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?…

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.

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. Or that in no way is creating a mini-me to carry on your genes selfish.