Nicola Moriarty's latest book 'Those Other Women' speaks to the often incomprehensible "otherness" of those who make choices that are different from our own.

It is broadly accepted, if not always applauded, that in the Western world today, women may do with their lives pretty much whatever they want. In many countries, including this one, they can even serve on the front line in war. So why, when society has become so relaxed about a multiplicity of female roles and identities, does it still baulk when we reject just one in particular - that of motherhood?

We may have moved on since the days when childless women were regarded as witches, but progress has perhaps not been as great as we'd like to think. This week, professional racing driver Leilani Munter, 44, who is fronting a new campaign for British charity Population Matters, spoke about the difficulties women can face in making the choice to be child-free. "It's something expected of people as if that is the natural chain of events: you meet your partner, get married and have kids," she said. "When you don't do that last step, people ask, 'Are you not having any kids?' I always answer, 'Actually my husband and I are child-free by choice. When you say 'childless' it sounds like you are missing something."

Munter's comments will no doubt resonate with Nicola Moriarty, who explores this theme in her new novel, Those Other Women - a title that speaks to the often incomprehensible "otherness" of those who make choices that are different from our own.

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In the book, two friends who have opted not to have children create a Facebook group for women like them. Set up in opposition to their local "mums online" forum, "Non-Mums Online" brings together those who have grown tired of watching their colleagues get pregnant and take months off work, only to return with "smug" smiles.

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It throws up an interesting question: why are we still so bewildered by women who are child-free by choice? I put this to Moriarty, surprisingly, a 36-year-old mother of two daughters aged nine and seven, when she speaks to me on the phone from her native Australia.

"Even though the world is changing and we are becoming more accepting of different types of family units, people will still [make assumptions about] women of a certain age," she says. "If they haven't got children yet, they're going to ask why. There's an assumption it's one of life's goals. I wonder if it's to do with that maternal instinct - the idea that everyone must have the biological clock ticking."

She points out that representations of women in popular culture tend to hammer the message home: once you are past a certain age, "everything you see paints the woman as a mother. If we do see [a child-free woman represented], usually it has to be all about someone who's focused on a career. It's never about a woman who just wants to live her life and enjoy it."

Childlessness is on the rise. Earlier this year, an international league table found that a fifth of British women remained child-free in their 40s, with the overall rate in this country up by almost 50 per cent since the mid-Nineties. In Australia, where Moriarty's novel is set, 16 per cent of women in their 40s have no offspring. Yet our attitudes appear to be lagging behind the figures, prompting a backlash among some women. The idea of Moriarty's non-mums Facebook group, for instance, is not fiction: on the contrary, several such online communities exist in real life.

One, called Women Without Children, features on its page a sardonic mock-up of a car sticker that reads "No baby on board, feel free to drive into me." Posts in the group include scientific research showing that mothers age faster, and a study that says women without children typically earn more.

Another article shared between members asks, "why are women without children still stigmatised by society?"

Moriarty's novel takes the theme as its jumping off point, making entertaining play of the rivalry between mums and non-mums - something she has experienced first-hand. "I recall that before having children, I did make judgments of other people," she admits. "I remember being part of the workforce and making judgments of mothers for having special privileges and [me] having to take up the slack."

She was 29 when she had her first daughter, having always known she wanted children. She also always knew she wanted to write - her childhood dream was to compose and illustrate children's books - but, unsure how to go about it, she bounced between various jobs in sales and marketing, ran her own gift hamper business, and became a swimming teacher.

"But I was always writing and I've got two sisters who are writers. Seeing them get published made me think, 'Wow, there are real life people who do this,'" she says.

One of these sisters is Liane Moriarty, the author of Big Little Lies. When her bestselling novel was adapted as an HBO television series starring Reese Witherspoon and Nicole Kidman last year, Liane's career attained a level of success most writers can only dream of.

"It's still surreal. You're like, 'oh my God, Liane, how did you get so famous?'" laughs her younger sister.

Is there ever any sibling rivalry?

"[It's] pretty good-natured," says Moriarty. "The only rivalry we have is fighting over family stories we want to use in our books."

Growing up in Sydney, Nicola was the youngest of six, and Liane used to read her bedtime stories. "It was definitely inspirational to have all this storytelling in our household," she says.

But her own first novel happened, almost by accident, about 10 years ago.

"One night I started working on something that I only ever intended on being a short story but when I got to the end, it didn't feel finished, so I thought I'd keep going," she recalls. "I didn't admit to myself that I was writing a novel due to fear of failure. I googled 'how many words in a novel' and came up with 100,000 and thought 'that's what I'm going to aim for.'"

The result was Free-Falling, a tragic-romantic comedy published in 2012. But only when an American publisher picked up her second novel, Paper Chains, originally published in Australia in 2013 and released in the US last year, did she realise she could carve out a future in writing.

"I thought, 'well, I've lived the dream but I'm unlikely to get published again," says Moriarty. She is currently working on her sixth novel.

Having published authors in the family has been a mixed blessing, however. "Over the years I've worried people have thought I'm only writing or getting published because of my sisters. People have said 'publishers are trying to cash in on her sisters' fame' - and that's very disheartening," she says.

Still, it must be tempting to ask her sisters for advice sometimes?

"I have in the past," she says. "It's nice to have someone to reassure you that yes, it's OK."

Such reassurance is what Moriarty is now offering in her new book - to women, like Munter and thousands of others, who have chosen to be child-free. It's assumed that "if you don't want this natural maternal thing there must be reasons for it," she adds. But those "other women" are really, as Moriarty points out, not all that different to anyone else.