Your personality directly influences your life expectancy, doctors now believe. And a positive mental attitude really can help you to resist illness. Jerome Burne reports on how to think yourself well

Just imagine you are worried about developing Alzheimer's or having a heart attack and you go to your doctor for a check-up. But instead of asking about the usual suspects - weight, smoking, diet, exercise and so on - he makes a sudden loud noise and then asks you to fill in a personality questionnaire.

Just imagine you are worried about developing Alzheimer's or having a heart attack and you go to your doctor for a check-up. But instead of asking about the usual suspects - weight, smoking, diet, exercise and so on - he makes a sudden loud noise and then asks you to fill in a personality questionnaire.

You might wonder whether he was having a breakdown - but in fact he'd be at the cutting edge of research. In recent months, a number of studies have given a hi-tech spin to the old idea that your personality can affect your risk of developing various disorders. What seems to be especially significant is how prone you are to negative thoughts and feelings - hence the testing of the strength of your startle reflex with a sudden noise. The stronger it is, the more susceptible to the damaging effects of stress you are likely to be.

That is certainly the case with rats who are "neophobic", or afraid of new things - a trait that goes with being anxious and fearful. A recent report found that such rats die sooner than their bolder relatives. But it's not just rats - similar trends appear in humans. A study at Rush University Medical Center in Chicago has just found that people who have negative emotions most often are twice as likely to develop Alzheimer's. And the connection has also shown up with Aids.

For years, it has been observed that shy and introverted Aids patients die sooner. Then, in December, researchers at the University of California Aids Institute reported that they had recorded a higher level of nervous-system activity in these patients, and this was what reduced their resistance to HIV. The effect was so strong that some standard drug treatments, which usually reduce viral load by a factor of 10, were ineffective.

The idea of a link between personality and disease goes back to the ancient Greeks with their doctrine of the four humours, which remained popular in Europe until at least the 17th century. In Victorian times, tuberculosis was believed to be the result of an overactive creative spirit consuming a frail body. However, once the tuberculosis bacterium was discovered and the disease could be treated with antibiotics, it became an example of how the theory that mind affects disease was simply a measure of our ignorance.

This highlights a split that runs through modern medicine, which explains why the idea of a link between personality and disease has always been a tricky concept. Scientific medicine developed in the shadow of the philosopher Descartes, who "solved" the mind/body problem by declaring the mind an ethereal thing, of a quite different substance to the body. At the time, it was a liberating idea, and medicine made huge strides by ignoring the mind and treating the body as a mechanical system. Antibiotics work, whatever your personality.

As a result doctors, by and large, have never been terribly keen on the links between psychological states and disease. They like to deal with what they can see and measure - a viral infection, a malignant tumour. This comes from an editorial in the New England Journal of Medicine a few years ago: "It is time to acknowledge that our belief in disease as a direct reflection of mental state is largely folklore."

Part of this suspicion has been fuelled by some of the wilder pronouncements of the psychotherapists. Freud, for instance, distinguished between real illnesses and hysterical ones brought on by unconscious emotional conflicts - asthma, for example, was explained as "the fear of losing parental love". As recently as the 1940s, the psychiatric explanation for Parkinson's disease was that it was the result of "conflict between an aggressive drive to action and an equally strong internal pressure to inhibit action". The specific conflict was the result of "the wish to masturbate in the ambitious moralistic man".

Despite these absurdities, evidence kept mounting that mental states can affect our health in very physical ways. Even people with tuberculosis can be free of symptoms for years and then succumb after suffering some kind of emotional trauma. Twenty or 30 years ago, the search focused on links between disease and personality types - a striving, ambitious Type A and a socially inept and anxious Type D. Both have been linked with a raised risk of heart attack.

