It's considered the ultimate taboo for any woman. In this day and age, to admit that you don't want children is seen as practically the same as saying Adolf Hitler had a point. It's best to keep your beliefs to yourself. After all, we live in a world where fertility and motherhood can practically elevate you to living sainthood - just ask Angelina Jolie.

But Cameron Diaz doesn't quite see it that way and, while publicising her first movie role as a mother in My Sister's Keeper, she defended those women who don't want to have children, saying that there are enough people already on this planet and that we don't need any more kids.

"I think women are afraid to say that they don't want children because they're going to get shunned," the actress said in an interview with Cosmopolitan. "But I think that's changing too now. I have more girlfriends who don't have kids than those that do. And honestly? We don't need any more kids. We have plenty of people on this planet."

While there are lots of reasons to not want to procreate, not many people say it's because children aren't eco-friendly.

But one woman who would agree with the actress is Toni Vernelli. The British woman works for an environmental charity and says she was sterilised at the age of 27 in order to reduce her carbon footprint.

Having accidentally fallen pregnant 10 years earlier, she had begged the doctor who was performing the termination to sterilise her at the same time.

He refused, but that didn't stop her hunting for someone who was willing to perform the surgery. Eventually, she found a surgeon who was willing to oblige.

"Having children is selfish. It's all about maintaining your genetic line at the expense of the planet," said Toni.

"Every person who is born uses more food, more water, more land, more fossil fuels, more trees and produces more rubbish, more pollution, more greenhouse gases, and adds to the problem of over-population."

This may be an extremist view, but lots of women are ambivalent about reproducing, especially those who put career ahead of family.

In 2006, a report was published by the US Census Bureau which showed that 20pc of American women from the ages of 40 to 44 do not have children -- double the level of 30 years ago.

The study also found that women with advanced degrees were more likely to be childless. Of women aged 40 to 44 with graduate or professional degrees, 27pc are childless, compared with 18pc of women who did not continue their education beyond high school.

With the current recession, one would expect birth rates to follow the trends of past economic recessions and decrease further again.

For Orlaith Young (26) not wanting children has nothing to do with either environmental or economic factors.

"I knew about two or three years ago that I didn't want to have kids," says Orlaith, a secondary school teacher from Greystones, Co Wicklow. "I just don't have a natural affinity with them and I would be too conscious I wouldn't raise them to have the social and moral capabilities that I'd want them to have.

"I'm not sure how to get the best out of them and I'd be stressed about not having nice children.

"I don't enjoy young kids and I'm impatient with them. I'd prefer to be handed a teenager if anything," Orlaith continues.

"Being a teacher probably has something to do with that but coming from a science background I'm also aware of what can go wrong genetically, and the possibility of having a child with a disability really scares me -- I just wouldn't be able to cope."

Orlaith says that the idea of adoption is more appealing, even though the thought of pregnancy and childbirth doesn't bother her at all.

"I just wouldn't want to have to give up my life. I've no problem being an auntie or whatever but I just don't want my own. Having said that, if I met someone and became involved in a serious relationship with them and they desperately wanted kids I'm not saying I wouldn't reconsider."

However, for Charlie Lewis (29) her mind has been made up for as long as she can remember, and fortunately her partner Eoghan is in agreement.

"I've always known I didn't want to have children but people always told me that that would change as I got older," says Charlie, who works as a project manager in Dublin city.

"I don't feel I'd want to take on the responsibility; it makes me nervous.

"Having seen my friends and cousins who have kids and observed what is involved, I wouldn't want the pressure of having to plan every little thing. I'm much more career-focused and I don't want to take away from that."

In contrast to Toni Vernelli's beliefs, Charlie has had people tell her it is a selfish decision not to have kids.

"A lot of older women are shocked by my decision and have told me it is selfish, but I don't take it personally. I love other people's kids, but I also love being able to hand them back.

"I come from a really close family and I'm the only one out of all of us who doesn't want children, but it's just a personal choice," says Charlie.

"My partner Eoghan feels the same, though I didn't know that until long after we'd met. We were together three years before I thought I had better broach the subject with him just in case he did want them.

"But he is very career-focused as well, and would rather concentrate on that as well as having the option to travel in later years without being tied down."

So the choice to have children is just that -- a choice.

Yet for many childless women there is still the need to defend the decision not to have kids.

In Japan, for instance, childless couples are labelled 'parasites' and accused of being unpatriotic.

In Germany and Russia, there have been discussions about introducing certain 'disadvantages' for couples without children, and in Slovakia there has even been reference to the introduction of a 'childless tax'.

And yet if you go to the other extreme and have a big family, you can also be considered selfish and in some cases sponging off society.

So apparently, unless you have 2.4 children, a husband who works in the bank and live in a solar-heated semi-D in the leafy suburbs with a hybrid Skoda parked in the driveway, you simply can't win.

And the final word goes to Cameron Diaz. When asked if she wanted children, the actress replied: "I never say never. I don't know what's going to happen. I could end up adopting half a dozen kids, or I could end up being the next 'octomum' -- who fricking knows!"