While many researchers generally credit the desire for smaller families for the decline in fertility rates in developing, low-income countries, new research suggests that prevention of unwanted births may actually be a larger factor.

The advent of safe and more effective birth control means that people have better control of when and if they have children, said John Casterline, director of the Initiative in Population Research at Ohio State University.

"While it is true that people now want smaller families, my research suggests a more important factor in the decline in birth rates over the past half-century is that people are now more successful than in the past in having the number of children they want," said Casterline, who is also a professor of sociology.

This fact has important implications on the effort by policymakers to limit global population growth in the coming decades, he said.

Casterline gave an invited talk on the contribution of family planning programs to stabilizing world population on Feb. 20 at the annual meeting of the American Association for the Advancement of Science in Washington, D.C.

His talk was part of the symposium "Estimating Earth's Human Carrying Capacity."

Recent projections suggest the human population will peak at more than 9 billion people by 2050.

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"The large increases in population that are predicted will only exacerbate other problems, such as resource depletion," Casterline said.

"We want to minimize the growth that occurs, and there is no viable way to do that except through more effective family planning."

Casterline said that the good news is that his research shows how the success of contraception has played a large role in the fertility decline in low-income countries, even more than the more widespread desire for smaller families.

In his analysis of fertility decline in 50 low-income countries from 1975 to 2008, Casterline estimates that about 44 percent of that decline is the result of prevention of unwanted births. About 13 percent was due to people wanting smaller families. The remainder was other factors, such as women marrying later and thus having fewer children.

"We are a lot more successful in family planning than we have ever been. It is one part of the whole medical and health revolution."

But the problem has been expanding that success, particularly in sub-Saharan Africa and South Asia, where the needs for family planning are greatest and where future population growth is expected to be the largest.

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Studies have shown that about one in four women (who are currently married or in a union) in Africa have an unmet need for family planning. That means that they report in surveys that they don't want any children in the near future, but are not using contraception.

In Asia and Latin America, 17 to 18 percent of women have unmet needs for family planning.

Just meeting this need could have a major effect on population growth. One study suggests that meeting the need for family planning could reduce the growth in population in developing countries by about 250 million people by 2050, Casterline said. There were slightly more than 4 billion people in developing countries in 2010, and one United Nations estimate puts the population in these countries at about 6.25 billion people in 2050. However, by meeting unmet demand for family planning, one estimate says that population will be held to about 6 billion people.

But especially in sub-Saharan Africa, there is also a need to generate more demand for birth control, he said.

Generally, women who don't want to use contraception have health concerns (such as fear of side effects), ethical or religious concerns, or are discouraged by others, especially male partners.

Casterline said effective family planning programs can address some of these obstacles, particularly health concerns.

"We commonly find in developing countries a belief that most modern contraceptives pose more health risk than either pregnancy or induced abortion, which of course is not true," he said. "We need to address those concerns."

The other major obstacle to wider use of contraceptives is simply getting supplies and services to women in places like sub-Saharan Africa.

"These obstacles to contraceptive use cannot be overcome without significant financial investment, and in the short run this will require international assistance," he said.

"It is distressing that international assistance for family planning has, if anything, declined during the past decade."

Research has clearly shown that international family planning programs are effective. Studies suggest that such programs can reduce the number of births women have by 0.5 to 1.5 over a lifetime.

"We know there are women in many parts of the world who would prefer to have fewer children. We need to do a better job in providing them the effective family planning services they want," Casterline said.