Confounding scientific projections, the world’s birth rate is falling—enough that in the coming decades, the populations of nearly all Western countries could start falling with it. Today, the global average birth rate is around 2.2 children per woman. It’s a number demographers call the replacement rate—the number that would keep the population flat. In most parts of the world, however, the birth rate is now lower than the replacement rate, with the global average bolstered by comparatively large families in many countries of sub-Saharan Africa.

Now, birth rates are below the replacement rate in North America, Europe, Russia, and China—and populations are already dropping in 30 countries, including even India and elsewhere in the developing world. Europe’s average birth rate is now about 1.5 children per woman; in South Korea, it’s 0.7. For the first time in human history, people across geographies and cultures have started wanting smaller families.

Meanwhile, governments all over the world have been trying to counter this trend, enacting policies designed to encourage bigger families—by providing longer parental leave, more and cheaper childcare options, and even direct tax incentives. But so far, the desire for fewer children seems nearly impossible to change. Why?

Jennifer Sciubba is a demographer, political scientist, and the author of 8 Billion and Counting: How Sex, Death, and Migration Shape Our World. For Sciubba, there’s no single factor driving the trend. Some reasons for people’s decisions on whether, when, and how many children to have are economic; some are cultural; some are technological; and they vary tremendously from region to region. Identifying them all, let alone weighing them all against one other, is difficult business. Still, researchers increasingly see one driver standing out among all others globally: a shift in social mores—in values. And as Sciubba says, values are highly resistant to change.

Michael Bluhm: What’s happening with these declining rates?

Jennifer Sciubba: There are different things happening, actually—different types of decline—and each type has a different driver. In some cases, rates fell from very high levels; in others, they fell from low to even lower.

Part of the phenomenon is a process called the demographic transition: Societies typically start with very high death rates and birth rates, but once they improve infant and child mortality, then death rates and fertility rates start to decline.

A major factor here is economic modernization. As incomes rise, the cost of living goes up too. As a population moves increasingly into cities, that leads to smaller homes and higher levels of education. With these changes, the time and money invested in having children will come with a higher opportunity cost—meaning, people might think they could invest their time and money in other things: more education, their careers, saving to buy a home, and so on.

When societies modernize, we also see a shift in values—a shift toward the preference for smaller families. That’s been a really important factor: Since the 1970s, a widespread desire for smaller families has accelerated the demographic transition from higher to lower birth rates.

In some countries, though, state policy has driven birth rates lower independently of economic modernization. Governments have used different incentives to push down birth rates—ranging from free contraceptives to forced sterilization. Improvements in public health enable the demographic transition, too, by lowering death rates and birth rates alike.

Most people have children because they want to have them; they believe their lives would be better if they had children. Now, it seems, more and more young people don’t believe that.

Another key variable is family planning. Since the 1960s, contraception has become commonplace in much of the world—on account of both economic modernization and state policy.

In most societies, birth rates are dropping because of a combination of all these things. In China, for instance, birth rates were already falling in urban areas on account of modernization even before Beijing instituted the one-child policy in 1980.

Bluhm: And what about countries where birth rates are falling from low to lower?

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