Victor Hugo once described melancholy as "the pleasure of being sad". Few now have the chance to experience that pleasure.

Sadness, according to a group of influential American psychiatrists, has taken on a clinical alter ego -- depression -- and is steadily being medicated into oblivion. In Britain, 31 million prescriptions for antidepressants were issued last year, a record high. The so-called 'Prozac nations' are now also steeped in Seroxat; as people increasingly turn to serotonin boosters to soothe their sorrow.

And yet, as a powerful book points out, sorrow is not a disease but a natural emotion, as vital to our wellbeing as happiness. The Loss of Sadness: How Psychiatry Transformed Normal Sorrow into Depressive Disorder argues that the ability to feel sad has survived hundreds of thousands of years of human evolution and must be of benefit.

"While depressive disorder certainly exists and can be a devastating condition warranting medical attention, the apparent epidemic [in depression] reflects the way the psychiatric profession has understood and reclassified normal human sadness as largely an abnormal experience," writes Allan Horwitz, professor of sociology at Rutgers University, and Jerome Wakefield, professor of social work at New York University.

Their book demonstrates how medicine has lost sight of the context in which people can become sad; the definition of depression requires only the appearance of symptoms such as insomnia, change in appetite and fatigue. As a result, people who are downcast for valid reasons, such as the end of a relationship or the loss of a job, can be mistakenly deemed depressive.

Despite their natural reaction to a misfortune, they are crowded under the same diagnostic umbrella as the poor souls who feel sad for no reason; it is only the latter whose brains are functioning abnormally and require treatment.

Horwitz and Wakefield are respected academics in the field of mental health; their book has already prompted widespread soul-searching about how depression is defined. "[The authors] make a persuasive argument that has major public health implications," comments Michael First, professor of clinical psychiatry at Columbia University Medical Centre and editor of the Diagnostic and Statistical Manual of Mental Disorders (fourth edition).

The DSM-IV, as this manual is known, is the checklist used by psychiatrists worldwide to diagnose all the mental disorders recognised by the American Psychiatric Association. The new edition, DSM-V, is set to be published in 2011; there is a growing call for major depressive disorder (which encompasses most depressive disorders) to be reframed in the terms that Horwitz and Wakefield suggest.

But perhaps the most surprising string to the book's bow is the author of its foreword: Robert Spitzer, professor of psychiatry at the New York State Psychiatric Institute. Spitzer is described by The New Yorker magazine as "one of the most influential psychiatrists of the 20th century". He was the driving force behind the third edition of the DSM; before its publication in the late Sixties, it was common for psychiatrists to differ wildly in their diagnoses of a condition in the same patient.

Spitzer's work resulted in a reliable, comprehensive dictionary of definitions that all professionals could use; under his stewardship, the diagnostic manual became a universal compass helping medics to navigate the complex territory of mental disease with confidence.

Spitzer admits that Dr Wakefield's book "caused me to rethink my own position..."

The book's central thesis is that sadness seems always to have existed. All cultures experience sorrow; human infants are born with a tearjerking ability to express it. Even the type of grief we experience at various events is remarkably consistent: those who miss a life goal, such as a wanted promotion, feel pessimistic and tired, while the bereaved tend to cry and report physical pain.

That chimpanzees, with whom human beings share a common ancestry, can appear mournful when life turns against them, indicates that sorrow has long been woven into the emotional fabric of human evolution.

These facts suggest that virtually everyone has an innate biological capacity to feel sad; so it is sensible to assume, as the authors do, that a good evolutionary reason lies behind the preservation of sadness as a psychological trait.

The Loss of Sadness points out that the way major depressive disorder (MDD) is defined takes no account of personal circumstances (except for bereavement).

For a diagnosis of MDD in a person, he or she should have displayed at least five of the following nine symptoms over a two-week period (the five must include at least one of the first two symptoms): depressed mood; a lack of interest in activities; change in appetite or weight; insomnia; or hypersomnia (excessive sleep); a physical slowdown; loss of energy; feelings of worthlessness or guilt; inability to concentrate and make decisions; recurrent thoughts of death or suicide. In the case of bereavement, the symptoms must endure for at least two months.

A similar range of symptoms can happen naturally to a healthy person after a stressful event, such as an adulterous betrayal, failing a college test, learning that a child has cancer, or a public dressing-down.

And without that context being specified, the authors write: "Contemporary psychiatry has inadvertently characterised intense normal suffering as disease."

In most cases of normal sadness, the suffering gradually subsides. But by then it is often too late -- he or she is already wielding a prescription for Paxil or Prozac.

The idea that depression is often an unjustified medicalisation of a normal human emotion is gaining currency. Earlier this month, Mark Rapley, professor of clinical psychology at the University of East London, organised a conference entitled "Demedicalising Misery", featuring speakers who believe that much normal behaviour is wrongly classified as disease, and/or the benefits of antidepressants are oversold. The conference was a sell-out.

Rapley does not use the word "depression"; he says that he believes in being "unbearably sad, so sad that you can't leave the house and you don't think life is worth living. I call that what it is: unbearable sadness. I see nothing wrong with using substances to make yourself feel better.

"What I object to is the intellectual trickery, and how the drugs industry has made us believe that, when we feel sad, we have something fundamentally wrong with us that needs correcting."

The Loss of Sadness: How Psychiatry Transformed Normal Sorrow into Depressive Disorder, by Allan V. Horwitz and Jerome C. Wakefield, Oxford University Press, £17.99

- Anjana Ahuja