Seligman describes this as an "epiphany". He vowed to change his own outlook, but more importantly recognised a strength — social intelligence — in his daughter that could be nurtured to help her withstand the vicissitudes of life. Looking back on "learned helplessness", he reflected that one in three subjects — rats, dogs or people — never became "helpless", no matter how many shocks or problems beset them.

"What is it about some people that imparts buffering strength, making them invulnerable to helplessness?" Seligman asked himself — and now he's made it his mission to find out.

Since its origins in a Leipzig laboratory 130 years ago, psychology has had little to say about goodness and contentment. Mostly psychologists have concerned themselves with weakness and misery. There are libraries full of theories about why we get sad, worried, and angry. It hasn't been respectable science to study what happens when lives go well. Positive experiences, such as joy, kindness, altruism and heroism, have mainly been ignored. For every 100 psychology papers dealing with anxiety or depression, only one concerns a positive trait.

A few pioneers in experimental psychology bucked the trend. Professor Alice Isen of Cornell University and colleagues have demonstrated how positive emotions make people think faster and more creatively. Showing how easy it is to give people an intellectual boost, Isen divided doctors making a tricky diagnosis into three groups: one received candy, one read humanistic statements about medicine, one was a control group. The doctors who had candy displayed the most creative thinking and worked more efficiently.

Inspired by Isen and others, Seligman got stuck in. He wanted to revolutionise psychology, but his weapon would be tough science. Clinical psychology was the science of how to get from minus five to zero. This would be the science of getting from zero to plus five. Seligman wanted experiments, he wanted statistics, he wanted proof.

He raised millions of dollars of research money and funded 50 research groups involving 150 scientists across the world. Four positive psychology centres opened, decorated in cheerful colours and furnished with sofas and baby-sitters. There were get-togethers on Mexican beaches where psychologists would snorkel and eat fajitas, then form "pods" to discuss subjects such as wonder and awe. A thousand therapists were coached in the new science.

Their holy grail is the classification of strengths and virtues. After a solemn consultation of great works such as the samurai code, the Bhagavad-Gita and the writings of Confucius, Aristotle and Aquinas, Seligman's happiness scouts discovered six core virtues recognised in all cultures: wisdom, courage, humanity, justice, temperance and transcendence. They have subdivided these into 24 strengths, including humour and honesty.

But critics are demanding answers to big questions. What is the point of defining levels of happiness and classifying the virtues? Aren't these concepts vague and impossible to pin down? Can you justify spending funds to research positive states when there are problems such as famine, flood and epidemic depression to be solved?

Seligman knows his work can be belittled alongside trite notions such as "the power of positive thinking". His plan to stop the new science floating "on the waves of self- improvement fashions" is to make sure it is anchored to positive philosophy above, and to positive biology below. And this takes us back to our evolutionary past.

Homo sapiens evolved during the Pleistocene era (1.8 m to 10,000 years ago), a time of hardship and turmoil. It was the Ice Age, and our ancestors endured long freezes as glaciers formed, then ferocious floods as the ice masses melted. We shared the planet with terrifying creatures such as mammoths, elephant-sized ground sloths and sabre-toothed cats.

But by the end of the Pleistocene, all these animals were extinct. Humans, on the other hand, had evolved large brains and used their intelligence to make fire and sophisticated tools, to develop talk and social rituals.

Survival in a time of adversity forged our brains into a persistent mould. Professor Seligman says: "Because our brain evolved during a time of ice, flood and famine, we have a catastrophic brain. The way the brain works is looking for what's wrong. The problem is, that worked in the Pleistocene era. It favoured you, but it doesn't work in the modern world."