Randolph Nesse continues The THES's debate on the impact of Darwinism by arguing that much of what medicine terms disease or breakdown is in fact a defence the body has evolved to protect itself from the possibility of even worse ailments

"Why should anything go wrong in our bodies?

Why should we not all be beautiful? Why should there be decay? - why death? - and, oh, why, damnation?"

Anthony Trollope in a letter Epigraph for "Easter Sunday, 1955," poem by Elizabeth Spires, The New Yorker, April 1995

Since the dawn of human inquiry, we humans have tried to understand why we suffer and why we die. While modern medicine has been remarkably successful in explaining the mechanisms that cause disease, when we ask why sickness and suffering should exist at all, we are referred right out of the clinic to the philosophy department. This is unfortunate because some such questions can, in fact, be answered. When we can explain how a biological trait increases Darwinian fitness, we can understand why it exists. It has seemed impossible to use this approach to explain disease because abnormalities obviously cannot be products of natural selection. Disease has seemed to result more from the weakness of natural selection than its strength, and the Darwinian revolution has therefore not yet come to medicine.

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In the past decade, however, a group of scientists have realised that a slightly different perspective opens up an entire new category of scientific questions about disease. While natural selection does not shape disease itself, it does shape vulnerability to disease. Thus, for each disease, we can seek an evolutionary explanation for why we are susceptible, why the body does not work better. Darwinian medicine is the new enterprise of proposing and testing such explanations.

The evolutionary reasons for our vulnerability to diseases fall neatly into just a few categories. We get infections because pathogens evolve faster than we do. We get atherosclerosis, obesity, myopia and the bulk of modern maladies because we live in an environment vastly different from the one that our bodies evolved in. While some genetic diseases result from defective DNA, many of the genes that cause disease have benefits that outweigh their costs. This explains the persistence of some genes that cause ageing and our inevitable death. Many problems, such as cancer, arise because of trade-offs, in this case the benefits of tissues that can repair themselves versus the risk of uncontrolled cell division. And finally, there are many problems, like the blind spot in our visual fields and choking on food, that arise simply because natural selection can never go back and start afresh to correct a fundamentally faulty design. But much of our suffering is not physical. Why do 30 per cent of us get mental disorders? Why are anxiety, depression and jealousy ubiquitous? Are mental disorders diseases?

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Perhaps the most important contribution of Darwinian medicine is an increased capacity to distinguish diseases from defences. Cough, diarrhoea and fever are not in themselves diseases. They are defences that have evolved to protect us when we need them. Doctors know that cough clears bacteria from the lungs; that is why they ask us to cough after surgery, even though it hurts. Patients with Shigella infections who take medication for their diarrhoea are more likely to have complications and a long illness. Rats and rabbits deprived of the fever response are much more likely to die from an infection. So, should you take medication to stop diarrhoea or aspirin for your cold? Astoundingly, the studies needed to offer definitive answers have yet to be performed.

What about morning sickness? If California researcher Margie Profet is correct, nausea may prevent pregnant women from eating things that might damage their foetuses. She has tested the hypothesis and found that mothers with less morning sickness turn out to be more likely to have babies with birth defects. What about pain? When we feel pain, we know something bad is happening, but the capacity for pain is valuable indeed. People born without the capacity for pain get severe joint disease in their teens and are usually dead by age 30.

But it is emotional pain, not physical pain, that bedevils most of us most of the time. We are often anxious, depressed or plagued with envy, yet somehow many moderns seem to believe that normal life is characterised by calm contentment. If that is true, then our fears and anguish are epidemics that need explanation and treatment. It is human nature to seek to place blame, and the blame for our shared suffering is placed by some on our abnormal society, by others on genes, and by still others on early trauma. But what if our mental suffering is, like physical pain, a useful defence?

Emotions researchers increasingly agree on one point: the emotions are adaptations shaped by natural selection. I tell my students to think of emotions as if they were computer programs that adjust all aspects of the machine to cope with a specific challenge. From this perspective there is no one primary aspect to an emotion. Physiology, cognition, behaviour and subjective experience all change in concert in ways that increase the ability to cope with a challenge.

Panic, for instance, is intense fear accompanied by increased heart rate, sweating, shortness of breath, and a strong urge to flee. When we face life threatening danger, these responses are useful. In fact, they are essentially identical to those described in 1929 by Walter Cannon as the "flight flight response". As Cannon noted, the shortness of breath results in increased ventilation and the high heart rate speeds the transport of glucose and lactic acid in and out of muscles. Sweating cools the body in preparation for flight.

This information has practical utility. Many of my patients suffer from panic disorder. On learning that the symptoms of panic are a useful response which, in them, goes off at the wrong time, they express profound relief at the understanding. More important, they quit interpreting their symptoms as possible heart trouble and this helps stop the runaway positive feedback cycle of panic.

