It seems that every few days new word arrives of birth rates declining around the world. The latest comes from Finland, where officials announced in late January that the national fertility rate had dropped to 1.4 children per woman — that’s more than half a baby short of the 2.1 babies per woman required for a society’s population to remain stable.

This was the eighth straight year that the fertility rate declined in Finland, despite the government’s efforts to reverse the trend; like many Nordic countries, Finland boasts extensive social programs to support parents. The reason for the drop may be something called the low fertility trap.

In poorer, largely rural societies, women face enormous pressure to marry early and to have children. The reasons vary: the men in their lives expect it of them; their kin expect it of them; religious leaders say God expects it of them; and often, the state expects it of them, because it needs men for the military.

However, almost everywhere in the world, people are moving from the countryside to urban areas. Fifty-five percent of humans now live in cities. Urbanization, it seems, is a powerful form of birth control.

Why? In the countryside, children can be assets — every child another pair of hands to work in the field — but in cities, they are more mouths to feed. Even more important, women who live in cities become better educated because of access to schools, media and other women. As women become better educated, they demand to have greater control over their lives and bodies. And, for most women, that means having fewer children.

At a certain point, society gets used to this. Getting married later becomes the norm. Having the first child later becomes the norm. Having only one or two children becomes the norm.

And something subtle but even more profound happens: the reason for having children changes. Couples no longer have children out of duty to the family, to God or to the state. People have children because they want to have children, because having a child fulfills their relationship and their sense of self. As it turns out, people having children for this reason are quickly fulfilled — why have more? — and the low fertility trap is sprung.