There is a term—my in-laws introduced me to it—for children born into a life of inherited wealth and unearned leisure. They are “fortunate sperm.” A newly published study gives that term new meaning. We all assume that the gender of a child is basically random—the result of a ferocious, wriggly race between tens of millions of sperm to get to and burrow into the waiting egg. But the new research suggests that mothers in a wide and diverse array of mammal species are able, perhaps unconsciously, to choose the gender of their offspring. Somehow the moms are rigging the game.
The idea that this might be the case was most famously laid out in a 1973 paper (PDF) by the evolutionary biologist Robert Trivers and the computer scientist Dan Willard. The central insight was that gender has a lot to do with one’s reproductive success. In many mammalian species, being male was either very good or very bad, reproduction-wise. Among lions, an alpha male, with an entire pride’s worth of females at his disposal, will have lots of offspring. But most male lions are not alphas, and they won’t have any offspring at all. Being female, on the other hand, is a safer, if less potentially lucrative, genetic bet. Since access to wombs is what all those male lions and bighorn sheep and elephant seals are competing over, most females will have at least some offspring.
Human social life is not lion social life, but there are parallels: Contemporary genetic testing of Asian men suggests that at least 16 million of them are descended from the marauding ultra-alpha Genghis Khan. Having a boy worked out very well, genetically, for the Khan family.
Evolution, Trivers and Willard argued, was unlikely to leave something this important up to chance. They hypothesized that mothers would somehow contrive to have offspring who were themselves likely to have the most offspring. For example, females likely to have dominant offspring—perhaps because their mate or they themselves are particularly big, strong, smart, or whatever confers an advantage in their surroundings—would be more likely to have male children. Those not likely to have alpha kids would hedge their bets and have daughters.
Trivers and Willard’s theory, like all provocative and influential ones, has triggered a raft of opposing and complementary theories about the logic behind biased sex ratios. There are lots of reasons, other biologists have pointed out, why mothers might prefer one gender over another: In primate species (including, in some cultures, ours), males tend to leave the community when they reach adolescence, so if a mother wants help raising other young, she should have a girl. On the other hand, if she doesn’t want another adult in the community hanging out competing for scarce resources, she should have a boy.
What all of these theories share, argues Joseph Garner, a Stanford professor of medicine and behavioral science and one of the authors of the new study, is that, clever as they are, “they are all somewhat post-hoc explanations.” There have been a few promising results, including one study that looked at the Forbes billionaires list and found that 60 percent of the children born to its male members were sons. But large-scale animal studies have proven difficult to do—in large part because lions and elephant seals are not good at keeping accurate family trees.
To get around that, the new study, published in the online journal PLOS One, looked at animals in captivity, at the San Diego Zoo. The researchers compiled data from breeding records on 38,075 individuals from 678 mammalian species—from the Sumatran tiger to the Kenyan impala to the Red River hog. Looking across three generations, they found that Trivers and Willard were right: Animal grandparents that skewed their offspring toward one gender had more grandchildren than those that didn’t. A lot more, it turns out. Animal parents that biased their offspring toward males had 25 percent to 29 percent more grandchildren than those that didn’t bias at all. Mothers that biased toward females had more grandchildren per daughter than those that didn’t bias. Those that most strongly biased toward males had particularly fecund sons, yielding 2.7 times more grandchildren than the sons of mothers that didn’t bias. In other words, being picky about the sex of one’s offspring is a very good idea in the animal kingdom.
All of which raises the question of how, physiologically, mothers could control this; the new paper is silent on that question. Various theories have been put forward: that the different shapes of “male” and “female” sperm allow them to be identified and selectively slowed in the female reproductive tract, or that hormones can trigger changes in the chemical composition of the womb to encourage the growth of male embryos over female embryos. Garner doesn’t find any of these compelling. “We just have no idea how they’re doing it, really,” he says. For him, that’s very exciting. He thinks the search for the answer will open up whole new research frontiers, and yield all sorts of real-world benefits: The dairy industry, he points out, has a huge incentive to ensure that its cows only have female offspring.
It’s not hard to imagine the implications for human reproduction, as well. When it comes to childbirth, we’re leaving less and less up to chance these days, and if there’s a way to control gender without resorting to in vitro fertilization, it’s sure to get used. And in the age of narcissistic parenting, the mere knowledge that the gender of a child says something about the parents may take on social significance. Garner cautions that there are explanations besides Trivers-Willard for why particular genders are favored in particular circumstances, and having a son isn’t de facto evidence of world-beating genes. Still, the more people hear about these results, the more likely it is that we’ll see sons, like summer houses and seats on museum boards, as a new status symbol for a certain kind of parent eager to show off the hand they’ve been dealt in the great genetic lottery.