Perhaps that's not so surprising. Having children can have a significant impact on finances, careers and the planet. More surprising is the growing body of evidence that suggests it can also make you less healthy and less happy. But can the situation really be that gloomy?

Children in the West can be a huge financial drain. The average middle-class American family would have spent more than US$245,340 on each child by the time they're 18. In Britain, the cost of raising a child has swelled 63 per cent since 2003, with childcare alone eating up 27 per cent of the average salary, according to the Centre for Economics and Business Research, in London. In Hong Kong, it costs parents an average of HK$6 million to raise each child, according to the Bauhinia Foundation Research Centre.

Luca Stanca, an economist at the University of Milano-Bicocca, in Italy, puts it bluntly: "On the basis of a purely economic approach, the optimal number of children for a rational agent is zero."

Children, though small, can come with a large environmental footprint. In the US you can recycle and bike to work all you want to reduce your carbon emissions, but those gains will be 20 times less than the carbon dioxide impact of having a child, according to a 2009 study from Oregon State University. The United Nations projects that "if current population and consumption trends continue, humanity will need the equivalent of two Earths to support itself by 2030".

Some have taken this message to heart. Environmentalist Bill McKibben struggled with the decision of whether to have children, and ultimately opted for one, defending his choice in his book . The Voluntary Human Extinction Movement even urges people not to add to the "burgeoning billions already squatting on this ravaged planet", seeing the only sustainable future as one without humans.

That's an extreme view. But there is now almost half a century of evidence on the relationship between having children and personal happiness that might give more people pause for thought. Contrary to what we might think, study after study has shown that having children does not seem to make people happier, and, in fact, may even make them a little less happy.

"The great majority of studies find no effect or a negative effect," says economist Andrew Oswald, of the University of Warwick, in Britain.

See also: How childbirth affects a father's well-being, and how to put it right

Having children makes couples less happy with their sex lives, is associated with depression, sleep deprivation and, as one study puts it, "hastens marital decline". One oft-cited 2006 study co-authored by Princeton University psychologist Daniel Kahneman found that a group of working US mothers ranked childcare 16th out of 19 everyday tasks in terms of positive feeling, just ahead of commuting to and from work, and work itself.

This year, Daniel Hamermesh, of Royal Holloway, University of London, and his colleagues published a study of more than 14,000 Australian and German couples finding that mothers reported a sharp rise in stress after the birth of a child - three times that of the father - and that it increased year on year until four years after the birth, when the study stopped. Another study published this year, which followed more than 2,000 first-time German parents, found that the average hit to happiness exacted by the arrival of an infant is greater than a divorce, unemployment or the death of a spouse.

On this basis, it might seem utter folly for couples to take the parenthood plunge.

"If you believe that having children will make you substantially happier, then, on average, you're wrong," says Oswald.

Sonja Lyubomirsky, a psychologist at the University of California, Riverside, in the US, began to doubt that black-and-white picture a few years ago.

"It didn't make sense that parenthood wouldn't make us happy," she says. "How would we survive as a species if no one wanted to be a parent?"

In 2012, she and her colleagues published a paper in the journal showing that having children made men (but not women) happier. It garnered a lot of press attention for suggesting that researchers had got the wrong end of the stick on parental happiness. But others questioned that conclusion. Saurabh Bhargava, an economist at Carnegie Mellon University, in the US, published a critique last year in the same journal. His criticism was that by comparing a largely married population with kids with a largely unmarried population without kids, Lyubomirsky's study failed to account for another factor that might be responsible for the happiness boost: being married.

"One of the most robust effects in the happiness literature is the effect of marriage on well-being," Bhargava says.

Lyubomirsky agrees that "marriage is one of the key alternative explanations". Still, in a response to the critique, she and her colleagues stated that they were not trying to prove that children make people happier.

"Motivated in part by media portrayals of parents who are 'miserable' and who 'hate parenting', we simply asked whether happiness and parenthood can coexist," they wrote.

This idea is backed up by research conducted last year by Angus Deaton, of Princeton University, and Arthur Stone, of Stony Brook University, in the US.

"We ask: Is it true that people with kids are happier than people who don't have kids? And the answer to that question is yes," says Deaton. "But the people who have kids have all sorts of differences from the people who don't have kids. They have more money, they're more religious, all these sorts of things." When Deaton and Stone accounted for those variables, the correlation between children and increased well-being disappeared.

Such niggles show just how complex parenthood and happiness are to study.

"If you want to understand the causal effect of sleeping pills on somebody's sleep, you can run placebo trials," says Oswald. "You can't for children." Kids can't be handed out at random to see what effect they have on people.

One way round this problem is with a before-and-after study of the same people. Their lesson seems to be that parents' happiness increases a year or so before the birth of the first child, and then returns to pre-birth levels by the time the baby is about one. So the true picture is clearly more nuanced than a blanket "kids make you unhappy". Stanca has recently found that parenthood tends to boost people's satisfaction with their lives apart from their financial circumstances - but for most people, the money woes associated with children were so great that any additional happiness they felt was swallowed up.

"Children do make us happy," he says, "provided we can afford them - or think so."

A parent's age may matter, too. In a study across 86 countries, Mikko Myrskylä, of the Max Planck Institute for Demographic Research, in Germany, and Rachel Margolis, of the University of Western Ontario, in Canada, discovered that for people younger than 30, children are associated, on average, with a decrease in happiness. From 30 to 39, the average effect on happiness is neutral, and at age 40 and above, it's positive. For them, it's the more the merrier, to a point - three seems to be the optimal number. Such effects suggest, say the researchers, that having kids may be a "long-term investment in well-being".

Then, of course, there's the question of where you live. Parents in the 20 to 29 age group tend to sustain a large hit to their happiness by having children, but Margolis and Myrskylä found the generous welfare systems in countries such as Sweden, Japan and France soften the blow. The most happy parents over 40 live in former socialist states such as Russia and Poland (China was included in the "developing countries" category in the study), where care of the elderly falls mostly to the family, so having children is a boon in later life. In countries with less generous welfare systems such as Britain, there is a slight indication of decreased happiness with the arrival of the first child.

Indeed, comparing happiness levels between parents and non-parents within a country, and then between countries, can serve as a sort of global barometer. In a study currently under review, sociologist Robin Simon, of Wake Forest University, in the US, and her colleagues looked at 22 countries and found that the happiness gap in the US between those with and without children is wider than in the majority of the other countries studied, where provision for parents is often more generous.

"Having kids in the US is brutal," Simon says. "The federal government requires that workplaces give six weeks maternity leave, but there is no requirement that it is paid. We just don't do anything to assist parents."

Simon believes it is this lack of support in the US and probably other countries that wipes out one gain you would expect to find even among less happy parents - a sense of greater purpose. In ongoing work looking at 12 indicators of well-being, including physical health, self-acceptance and sense of purpose, Simon and her colleagues found that none, except lower alcohol use, was associated with parenthood in the US.

"I thought at least purpose and meaning in life would be higher for parents," she says, "and we find it's just flat."

Parental well-being, then, would seem to be a lottery. Children themselves probably don't make you less happy - but external factors associated with raising them might. If you're married, well-off or a resident of a Nordic country with generous social provision, you have a better chance of enjoying parenthood. For the rest, it may not be the experience they had hoped it would be.

Despite the gloomy picture, the vast majority of people still would like to have children, so, for Simon, the solution is to create a society that allows more of us to reap the rewards of parenthood.

"There is joy to having kids," she says. "But I think that for most people, the stresses that are associated with having kids overshadow those joys."

New Scientist