Recently, researchers have focused more on specific emotions such as anger, which has been linked with a worsening of irritable bowel disease, ulcers, headaches and sensitivity to pain, and even with backache, arthritis and a greater chance of becoming addicted to nicotine. A few months ago a report said that sulky, bad-tempered men were 30 per cent more likely to develop a dangerously irregular heartbeat.

What makes these findings different from the claims of 20 years ago is that there is a much clearer idea of the sort of mechanism that might be involved. "People who are hostile have a chronically 'turned on' sympathetic nervous system," says Professor Catherine Stoney, a psychologist at Ohio State University. "This gives them higher blood pressure, higher heart rates and higher cholesterol." The stress of being angry and hostile also increases the amount of inflammation in the blood vessels, which in turn encourages the build-up of damaging plaque.

Stoney found something else at work that suggests an even more direct link between anger and heart problems. Angry men and women have higher levels of an amino acid called homocysteine in their blood. High homocysteine is increasingly recognised as being directly linked with heart attacks.

So the way the link between personality and health works is that negative states are more stressful, and long-term stress makes changes to the immune system, which in turn can have damaging effects on the body and the brain. The link with a more fearful outlook and Alzheimer's, for instance, may come via the damage that stress can do to an area of the brain known as the hippocampus, which is involved with forming new memories.

But emotions don't just make you ill; they can also have a protective effect, and here the mechanism involved is different again. A link between happiness and a boosted immune system was dramatically demonstrated last autumn by a team from the University of Wisconsin-Madison. Brain-scan studies have already shown that people with a more positive outlook also have more activity on the left side of their brain in the prefrontal cortex (just behind the eye). The Wisconsin scientists ran brain scans on more than 50 people and found that same thing. But then they gave all the subjects a flu vaccination. Six months later, those with the most left-brain activity had the highest level of flu-fighting antibodies in their blood.

One of the most intriguing discoveries about optimism is that it seems to be beneficial even when it is unfounded. A study of men with Aids discovered that those who held a realistic view of when they were likely to die in fact died an average of nine months earlier than those who were absurdly optimistic about their survival chances.

"Optimism seems actually to change the nervous system and boost the immune response, just as being gloomy depresses it," says Dr Shelley Taylor, a University of California psychologist. She found that optimistic law students had higher levels of disease-fighting killer cells in their blood than did the pessimists. "Optimism is an underrated resource," she says. "It gives you much more than people imagine it does."

All this suggests not only that psychological tests may be able to predict your risk of disease, but that dealing with psychological issues may make you healthier. The Wisconsin researchers, for instance, have shown that regular meditation can increase the amount of activity in the positive left side of the brain.

It is also possible to use psychological techniques in order to encourage optimistic thinking. "Try thinking of pessimism as a sort of behaviour," says Professor Martin Seligman of the University of Pennsylvania in Philadelphia, "rather than a way of thinking. Pessimism is how we explain to ourselves and to other people why certain things happened."

Research by Seligman has shown that when something goes wrong, optimists and pessimists explain what is happening in opposite ways. "The pessimist blames himself, and thinks not only that it is likely to happen again but the rest of his life will be affected by it too," he says. "The optimist, on the other hand, puts it down to outside forces that are temporary and have no long-term significance."

A classic optimistic sportsman's response to losing, for instance, would be to blame the weather. But when explaining a win, he'd do the opposite - it would be all due to his training, he was getting better all the time, and so on. A psychological technique known as cognitive behavioural therapy (CBT) can be very effective in changing your outlook from pessimistic to optimistic.

CBT can also make a big difference to the way you handle anger by teaching such techniques as taking deep breaths before reacting, keeping a log of what pushes your anger button, and not buying into the notion that the best way to deal with anger is to let it out.

So could changing your psychological state actually lower your risk of heart disease? A study by Duke University Medical Centre suggests it might well do. Heart patients who were taught how to manage stress had an average of 0.8 cardiac incidents after five years, while those who were given "usual care" had 1.3. And the annual cost of treating these patients dropped from $4,523 to $1,228.