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General anxiety is more pernicious. With no obvious cause, many of us worry constantly and spend much of our life's energies trying to protect ourselves from imaginary dangers. The psychoanalysts tell us, rightly I think, that many such anxieties arise from hidden impulses of which we are only dimly aware. The utility of these most distressing feelings is best observed in people who lack them. Such people follow their drives, betray their friends, annoy their relatives, and, unless they are very clever, get in terrible trouble. The psychiatrist Isaac Marks has coined the term "hypophobics" to cover this heretofore unrecognised disease of insufficient anxiety, but it may be some time before people line up at anxiety clinics for medications to increase their anxiety.

Can sadness, like pain and anxiety, be useful? This is the single question that most preoccupies me. Because certain experiences (losses) arouse sadness in all of us, and because sadness is so consistent a syndrome, it seems likely that it has a function, but what could it be? Well, what behaviour is useful after a loss? First, it is wise to stop what you are doing to prevent future losses. Then it will be wise to mull over the loss, even to ruminate about how to avoid a recurrence. This may apply even to grief. The mother in my community whose child choked when a string on a hood got caught in playground equipment has not just protected her other children from the risk of a similar tragedy, she has started a national campaign to eliminate this danger by changing clothing design.

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Notice that I address sadness, not depression. Some depression is obviously a result of pathological brain mechanisms, but could some depression be useful? The Swedish psychoanalyst Emmy Gut argues that when one of life's paths peters out in the woods, the wise action is no action. She argues that it is wise, in such situations, to stop, to reassess internal and external resources. The idea follows that of Bibring and wise counsellors across the ages. When I see someone with depression I always look for unreasoning commitment to hopeless enterprises, whether a marriage, a job, or a dream, and more often that not, my patient and I come quickly to a better understanding of their depression.

So, is depression a disease? Of course it is, in some cases. Just as panic disorder can be caused by a regulation mechanism that is awry, depression can come from brain abnormalities. Should we then look to defective genes for the explanation? We know blood relatives of patients with anxiety or depression are many more times more likely to have the same problem. Genetic factors are certainly involved, but once we recognise anxiety and sadness as defences, we can see that there must be genetic variability that affects their regulation, just as some people cough or vomit especially easily. When does this dysregulation constitute a defect, and when is it just a variation? We do not know.

As new pharmacological agents allow us to control our emotions with ever greater specificity and safety, people will demand a better understanding of the functions of the emotions. If a safe, non-addicting anti-anxiety medication were invented today, how would we use it? Based on history, the answer must be "excessively", but we really have no criteria at present.

When I first realised the utility of cough, fever and anxiety, I wondered how on earth we doctors could get away with blithely blocking defensive reactions. If natural selection is so powerful, then why has it not fashioned regulation mechanisms that already express the optimal degree of response? As has happened so often before, my investigation soon led me to greater respect for the wisdom of the body. If one approaches the regulation of defensive responses as a problem in signal detection, one quickly realises that the cost of a false alarm is tiny. Vomiting, for instance, may cost only a few hundred calories. Not expressing the defense, however, may be fatal. So if there is even a 5 per cent chance the chemical in the stomach is a serious toxin, the normal mechanism should initiate vomiting. By this reasoning, I finally realised why so many of us fear so much, so often. Fear, while painful, is cheap. If there is even a small possibility of avoidable danger, a fear response will be worth it. Thus, we can block fear responses in most instances without incurring any cost. There will be, of course, some instances when the fear will be useful, but that is another matter.

Psychiatry has been trying, for nearly 20 years, to find its proper place in medicine. By brain scans, diagnoses and drugs it has come part way, but by loudly asserting that mental disorders are all diseases like medical diseases, it paradoxically may have been missing exactly the principle that has long unified medicine. When a patient comes to the internist with kidney failure, the internist knows the functions of the kidneys, and thus some supportive measures, even before the cause of the failure is ever determined. If psychiatrists likewise knew the functions of anxiety, depression, envy, love and jealousy, perhaps they too would have an understanding of the functions of the emotions that constitute much psychopathology. As we come to recognise anxiety, sadness and jealousy as defences, perhaps we can put aside all the arguments about the "medical model" in psychiatry, and get on with the job of trying to relieve human suffering just as the general physician provides relief from cough, fever and physical pain.

Randolph Nesse is professor and associate chair for education and academic affairs, department of psychiatry, University of Michigan Medical School, and co-author (with George Williams) of Evolution and Healing: The New Science of Darwinian Medicine (Weidenfeld and Nicolson, 1995), which will be reviewed in The THES next week.

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