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By Christine Grillo

For every species, population growth is good—up to a point. When a species passes that critical tipping point, though, challenges arise: food and water shortages, diminished habitat, the spread of disease and increased conflict. For most species there is a system of checks and balances. When resources are limited, fertility declines. Honeybees and birds, for example, regulate egg-laying to food and habitat availability.

But other species don’t seem to shut down reproduction when resources max out—rabbits, lemmings and deer, for example. Instead of leveling off, their population exceeds carrying capacity and then drops dramatically during die-offs.

Which type of species are humans? “Do we overshoot and collapse, or do we reach a limit and stop?” asks demographer Stan Becker. Some ecologists have estimated that the human population has already surged past sustainability. Becker says that population growth will come to zero—somehow. “Whether it’s 10 billion, or 8 billion, who knows, but it will stop.”

Ideally, human population growth would level off through a decline in global fertility to a rate in which the average woman bears 2.1 children. In some regions of the world—Europe, for example—the fertility rate is already much lower. In other parts of the world—sub-Saharan Africa, most notably—the average woman gives birth to five children. The challenge is to reach families in those parts of the world.

A recent study in Kenya and Ghana, by Becker and fellow demographer Agbessi Amouzou revealed two important findings. First, that family planning ideation, defined as the sum of a person’s knowledge of family planning and exposure to the concept, is a strong predictor of a woman’s desire to stop or limit childbearing. Second, a predictor almost as strong is the woman’s economic status. Women with more knowledge and more wealth desire fewer children.

In addition to educating women about family planning, it’s critical to involve men in the decision-making and contraceptive use. When both members of a couple are involved in family planning, says Becker, better results ensue. Women—or couples—who decide to space births can do so by using contraceptives or by exclusive breastfeeding, and those who want to stop childbearing may choose sterilization. Many women without access to effective contraception resort to abortion to limit childbearing. Later marriage is another route to lower fertility.

The good news is that the average age of marriage is going up globally, and contraceptive use is going up as well. But there are still 215 million couples who want to limit childbearing and don’t use contraception, and safe abortion is still unavailable for many who choose it.

“In family planning,” Becker says, “culture is important, but government is more important. Some governments do things, and some don’t.” He cites the example of Bangladesh, where the government applied a family planning policy. Beginning in the late 1970s, the Bangladesh government began door-to-door delivery of contraceptives, supported by a nationwide advertising campaign that announced, “Small family, happy family” on billboards and posters. Today, small families are the norm in Bangladesh, and the average desired family size is 2.3 children.