Got the evolution blues...

We are accustomed to attributing depression, anxiety and other mental disorders to faulty brain chemistry or childhood trauma. But there may be another explanation: the legacy of the apes.

Imagine you went to your doctor complaining of a cough and he asked if you would be willing to take part in a "cough disorder" research programme. "You've been coughing at a rate of more than twice an hour for over two days so you've got cough disorder," he tells you, and goes on to talk about how the programme aims to categorise subtypes of the disorder - those linked with a runny nose, those linked to allergies, those associated with smoking and so on.

The research vista widens. He describes how, following the discovery of a cough control centre in the brain, researchers are looking at how abnormalities in brain chemistry cause "cough disorder". A promising lead, he enthuses, is that, because we know codeine stops coughing, "cough disorder" is likely to be the result of a deficiency in the brain chemicals that codeine mimics.

This is clearly madness because we know that a cough is a useful defence mechanism aimed at keeping our breathing passages clear. But if you replace "cough disorder" with "anxiety disorder", then suddenly the research programme seems awfully familiar. However, according to the burgeoning new field of evolutionary psychiatry, this is equally misjudged. Anxiety and most other forms of mental illness can also be seen as normal defence mechanisms, shaped by natural selection, that have started functioning in a way that is no longer useful.

The Darwinian perspective has changed the way we think about nearly everything, from the survival of universes to themes in literature, but until very recently psychiatry had been largely unaffected by its insights. At the moment, if you want to understand why someone has a psychiatric problem there are only two avenues open: the psychotherapeutic (traumatic experiences in childhood), or neurological (brain chemistry gone awry).

Evolutionary theory offers a third way. "Mental illness is ultimately a product of the brain, and the brain is obviously a biological organ," says Dr Simon Baron Cohen, of Cambridge University. "It has been taken for granted in all other areas of biology that to understand biological phenomena evolutionary factors must be considered." In his new book The Maladapted Mind, Baron Cohen pulls together some of the strands of this new approach, which is set to change the way we think about mental disorders.

Take panic attacks. These first happen when a person is away from home. There is a sudden surge of alarm that seems to come out of the blue, followed by a sense of doom and an unbearable urge to escape. The victim's heart pounds; they pant, feel dizzy. The attack passes, but days or weeks later another strikes, and then another. The sufferer goes out less and less.

So why do people get panic attacks? Freudians can detect their origins in both sexual arousal and birth trauma, despite the fact that the two are quite different. Learning theorists are far less fanciful, and will talk about a conditioned response, but it is rarely obvious what triggered the attack and this doesn't explain why patients with quite different histories have exactly the same symptoms. Neurochemical research can tell us what goes on in the brain, but not why.

The Darwinian account of panic attacks, put forward by Rudolf Nesse of the University of Michigan, sees the whole thing differently. "What's going on in a panic attack is that a normally valuable defence mechanism has swung into action. It's a physical, mental and emotional package, with great survival value, designed to get us out of life-threatening situations fast. It's a behaviour pattern typically found in animals that rely on homes and kin for protection."

He draws an analogy with the immune system. This, too, is a co-ordinated set of responses shaped by evolution that is triggered by a perceived threat from a chemical or a pathogen. But occasionally, as in the case of hay fever or auto-immune diseases, the response is inappropriate.

"The panic response is an example of prepared learning," says Nesse. "Because of the way our ancestors lived we are disposed to be worried about open landscapes with no refuge or closed-in spaces, while being alone, away from home and with strangers, is seen, rightly, as dangerous."

The evolutionary approach also suggests a reason why panic attacks are more common in females: not only were they less able to defend themselves, but they would have found it more difficult to flee because they were often caring for children, and they were likely to be raped.

The Darwinian view of mental illness doesn't replace the other approaches - drugs can be used to reduce the attacks, giving the patient a chance to use behavioural techniques to change their responses - but it does put them in a wider framework. "At a clinical level there are also benefits," says Nesse. "Most patients arrive in a clinic worried that they are crazy, weak or medically ill. However, once they understand this is a natural response that has gone off inappropriately, the whole issue of whether this is a real illness or `just psychological' disappears. That helps them to co-operate with the cure without feeling responsible for the illness itself."

Depression is another condition that can usefully be put in an evolutionary framework. At first sight it is hard to see what the benefits of sitting around all day hating yourself and the world might be, but going to sleep and hibernating are even less productive, and they are both adaptive strategies. One idea is that depression is a way of surviving a loss of status. "When primates battle for food or mates," says Dr John Price, "they rarely seriously hurt each other because it's a ritual. When one submits, the other stops. Depression is a psychological package of thoughts and feelings that has evolved to stop us carrying on a hopeless battle."

When you're confronting someone - it could be a disagreement over a new marketing strategy or spilt beer in the pub - you both instantly have a pretty good idea of who is the more powerful, whether by the zeros on the pay cheque or inches round the biceps. Animals, according to ethnologists, have much the same ability, and it is known as "resource holding potential" (RHP). "In humans, RHP evolved into self-esteem," says Dr Price. "Much of the time, high self-esteem is good for getting things done, but wildly overestimating your abilities can lead to serious damage or death."

Once you have been fired, for instance, the immediate depression stops you wasting your time and further damaging your reputation by going back to the office to fight for your job.

Thinking about depression in this way does suggest a different approach to treatment. At the moment you are likely to get, as well as drugs, a psychotherapy based on the idea that your thinking processes have somehow gone awry - you are drawing conclusions without any evidence, over-generalising, magnifying failures and ignoring successes. A cognitive psychologist will try to argue you back into a more rational state.

That fits well with the Darwinian approach - such negative thinking certainly stops you doing much - and if cognitive therapy works, fine. But explaining what is happening in terms of conflict and status suggests an alternative as well. "We help the patient to identify the conflicts that are causing trouble and then work on ways of resolving them," says Dr Price. "Sometimes negotiation and compromise works; sometimes self-assertion training actually helps the patient to win the conflict; other times we encourage them to give up the fight consciously rather than being forced to do it by their depression programme."

Looking at depression in terms of status also throws new light on the link between hostility and depression. Researchers have long disagreed about whether depressed people are more or less angry with those around them - some studies have found they are, some not. "The purpose of depression is to stop you challenging higher status people," says Dr Price. "The confusion arises because no one has been looking at who the hostility is directed at." Which explains why the patients who appear inert and passive to the consultants can also be laying into cleaners or porters.

At its most ambitious, seeing mental illness through the evolutionary lens provides an escape from the mind-body split that has bedevilled psychiatry all this century. Our emotions and their associated thoughts, for instance, are seen as part of programmes designed to deal with particular challenges - fear with immediate threats, love with mating, anxiety with future threats and so on. As such, they are shaped by the same selection processes that have determined the way the rest of our bodies function. "Understanding the emotions and how they are normally regulated," says Nesse, "would provide for psychiatry something comparable to what physiology provides for the rest of medicine"n

The Darwin Seminar at the London School of Economics today will be devoted to the topic of evolution and psychiatry. Simon Baron-Cohen and Randolph Nesse among others will be discussing their work.

`The Maladapted Mind' is published this week by Psychology Press, pounds 44.95.