At a bright, arty youth hostel in Cape Town, my teenage daughters crossed paths with a young man who seemed to spend most of his time watching late night television. At one point he said, “I want to have twelve children.”

Both girls were indignant. “Doesn’t look like he’s doing anything to start his savings account,” one sniped later.

“I suppose he’s going to change all the diapers,” said the other. They missed the point.

Or maybe they didn’t. They perceived that his desire for kids was not about wanting to be a parent—to love and nurture them and watch them grow. It was the number that mattered.

Reproductive health service providers and researchers are just beginning to look at the way that men’s status needs play a role in women’s pregnancies. A study in the U.S. found that a significant number of pregnant teens had been pressured or coerced by men who made power moves that can be described only as classical machismo. (See When Teen Pregnancy Is No Accident.) Some hid pills or damaged condoms to show they could get girls pregnant. Others manipulated or pressured a girl to do things that made her uncomfortable. Her resistance to unprotected sex made it more desirable. Sometimes the desired outcome was simply sexual compliance; sometimes it was pregnancy (which could then be terminated); sometimes it was offspring. Teen health clinics are starting to ask young women more explicitly about these kinds of experiences so that they can offer counseling and support in cases of actual abuse or more discreet contraception in cases where the clients simply want control of their own fertility.

A primatologist or most any wildlife biologist probably would find these behaviors unsurprising. In our travels through Africa, my family recently paid a visit to Rwanda’s mountain gorillas in the north and chimps in the south. The gorillas live in kin groups ranging around fifteen to thirty individuals. The dominant male gets the privilege of sex with the females and so gets to claim the group’s offspring as his own. The females and young are both companions and possessions. Chimps are less hierarchical; adult males within the group get to compete for conjugal rights, each seeking to persuade/pressure females into bearing his offspring. In the absence of higher order reasoning skills, our primate kin instinctively solicit sex, seek to control mates, and try to out-compete others by producing more offspring.

Instinct often becomes justified and encoded in powerful, persistent cultural traditions. In some parts of Madagascar, a young woman needs to demonstrate her fertility before she gets married. Pregnancy at age 13 or 14 is a way of proving that a girl is marriage material. She hands the first child or two off to her parents to raise. Traditionally, Malagasies see children, like Zebu (the local cattle), as a visible, tangible form of wealth. So do many Africans. The president of South Africa, Joseph Zuma (of Zulu ancestry), has several wives and twenty children with another on the way. The king of Swaziland can do him one better—he has sixteen wives and adds to his harem through an annual ceremony in which the maidens of the kingdom dance before the royal family. These country leaders, one elected, one hereditary, are modern recipients of cultural traditions in which high status men are polygamous and have more offspring, and low status men aspire to both.

Such traditions once ranged widely. King Solomon, renowned for wisdom and favored of God, had 700 wives and 300 concubines according to the Biblical account. His offspring remain unnumbered. The prophets Abraham and Mohammed both were polygamous, with Mohammed’s youngest wife, Aisha, said to be nine years old at the time of marriage. Chinese emperors and Indian moguls collected women, including foreign exotics. Yahweh, god of the Hebrews, gave his people permission to keep virgins as prizes of war. In all of these cases, women were more than mere sex objects. They were fields to be ploughed, yes, but there were seeds to be sown, and a harvest of offspring to be reaped.

Culturally, the West has largely moved away from prizing high numbers of children and toward prizing children who prosper; a couple of highly successful offspring gain a parent more respect than a large brood. But there are notable exceptions, often driven by religious ideologies that seek to gain population share by sanctifying traditional reproductive instincts. The Mormon sect is a great example, as is the Catholic prohibition against contraception and abortion. (To read about a resurgence of Christian competitive breeding, bing “Quiverfull Movement.”) One of life’s ironies is that our spiritual hubris—our insistence we are not animals but rather souls made in the image of a god—so blinds us to our creature instincts that it strengthens the very animal nature we seek to deny.

