Life expectancy is up and poverty is down worldwide, even with our rapidly expanding global population. Are our fears of catastrophe overblown?

However, the attempt to obtain reasonably reliable estimates of both the benefits and costs of these programs remains very challenging.

Kohler draws on recent estimates to find that expanding family-planning services to all women with unmet needs—215 million women—would require an additional annual expenditure of $3.6 billion, bringing the total cost to $6.7 billion annual. Three-quarters of these additional expenses would be required for program and other systems costs related to expanding family planning services, while only 16 percent would be required for the supplies and contraceptive commodities.

The benefits are large. Reduced fertility, increased child spacing, and possible reductions in unwanted fertility are likely to reduce infant and maternal mortality, each year leading to 150,000 fewer maternal deaths and 600,000 fewer motherless children. These effects alone, Kohler estimates, are worth more than $110 billion, meaning that each dollar spent will achieve $30 to $50 of benefits.

But moreover, it is also estimated that reduced fertility will lead to higher levels of female education, increases in female labor force participation and earnings. At the same time, fewer children and more men and women in the work force will increase economic growth over the coming decades. Essentially, reductions in fertility and population growth rates would result in sustained increases in GDP per capita over several decades. This could lead to an extra benefit of perhaps $60 for every dollar spent.

With the caveat that knowledge about the interactions between population and development remains limited and heated discussion takes place about many assumptions, Kohler’s research suggests substantial benefit-cost ratios for family planning programs. Altogether, he finds that every dollar spent in this area could result in benefits worth about $90 to $150.

Kohler’s analysis adds further weight to the argument that family planning programs are a good economic investment, especially in light of continued population growth in the world’s worst-off countries: That upward of one-quarter of women want to limit their fertility but are not using any contraception points to a real need for greater emphasis on this area.

Today we only look at one solution, but you still have to rank it, keeping in mind that many other solutions—from many other challenges—vie for our attention and limited funding. Where should family planning fit in?

Tomorrow, we will look at natural disasters like floods and earthquakes. These often capture more media attention than the sorts of ongoing challenges we have looked at to date, but are there cost-effective ways to increase community resilience and save more lives?

In this series, Bjorn Lomborg explores the smartest investments to respond to global challenges like hunger, disease, sanitation, and climate change—and readers get to have their say. See the other  articles here. And find out which investments are currently at the top of the Slate readers’ priority list here. Be sure to vote in the poll at the end of each article.