These are the conclusions of a burgeoning happiness industry that has published 3 000 papers, set up a Journal of Happiness Studies and created a World Database of Happiness in the last few years. And happiness has finally burst out of the academic closet, with several weighty volumes on the subject having been published in the last year.

But can scientists tell us what happiness is? Economists accept that if people describe themselves as happy, then they are happy. However, psychologists differentiate between levels of happiness. The most immediate type involves a feeling: pleasure or joy. But sometimes happiness is a judgement that life is satisfying, and does not imply an emotional state.

Public surveys measure what makes us happy. Marriage does, pets do, but children don't seem to (despite what we think). Youth and old age are the happiest times. Money does not add much to happiness; in Britain, incomes have trebled since 1950, but happiness has not increased at all. The happiness of lottery winners returns to former levels within a year. People disabled in an accident are likely to become almost as happy again. Psychologists know that increasing the number of social contacts a miserable person has is the best way of cheering them up. And happiness levels are probably genetic: Identical twins are usually equally bubbly or grumpy.

Perversely, happiness has a negative image in our culture. Influenced by a sceptical European philosophical outlook, we think of happiness as a trivial pursuit for the Oprah generation, a Shangri-La perpetuated by self-help gurus. Isn't it selfish to try to increase our happiness, while much of the world faces suffering and premature death?

America has pursued the chimera of happiness vigorously, not least through the insatiable consumption of self-help literature. So it's no surprise that it's an American who's making happiness a subject of scientific study. At first glance, Martin Seligman's best- selling book Authentic Happiness, with its sunshine-yellow title on a sky-blue cover, blends with other manuals on the pop-psychology shelves. But America's latest guru of feeling good is not a stage hypnotist, an evangelical preacher or even a business visionary. Seligman is an eminent professor of psychology with a string of degrees.

The idea for positive psychology came to Seligman in 1997 while gardening with his daughter Nikki. She was throwing weeds around and he was shouting. She reminded him that she used to be a whiner but had stopped on her fifth birthday. 'And if I can stop whining, you can stop being a grouch.'

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

Seligman 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 babysitters. There were get-togethers on Mexican beaches where psychologists would snorkel and eat fajitas, then form groups to discuss subjects such as wonder and awe. A thousand therapists were coached in the new science.

But critics are demanding answers to big questions. What is the point of defining levels of happiness? 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's anchored to positive philosophy above and to positive biology below. And this takes us back to our evolutionary past.