Not everyone does it for the kids.

“The Big Bang Theory” revealed on an episode last week that Penny (Kaley Cuoco) doesn’t want children — a decision that came as a surprise to her husband, Leonard (Johnny Galecki), and drew initial dismay and condescension from her female friends. “You know, not everyone needs to have kids to be fulfilled,” Penny told one friend.

The reluctance to procreate extends far beyond fiction. 2017 saw the lowest number of births in 30 years, according to a National Center for Health Statistics report this past spring, in a 2% dip from 2016. And the fertility rate for women aged 15 to 44, also a record low, declined 3% from 2016 to 60.2 births per 1,000 women.

Why the shift away from babymaking? Among adults aged 20 to 45 surveyed this summer by Morning Consult for the New York Times, 36% of those who said they didn’t want children or were unsure cited wanting leisure time; 34% said they haven’t found a partner, 31% said they can’t afford child care, and 30% said they had no desire for children.

“I love my life as it is,” Amy Blackstone, a sociology professor at the University of Maine, told Moneyish of her own decision not to have kids. “And I feel that parenting is such an important job, and such a hard job, that to ask anyone to do it who is not 100% into doing it is not in the best interest of children or of parents — or of really anybody.”

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Blackstone, 46, now a prominent researcher in the “child-free” movement, grew up assuming she would become a parent — and played a considerable role in socializing herself toward that role. She became a certified babysitter at age 11, served as head of her church nursery in high school and worked as a nanny throughout college. But around her mid-30s, as many of her friends were having kids or planning for a family, she realized she wasn’t feeling that same pull toward motherhood.

Blackstone set out to understand what was “wrong” with her, she said, only to discover there was “absolutely nothing wrong with me.” “Everything about the way that we’re socialized — especially women — tells us that no, it’s not normal to not want to have kids.” (Some research has also suggested that both men and women experience so-called “baby fever.”)

What started as a personal quest transformed into Blackstone’s chosen field of study. She now runs the blog We’re Not Having a Baby! with her husband, Lance Blackstone, and authored the forthcoming book “Childfree by Choice.”

“I think that historically, our economic system (and) our social structures are set up based on the assumption that women and men will partner with each other, first of all … and that those partnerships will result in new members of the church, new members of society, new workers to enable society to carry on,” Blackstone said. Women have historically been valued for their contributions as mothers, she added.

Blackstone isn’t alone in leading an intentionally child-free existence. “I don’t have the money, I don’t have the community support, and I don’t feel like I fit into our parenting culture,” Tara Eisenhard, a 39-year-old divorce coach and author from Harrisburg, Pa., told Moneyish. She also cited health concerns like the maternal mortality rate — the U.S. has the highest of any developed nation — as well as health-care costs.

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Stephanie Bousley, a 35-year-old author and personal finance consultant in Los Angeles, drew lessons from her own childhood in Minnesota, where money troubles served as a constant source of fighting. Bousley thought her parents would’ve been more comfortable without kids, she told Moneyish, and maybe even happier.

As she entered her 30s and watched many of her friends have kids, Bousley added, she never found herself wanting the same. There was also her roughly $200,000 in student loan debt after grad school. (Women claim nearly $900 billion of the country’s $1.4 trillion student loan debt, according to an American Association of University Women report; meanwhile, they face the setback of making an average of 80 cents on a man’s dollar.)

Rachel Johnston, a 32-year-old publicist and roller derby athlete living in Los Angeles, recounted constantly struggling with “the checklist of things that women are expected to do with their lives.” “I never connected with women’s stories of pregnancy or childbirth,” she said. “I sort of always watched them in movies and in culture and friends and things like that, sort of observational and never with a desire to participate in it.” She figured that if she ever did raise a child, it would be in a non-biological fashion like adoption.

“I’ve sort of built a career around things that I love and with people that I love, and I’ve created this incredible family of friends and chosen people,” Johnston said. “I can’t imagine that conceiving would bring something to my life that I don’t already have. And maybe I’m wrong, but also it’s my choice to be wrong about that.”

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But research shows that people face stigma for not wanting kids: A study last year, in fact, found that voluntarily child-free people inspired “significantly greater moral outrage” than those with two children. (Meanwhile, studies have shown that parents tend to be less happy than non-parents. Research also suggests that parents in the U.S. face the largest “happiness gap” compared to people without children, a disparity explained by the presence of family-friendly policies.)

Blackstone recalled an Uber driver last year telling her she would “regret” not having kids after learning of her area of study and child-free preference. “I have encountered people who have suggested that my life is not complete, that I’m not complete — that it’s impossible to know what love is unless you have children, or that it’s impossible to be a responsible human being unless you have children,” Eisenhard added.

“People say things like, ‘You could change your mind’ or ‘Are you sure?’” said Lindsey Smith, a child-free 30-year-old Pittsburgh cookbook author who says she draws fulfillment from mentoring and spending time with kids in her neighborhood. “It is weird, because you feel like there is a societal expectation that you’re going to miss out.”

These conversations aren’t always fraught. “One of the things that’s really awesome and refreshing about being so vocal about my choice,” Blackstone said, “is it’s provided the opportunity for me to have real, honest conversations with my friends who are mothers. I think that there’s been some eye-opening on both sides.”

The decision to have kids isn’t one to take lightly, Smith said. “I take it very seriously, which is why I decided not to,” she said.

“It’s a hard enough job, and we do a poor enough job in this country supporting parents,” Blackstone added. “I think the last thing we want to do is urge more people who know they don’t want to do it, to do it.”