As baby fever swept her friends, and bumps and booties became staples of their conversations, Kristen Bossert stayed cool and distant. She felt a burning desire to pursue graphic design, to marry her high school sweetheart. But kids? Meh.

In her early 20s, she told her then-boyfriend plainly that she had no interest in being a mom. As a little girl, she’d never played with dolls, preferring to paint instead. She liked kids, but couldn’t imagine herself birthing one. She valued the freedom to spontaneously travel the world or sleep in on Saturdays, to hone her skills as an artist. Twenty-three years later, the happily married couple has no regrets about their family of two.

“It’s the best decision we ever made,” says the New Jersey native.

Since the dawn of birth control, more women have opted against having kids. Nearly one-in-five American women now ends her childbearing years without giving birth, up from one-in-10 in the 1970s, according to a recent Pew study. The percentage has risen for all racial and ethnic groups.

The top reason women give for not wanting kids is simply loving their life as it is, says Laura Scott, author of Two Is Enough: A Couple's Guide to Living Childless by Choice. From 2004 to 2006, Scott conducted a survey of 121 self-selected childfree women. Other leading reasons included valuing freedom and independence and not wanting to take on the responsibility. And 74 percent said they “had no desire to have a child, no maternal/paternal instinct.”

This growing community—which refers to itself as “childfree” (emphasis free) or “childless by choice” (emphasis choice)—raises a compelling question for women on both sides of the maternal divide: Why do some feel a seemingly innate, almost primal desire to procreate, while others don’t?

While we know that 1.9 million American women ages 40 to 44 were childless in 2008, it’s tough to quantify the number of childfree, Scott points out, since most studies don’t distinguish between being childless by choice and by circumstance. But in a recent study, Kristin Park, a sociologist at Westminster College, found that childfree women (and men) are more educated, more likely to work in professional occupations, more likely to live in urban areas, less religious, and less conventional.

Surprisingly, given how fundamental the question might seem to the perpetuation of the human species, the reasons for why some women want children and others don't remain fuzzy. Few scientists have actually studied women’s so-called biological drive to reproduce, so no universal explanation has emerged in the literature. Some attribute it to basic genetic variety; some women are into kids, some aren’t. But with relatively few women falling into the “aren’t” category, the question of why these few women aren’t interested in kids becomes even more interesting. The only consistent biological theory is that women with no desire to parent either have a high testosterone level or were exposed to above-average testosterone in the womb.

Many scientists believe the seemingly biological drive some women feel isn’t triggered by biology, so much as culture—combined with a fertility deadline. Not only is having children more socially acceptable, says evolutionary biologist David Barash, author of Strange Bedfellows: The Surprising Connection Between Sex, Evolution and Monogamy, but for many, as a life goal, it represents a source of happiness and belonging in the same way that attending college or pursuing a career might. Evolution has bestowed upon women a desire for sex and the equipment to have a baby; from here, free will steps in.

“I imagined a ticking time bomb set for some random day in my 35th year,” Laura Scott wrote in her book, “an incendiary mix of hormones and longing that would explode my being and rewire my brain.” That day never came.

One of the more intriguing notions is that, for many of these women, the lack of interest in children appears to have been in place well before their 30s—in some cases, before they could have even fully grasped the concept of parenthood.

Take Elizabeth Mannering, now 48. At 6 years old, she was already showing a lack of desire for all things infant. She felt no draw to babies or younger kids, and confesses even feeling a little repulsed by them. And she shunned dolls, “except for Barbie,” she says, “who had a glamorous life with fabulous clothes, a cute boyfriend, and no kids.”

“Over the years, people dismissed me. They would look at me knowingly—often condescendingly—and tell me I would change my mind,” she says. “People who barely knew me (mainly men) would say I'd make a wonderful mother.”

For these “early articulators,” not having kids hardly feels like a choice at all. Instead, it’s a defining—unchangeable—part of their identity. Many early articulators are so sure of their decision, they seek sterilization in their 20s, yet report experiencing pushback from physicians who fear they’ll regret their choice. “Fortunately birth control pills worked well for me and my age has finally convinced the medical establishment that I will not, in fact, change my mind,” Mannering wrote of her attempt at sterilization, which began at age 18.

Despite their growing visibility, such women still report feeling stigmatized. Bossert says she often doesn’t tell people about her childfree status until she gets to know them. Insensitive friends will suggest that their choice is a sign of immaturity, even selfishness—to which the childfree often point out the myriad problems with conceiving a child they don’t want.

“Being a parent is as much of a choice as being childfree,” says Alicia Marsh-Evans, 30, a childless-by-choice seminary student in Texas. “There shouldn’t be a default.”

To offer support and like-minded companionship, a thriving subculture of websites, forums, and meet-up groups has emerged. On TheChildfreeLife.com, discussion topics include childfree issues at work and “non-children” (i.e., pets), among others. The social group No Kidding boasts dozens of chapters in the U.S. and abroad. While most espouse a “live and let live” mentality, some groups take a more in-your-face approach to living childfree—a message perhaps best illustrated on this T-shirt, emblazoned with “ Why would I want kids? I’m ENJOYING my life.”

Among the many questions parents ask the childfree, a common one is: “But who will take care of you when you’re old?” Some cheekily respond that there’s no guarantee that kids will fill that role. Or that they’ll have saved plenty of money (that they didn’t spend on, say, diapers and private school) to pay for a caregiver. Several half-jokingly suggest that they’ll move to a compound in Mexico with all of their childfree friends.

But perhaps L.T. Ciaccio, a childfree lawyer in Manhattan, sums up her comrades’ ethos best: “I’d rather live the life I want to for 70 years than sacrifice that time just to hedge my bets at the end.”

Danielle Friedman is a homepage editor and reporter for The Daily Beast. Previously, she spent five years working as a nonfiction book editor for Hudson Street Press and Plume, two imprints of Penguin Group. She is a graduate of the Columbia University Graduate School of Journalism.