Aisling Irwin, continuing our series on the intellectual impact of Darwinism, talks to psychologist Paul Ekman and discovers how he was inspired by the Victorian biologist to classify the facial expression of emotion.

Here's a simple test of the strength of your marriage. Research shows that when a husband and wife meet up after work, and their interactions are videoed, there is a certain sequence of facial expressions that predicts a future marital breakdown: the wife displays anger; the husband no emotional response; then the wife displays disgust. Facial expression is a window on emotion, according to Paul Ekman, professor of psychology at the University of California in San Francisco. He is the grandfather of the study of facial expression and he has gradually given respectability to the study of emotion.

With his painstaking study of facial expressions across cultures and his resulting assertion that there are only a few basic emotions, which are common to all humankind, he has been controversial but he has also raised new ideas in psychotherapy, pharmacology, lie detection . . . and advertising.

Advertising? Picture this: you see an advert; as you react to it your emotions flicker across your face; a video traps them; later, the advertisers, using a facial expression monitoring system developed by Ekman, reconstruct your reactions. Within three years, says Ekman, a combined computer/camera should be able to decipher anyone's instantaneous emotional reactions.

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It was reams of film of a New Guinea society that set Ekman testing ideas about facial expression, back in the 1960s. He spent a year, watching the films, which had been taken in a "stone age" village. It was not the differences in culture that Ekman found fascinating but the lack of differences in facial expression. "I saw nothing I hadn't seen before," he says. He visited the village himself and published work in the late 1960s that attracted a lot of attention. He claims that there are six basic human emotions: anger, fear, disgust, happiness, sadness and surprise. They belong to our evolutionary, rather than to our cultural, heritage. And they are fleetingly and involuntarily expressed on our faces using combinations of facial muscles that are particular to each one. This challenged the traditional view, which is that there are hundreds of emotions lying along many "dimensions", such as the pleasantness dimension, which has happiness at one end and sadness at the other.

Reaction to his results was predictable: to biologists it was obvious, to psychologists it was becoming acceptable, to anthropologists it was outrageous. The ideas violated behaviourism, in which all relevant behaviour is learned. They also challenged relativism, by setting up rules that apply to all cultures. "I had a paradoxical reaction from lay people," he says. "They would say 'of course everyone's expressions are the same' but they would then say 'but I can't figure out those Japanese'."

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Ekman fought his antagonists in a painstaking way, designing new experiments to meet every objection to his methodology. His inspiration was Charles Darwin, who had published a sell-out book on the expression of emotion which had then lain in obscurity for 100 years. Darwin had explored the emotions we share with other primates, and the expressions we share with other cultures. "I was totally convinced," he says. "But I had the problem of explaining why the anthropologists had come to the opposite conclusion. After all, they were not dumb."

In an ingenious experiment he took two groups of students, one American and one Japanese, and showed them a film which elicited expressions of disgust in both groups. Then he showed the film again, but placed a man in a white coat in the room. This time, though the Americans continued to express disgust, the Japanese showed none. The presence of an authority figure, decided Ekman, caused a cultural reaction in the Japanese that led them to manage their facial expressions. He believes that there are important cultural and individual differences in facial expression but that, at root, evolution has programmed us with a common emotional software.

An alternative theory to explain Ekman's findings is "species constant learning": there may be some universal environmental factor that drives every member of a species towards the same facial expression. For example, we raise our eyebrows in surprise; but raising the eyebrows lengthens the visual field, so it could be a handy response that everybody learns, rather than one which is innate. To refute this theory Ekman has shown that people born blind also raise their eyebrows.

Ekman's method of measuring facial muscle movement, and therefore emotional expression, is now used by hundreds of people for disparate purposes: the marriage analysis, and subsequent marriage guidance described above; the study of pain, sign language, politicians speeches.

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His ideas are still controversial but he has moved onwards to match discrete emotions with specific physiological responses. Ekman has found that each of his few, discrete emotions causes a different, reproducible physiological response. Each response fits with an evolutionary explanation of why the emotion is necessary for survival. So, if you feel anger, blood rushes to your hands; at fear, the blood goes to your legs. The responses are the same across cultures.

Even more controversially, Ekman says that if you change your facial expression you will change your physiology (it works for 70 per cent of people, he says). "To those who have a linear model of the mind this makes no sense but if you think of emotion in terms of networks it does make sense, so the neurophysiologists all agree with me." So, pull your face into a smile - but it must be a proper smile that uses all the muscles you would use if something made you happy - and you will, albeit briefly, feel the physiological changes that correspond to pleasure.

Ekman is motivated by an idea that captivated him in his youth. After graduating in clinical psychology, he wanted to be a psychotherapist. But he swiftly became disillusioned at psychotherapists' ignorance about emotion. Now he feels he can help them.

Ekman argues that understanding the evolutionary origins of emotion, far from leaving us helpless, can enlighten us about our behaviour, which has evolved to be optimum for some primitive state before our development overtook our evolution. "We're very well adapted emotionally for the things we needed then," he says, for example the mother-infant reaction, and emotions involved in courtship, cooperation and friendship. "We're least adapted for the changes that tools have provided. Anger is a very socially good response. It motivates us to change things for the better." But anger, in the past, had time to die away before we did something regrettable. "It takes a long time to kill somebody with our hands. Now we can now kill people instantly."

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Ekman believes that emotion is the missing link in psychology. "Theories of cognition have left out emotion. The theories we are going to develop will have to be much wider". In his latest book, The Nature of Emotion, he says: "Just as the study of cognition provided a unifying theme for psychology in the 1970s, we believe that emotion can serve a similar role in the 1990s . . . In an era when the discipline of psychology is becoming increasingly fractionated . . . Emotion is a topic that holds out such a promise."