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. Our Prozac nation is now also steeped in Seroxat; we increasingly turn to serotonin boosters to soothe our sorrows.

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,” write 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 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.

He calls The Loss of Sadness a “brilliant tour de force” and a “water-shed” in the development of the field. More tellingly, Spitzer admits that “Dr Wakefield has critiqued my efforts in ways that I have largely become convinced are valid . . . (the DSM’s) diagnostic criteria specified the symptoms that must be present to justify a given diagnosis, but ignored any reference to the context in which they developed. In so doing, they allowed normal responses to stressors to be characterised as symptoms of disorder.”

The book, Spitzer reveals, “has caused me to rethink my own position . . .” It is probably the closest you will ever come to hearing a doctor of Spitzer’s stature admit that he was wrong. 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.

Dr Dylan Evans, an evolutionary psychologist and author of Emotion: The Science of Sentiment, says it is clear that the ability to feel sad has been honed over millions of years. “It must be an innate capacity that has come to us through evolution, because it is a basic, universal emotion,” he says. “People don’t have to learn to be sad; blind people who have never seen faces produce the same facial expressions. Just like other basic emotions, such as happiness, fear and disgust, sadness has all the hallmarks of an adaptation.”

Evans accepts, though, that the benefit that sorrow brings is not clear. “Disgust helps you to avoid rotting food, and fear makes you avoid wild animals and the dark. The function of sadness is more difficult to work out.

“One possibility is that it makes you stop and reflect on what you’ve done, and it’s a way of stopping mistakes in the future. The problem with this is that sad people often don’t think logically or creatively, and don’t learn from their mistakes. Another theory is that it initiates support from other people.”

People who cry, for example, are usually quickly surrounded by sympathetic individuals. Evans elaborates: “Some people have speculated that clinical depression is an attempt to extort, rather than elicit, support.”

Another theory suggests that sadness helps to protect the hierarchical nature of human society and thus prevents anarchy: “When you see people higher up the pecking order you feel sad, and that sadness stops us from making risky challenges to those higher up.”

Only when intense sadness has no clear cause, or lingers longer than usual, should it be considered dysfunctional, and be treated medically. The Loss of Sadness points out that this distinction is not made in the way that major depressive disorder (MDD) is defined. In particular, it 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.

Evans agrees with the idea that normal sadness is overmedicalised; he sees this as a chance to discover the true function of sadness. “By giving Prozac out on a massive scale, we are, in effect, creating a huge social experiment in which sadness disappears.” Knowing what happens in its absence should give us a clue as to why we are born to feel sad.

Randall Nesse, a psychiatrist at the University of Michigan, once speculated that economic booms and busts were becoming more extreme because so many stressed-out investors and traders were on Prozac, which blunted their sense of caution. When the gambling is heavier, Nesse argued, the bubbles are more voluminous and the busts more spectacular.

“Anecdotally, people on antidepressants say that they feel untouchable,” Evans says. “They don’t seem to have that emotional immediacy that most of us have.” For severe depression, that is a good thing but in healthy individuals the antidepressants may be desensitising people to vital emotional cues from others. The idea that depression is often an unjustified medicalisation of a normal human emotion is gaining currency in this country, too. 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 regards the current epidemic in depression as a social and cultural one, not a bona fide clinical one. “How is it that we have become so bamboozled that we fail to recognise certain human experiences, such as grief or sadness, for what they are?” Rapley asks.

And if depression is just normal sorrow, he rails, it’s reasonable that antidepressants are not correcting some fundamental, underlying brain deficiency: “If I have a severe headache and take aspirin to make me feel better, I don’t think of myself as having an aspirin deficiency. If I drink a couple of glasses of wine before I go to a party and then become the life and soul, I don’t regard myself as having an alcohol deficiency. If we don’t believe it for aspirin and Shiraz, why do we accept it for SSRIs (selective serotonin reuptake inhibitors, such as Prozac)?”

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