Our insistence that we are rational is another blinder. For decades economic theorists argued that people are rational maximizers of self interest. They thought rationality drove our political inclinations and voting behavior. They were wrong. In recent years, social scientists have shown that religious narratives, classical cognitive distortions, inexplicable likes and dislikes, and other non-rational factors drive how we vote. As we learn more and more about the human mind—not just voting behavior– we find that rationality is a thin veneer covering a whole host of impulses and instincts that operate below the level of conscious analysis. The reptilian and mammalian parts of our brain are in charge, and we make reasoned decisions driven by the cerebral cortex only when they aren’t dragging us in one direction or another.

And yet, many forward-thinking people continue to assume that large family size is driven by rational and economic factors: lack of information, lack of contraceptive access and economic maximization–meaning poor farmers need children to work the fields, people have lots of kids because they expect some to die. To some extent each of these assumptions is demonstrably true. But as governments and NGO’s have provided immunizations and sanitation services, child feeding programs, antibiotics and, most recently, anti-retrovirals around the world—along with condoms, Depo-provera, and schools—the result has not been a stabilization in the number of surviving adult offspring. People don’t do some calculus that says, “I want two teenage daughters to help plant and harvest the beans and feed the sheep. My mother needed to have eight kids to have two grow up. I can get by with three or four.”

The result instead has been a population explosion that can be devastating at the level of the family and village as well on a global scale. In Tanzania, where I now sit, the population has swelled from 15 million to 45 million since the 1970’s. Bangladesh has attained an average human density of 1200 people per square kilometer. India, which has been aggressively promoting family planning for two generations still grows at the rate of approximately one person every three seconds. The resource-greedy U.S. spends billions on weaponry to protect our oil supply as gas hungry consumers sprawl onto once fertile farmland.

The personal cost is painful. In the U.S., as people spread out to get a little space from each other, middle class suburban teens started dying at higher rates than urban teens. (They drive more.) Average commute time (correlated with obesity and lower happiness ratings) grew to fifty minutes. Green spaces disappeared and doctors found they could barely afford homes that once belonged to mechanics and postmen. But these costs seem minor compared to many places.

In mountainous countries like Ecuador and Guatemala, subsistence farmers are forced onto steeper and steeper pitches. With the forests that once anchored the soil gone, topsoil pours down the slopes in heavy rain, making the land ever less productive and life ever more precarious. In 2005 a Guatemalan mudslide killed over 650 people, including 35 rescuers. In Eastern Africa, traditional houses bulge with children, and improved agricultural practices –modern beehives, better seed varieties, etc.—are offset rapidly by more hungry mouths. Rwanda’s high population density and ever growing competition for scarce resources has been cited as a factor in the 1994 genocide that left the country in ruins. Today, population pressures in Eastern Africa may be setting the stage for war as tensions heat up about how to divide the precious waters of the Nile.

All of which is to say that the stakes are high.

In a sense, reproductive issues are getting better. The global fertility rate – meaning number of live births per woman – has been in decline since the 1960’s (though thanks to exponential mathematics, the absolute number of humans added each year continues unabated). Girls with access to school start bearing children later. Human population is projected to peak out around 9 billion. Hard working families, even in desperately poor rural villages, often have the option to limit their family size so their children can flourish. Many do. The dream of a better future for our children is a powerful one.

Even so, I can’t help but wonder if we need to revisit our assumptions about why people get pregnant in light of what we now know about decision making: group dynamics, culture and especially the emotional, intuitive part of our brain that is guided by what worked for our human and pre-human ancestors. Each of these fields of study potentially offers information about why we humans are so slow to alter our reproductive behavior, and so make change agents more effective. What exactly are we up against? What are the non-rational factors that make people reluctant to use contraception independent of access, education and economics? Which aspects of religion, culture, and instinct are on our side? Which are against us? One smart NGO, Engender Health, has started engaging men who are respected religious and community leaders to spearhead conversations about sex, childbearing, and child rearing.

If we care about peace, health, the end of poverty, the global web of life, the wellbeing of future generations—any or all–we need to figure this thing out. As population pressures have grown, so have our tools for solving the problem. We have medical technologies, research methods and information about human motivation that didn’t exist even ten years ago. The question is whether we are willing to ask hard questions, accept hard answers, and work hard to create a future based on our yearnings rather than our instincts.