Critics of evolutionary psychology often point to a number of standard puzzles. If the driving force of human behavior is genetic replication, then why are there homosexuals, and why do people adopt others’ children, and why are fertility rates so low in modern developed societies?

These are all real puzzles. But they can’t somehow render evolutionary approaches irrelevant. Really, just flip it around. Does anyone have a satisfying way to explain, without resorting to some kind of evolutionary account, why the vast majority of young adults want to have—and the large majority in fact end up having—their own biological kids? If modern life in developed countries is about selfish pleasures, or consumerism, or going to interesting places, or intellectual enrichment, or whatever, then why have kids at all?

Especially for women. I mean, seriously—the idea here is that they’re going to grow little humans inside their bodies, which I hear is often a very uncomfortable thing to do. Then they’re going to squeeze a wriggling, wailing watermelon out of their lady parts (or perhaps have a physician literally slice open their abdomens to remove it).

Then the new parents will spend years losing sleep, cleaning up feces, fretting and obsessing, all while surrounded by some of the worst music, videos, and books in human history, material that will be listened to and relistened to, watched and rewatched, read and reread in a soul-squashing spectacle of mind-melting monotony. And it’s expensive, both in terms of mothers’ lost income and the money pit of pregnancy, basic necessities, childcare, Disney World, sports, summer camps, cell phones, cars, college, and so on. All to produce beings for which many parents would literally lay down their own lives if needed.

People don’t have kids to maximize modern pleasures; they have kids because they’re powerfully motivated to bear the specific burdens of reproduction. It’s in fact a triumph of evolutionary psychology that it’s perhaps the only non-hand-waving way to make broad sense of the rather extreme sacrifices of parenting.

Yet, no doubt, there are puzzles. Some are bigger deals than others.

Minor puzzles: Homosexuality and adoption

Homosexuality is an evolutionary puzzle, but a minor one. First, same-sex behaviors are not limited to humans, so this isn’t something about how humans have broken the mold of evolution. Second, the most theoretically puzzling forms of human homosexuality are rare, and at any rate just a small fraction of a larger puzzle of non-reproductive lives.

If we look at Baby Boomers, for example, according to U.S. General Social Survey data of those ages 42 and older (i.e., past the ages when people typically become first-time parents), only 0.8% of men and 0.4% of women—less than 1% of each—report having had sex only with members of their own sex since age 18 and not having had any children. Even if these are on the low side given underreporting, it’s still the case that a paradigmatically homosexual life-history is pretty rare. By comparison, about 3% of both men and women reported having had sex with both men and women and having had children.

If we focus on Boomers who didn’t end up having any children, while the vast majority have had sexual partners, only around 15% of childless men and 13% of childless women report ever having had a same-sex partner since age 18—that is, the large majority of childless Baby Boomers have had exclusively heterosexual adult sexual histories. What this suggests is that, even when focused on the puzzle of childlessness, same-sex patterns just aren’t the major theme.

Adoption presents a similar story, though I don’t have exact numbers at my fingertips. If we focus on de facto adoption and not just legal adoption—that is, cases in which folks who are not the biological parents of children nonetheless assume the ordinary responsibilities of childrearing for those children, whatever the legal category of the relationship—the large majority of cases are ones in which the caregiver is either a close biological relative of the child (e.g., a grandparent) or is a romantic partner of one of the biological parents. Neither situation is an evolutionary puzzle. Further, for the rarer cases in which there is no preceding biological nexus between adopting parents and their children, it seems clear that such cases often follow from circumstances in which couples would have preferred conceiving their own children but couldn’t.

Both homosexuality and adoption present interesting issues that should be explored. But neither represents some kind of fundamental challenge to evolutionary perspectives on modern humans. In fact, there are large and growing bodies of research on both topics from an evolutionary perspective, research that often spots important themes that others miss.

A bigger puzzle: Low modern fertility

A bigger puzzle is that it’s apparent that folks are just not having that many kids in developed countries these days. And they could be having a ton. Rich societies have never been richer. Child mortality rates are as low as they’ve ever been. Developed societies have in place more-and-less-generous welfare systems that, at a minimum, aren’t generally going to let (many) of their children starve or go without basic medical care or education. If we are evolved creatures bent on maximizing the head-count of our surviving offspring, there’s never been a better time to cry Nature! and let slip the dogs of fertility.

In an often-quoted phrase in evolutionary circles, back in the 1980s Vining called low modern fertility “the central theoretical problem of human sociobiology.” It’s not that it’s an existential threat to evolutionary psychology, but it is a tough nut.

Recent approaches suggest potential ways to get one’s arms around the puzzle. Advances in life-history theory and sociosexuality, for example, have introduced various ideas involving complex and contingent sexual and reproductive strategies, though I haven’t seen much work systematically applying these advances to the specifics of low modern fertility.

That’s a gap I’m going to try to help fill in. This is the first in a series of posts I’ll do on the low-fertility puzzle. It won’t lead to The Big Answer to Everything. But I’ll make progress. To help give focus to the puzzle, I’ll provide some background on how the modern U.S. differs from the past century, and I’ll show where within the modern U.S. very low fertility rates do and do not appear. I’ll offer some suggestions about how to think through and expand existing notions of life-history trade-offs in the specific context of modern developed societies. It won’t be everything, but it really will be something.