Married life is the key to happiness but having children can ruin it all, a psychologist claims.

Couples only recover their blissful existence once their offspring have left the nest, says Daniel Gilbert, a professor of psychology at Harvard University

He told a conference in Sydney that studies in America and Europe had shown that feelings of contentment spike during the early years of marriage, but fall heavily after having children.

Prof Gilbert said that a desire by couples to get a return on the time and money they have invested in their children is part of the reason they persuade themselves that their offspring are enhancing their lives.

"Figures show that married people are in almost every way happier than unmarried people - whether they are single, divorced, cohabiting," he said.

"Married people live longer, married people earn more money per capita, married people have more sex and enjoy it more."

However, when couples have children, happiness levels plummet. "Children do seem to increase happiness [while] you're expecting them, but as soon as you have them, trouble sets in," he told the "Happiness and its Causes" conference. "People are extremely happy before they have children and then their happiness goes down, and it takes another big hit when kids reach adolescence.

"When does it come back to its original baseline? Oh, about the time the children grow up and go away." Money was part of the reason that couples believed their children brought them happiness, he said.

Psychological studies have shown that people appreciate products more if they pay more for them. "We pay for [children] in time, attention, blood, sweat and tears - what kind of idiots would we be to devote all of that to the rearing of our young if they didn't bring us some happiness?" he said.

Repeated studies have shown that the period immediately surrounding childbirth can be the most stressful that couples experience. Psychologists have also found that couples with children are less satisfied with their marriage than those without.

Research in the Netherlands in the 1990s showed that couples who had two children were less happy than those with none. But some experts believe that it is not having children itself that breeds the discontent.

Richard Tunney, of the University of Nottingham, said: "From an evolutionary point of view we are programmed to procreate. It would not make sense for having children to make us unhappy.

"However, in countries like Britain, having children is hard - your finances are hit, childcare in this country is appalling and for women especially, their careers suffer.

"That is not the fault of having children per se, but of society."