave an account of his bipolar disorder, he was accused of "normalising" what was a "severe mental illness", that he had the luxury of shunning treatment due to his privilege and wealth. These sorts of criticisms missed the point: his wealth was earned largely because of his so-called disorder. The years when he was contributing to hit TV shows such as A Bit of Fry and Laurie and Black Adder were largely fuelled by protracted periods of elevated mood - in the form of mild mania, or "hypomania".

On the other hand, straightforward depression (unipolar depression) has no elated, productive, hypomanic side and, as a consequence, very few people have written about the positive aspects of the condition. The focus has been on the acute symptoms of what can be an incapacitating illness, while forgetting that depression has meaning, and that there is a life after recovery.

The assumption that depression is a disease has been reinforced and perpetuated by biologists, psychiatrists and pharmaceutical companies, all of whom have a vested interest - consciously or unconsciously - in the clinical perspective. This might be an appropriate model for the more severe "melancholic" forms of depression that psychiatrists tend to see, but not for the majority of cases of depression. Most depression is dealt with in the GP surgery or resolves itself without a single medical consultation. Most of the time, depression is hidden from view because of the stigma attached to it.

Although technological advances in antidepressant treatments have undoubtedly been responsible for the alleviation of much suffering, strict adherence to the medical (disease) model is preventing a more complete understanding of why we as a species are so susceptible to depression, with at least 20% of men and 25% of women experiencing the condition in their lifetimes. The disease model may also be engendering a sense of powerlessness in those with depression or ex-sufferers. What so commonly goes along with this perspective is the implication that the condition is due to some unusual constitutional weakness. The only solution, therefore, is chemical.

It is a complete nonsense to talk of depression being unusual when it is plainly common. The search for a "depression gene" has foundered because the genetic underpinnings are spread across the population, like the genes determining height. Most of us probably have a moderate susceptibility to the condition under certain stressful circumstances. Nevertheless, we see GPs overprescribing antidepressants, and the World Health Organisation talks of increasing access to "treatment" to deal with the global epidemic in depression-related disability - predicted to be second only to heart disease as the most important cause of disability by 2010. All of this ignores the "ultimate" cause of depression.

My recent review of theories and personal observations suggests that depression might serve some useful functions. It also highlights how ideas that have been around since the 1960s have been largely ignored by the mainstream media. The "war of depression" - between the view that depression is enlightening and the view that it is hindering - has raged for a long time.

The truth is that short-term pain can lead to longer-term gain. A recently published follow-up study of depression in Holland - the Netherlands Mental Health Survey and Incidence Study (Nemesis) - used a sample of 165 people with a major depressive episode, and provides some preliminary scientific evidence to suggest that depression is indeed helpful in the longer term. Researchers who were looking for evidence to suggest that depression leaves people chronically disabled were surprised to discover the opposite.

Life's trials

The population they followed from before illness to the period after recovery showed that people seemed to cope better with life's trials after depression than they were doing before its onset. In the group as a whole, averaged ratings of vitality, psychological health, social and leisure activities, occupational performance and general health all significantly improved upon recovery from depression, compared to functioning prior to the depression. A minority of individuals got worse after a depressive episode, mostly in the realms of general health, vitality, and physical functioning. However, much to the researchers' surprise, severity of depression and availability of treatment were not significant predictors of this decline. It was more to do with additional problems such as social isolation, having a physical illness, or drug or alcohol addiction.

Depression can lead to great insights and achievements. The precocious philosopher John Stuart Mill wrote his famous work, Utilitarianism, in 1861 at the age of 19 and became depressed at the age of 21. Upon recovery, he confessed that the experience had taught him an important lesson - that he should not sacrifice his social and emotional development to intellectual ambition. More than 2,000 years ago, Socrates warned of the same danger: melancholia was thought to be a consequence of rigorous philosophising. However, Aristotle believed it to be a state of immense moral and spiritual value because of the insights it could bring.

Evolutionary theories of depression explore why the condition has apparently persisted so commonly and universally since ancient times. Culturally sensitive surveys of traditional communities of the world suggest that the "core elements" of depression are indeed universal. Certain communities in Africa regard depression as a natural consequence of an inter-dependent way of life, and those with depression are given time out from normal responsibilities until they recover.

Furthermore, we see the behavioural manifestation of depression occurring in other mammals that have the same capacity for maternal bonding, intimate relationships, and play, and that have similar brain biology.

To explain why depression has not been "bred out" through Darwinian natural selection, theories have suggested that rather than being a defect, depression could be a defence against the chronic stress that misguided people can put themselves under. It is possible that depression defends us against the tendency to deny our true needs by chasing unobtainable goals and helps to bring these needs into sharper focus. More specifically, the proposed benefits are as follows: removal from a stressful situation, introspection, problem solving, the development of a new perspective, and reintegrating this with the community upon recovery.

This sequence is repeated in the legends, myths, tribal beliefs, and religions of the world that tell the story of solitary exploration in adversity leading to personal growth.

Depression may bring about a "rebirth" because it removes self-delusion. There is some evidence from scientific studies to show that depressed people are rather more realistic in their thinking than "healthy" individuals - the phenomenon of "depressive realism". It prompted the scientific journalist Kyla Dunn to write: "One cognitive symptom of depression might be the loss of optimistic, self-enhancing biases that normally protect healthy people against assaults to their self-esteem. In many instances, depressives may simply be judging themselves and the world much more accurately than non-depressed people, and finding it not a pretty place."

With recovery, and with the lifting of mood, a new kind of truth could emerge that lies somewhere between the overly optimistic and the overly negative. The new truth would be devoid of blind optimism: a more humble assessment of the depressed person's own capability, containing a more balanced picture of his or her perceived strengths and limitations.

Other functions have been proposed by evolutionary psychologists. Ed Hagan has suggested that getting depressed is like going on strike for better pay and conditions - it is a way of making the wider social network more aware of your needs. Paul Watson has suggested that the sobering quality of depression makes us more aware of any changes that might need to occur in that social network.

Coping strategies

Depression may have forced our ancestors to look again at their strengths and their limitations, their coping strategies, their direction, their priorities, their supports. Regardless of the reason for falling into depression, the journey has the potential to make us better equipped, in a general sense, for life. If we are too busy to think and feel, to be mindful, depression might represent the first opportunity to take an honest inventory of ourselves. If the modern world prevents us from learning from depression, perhaps it is the fault of the modern world and not this ubiquitous human condition.

· Paul Keedwell is an honorary consultant psychiatrist at the mood disorders clinic at the Maudsley hospital, London, and a lecturer in the neuroscience of emotion. His book, How Sadness Survived, is published by Radcliffe, RRP £12.99. To order a copy for £11.99 with free UK p&p go to theguardian.com/bookshop or call 0870 836 0875.