By Michelle Goldberg: There are two ways to look at world population numbers.

There are two ways to look at world population numbers.

By one measure, the world has grown beyond its capacity. As Hillary Rodham Clinton's science adviser, Nina V. Fedoroff, recently told the BBC: "The planet can't support many more people."

But in parts of Europe and other developed countries, the problem isn't too many people but too few: Dwindling birthrates have prompted concerns about whether a shrinking pool of young people will be able to maintain the social safety net for the previous generation.

Politically, the discussion about population is deeply polarized. Conservatives talk about falling birthrates in almost apocalyptic terms, suggesting Europe is being punished for its sins of secularism and feminism. Best-selling author Mark Steyn has predicted "the demise of European races too self-absorbed to breed," while 2008 Republican presidential candidate Mitt Romney warned that "Europe is facing demographic disaster." Liberals, meanwhile, tend to see the Malthusian specter of overpopulation as a far greater threat.

So who is right? Is our future endangered by overpopulation or underpopulation? The answer is both. But in an elegant irony, the two problems have the same solution: giving women more control over their fertility and their lives. Both very high birthrates and very low ones threaten social stability, and both, it turns out, are symptoms of countries' failures to meet women's needs.

Right now, the world's population is growing at the unsustainable rate of 78 million people a year, and according to the United Nations, it will probably keep growing at 70 million or 75 million a year through 2020. Almost all of that growth is in the slums of the Third World. As former CIA Director Michael V. Hayden said in a speech last year, "By mid-century, the best estimates point to a world population of more than 9 billion. That's a 40 percent to 45 percent increase — striking enough — but most of that growth is almost certain to occur in the countries least able to sustain it. Places where swelling population is likely to fuel instability and extremism — not just in those areas but beyond them as well."

The ethical and effective way to counter rapid population growth is to bolster women's rights and improve their access to family planning. Education is crucial — study after study has found that girls who go to school marry later and have fewer, healthier children. Access to contraception is also key. According to the Guttmacher Institute, almost one-quarter of married women in sub-Saharan Africa have an unmet need for birth control. In a number of Latin American and African countries, more than 40 percent of recent births are said to be unwanted. Meanwhile, high rates of unsafe, illegal abortion — responsible for 13 percent of maternal mortality globally, according to the World Health Organization — speak to women's desperation to control their fertility.

At the same time, several developed countries, including Japan, Russia, Italy and Spain, have what appears to be the opposite problem. A stable population requires each woman to have an average of 2.1 children. (The extra one-tenth of a percent accounts for early deaths.) Demographers say that countries can adapt without much trouble to fertility that is a few tenths of a percent less than that. However, below about 1.7 children per woman, economic growth, pension systems and general cultural viability all come into question as a shrinking pool of young people is forced to support a growing number of the aged.

Italy, for example, has a fertility rate of 1.3 children per woman.

One leading demographer estimated that if Italy's 1995 birthrate remained stable over 100 years, then without immigration its population a century hence would be a mere 14 percent of what it is today.

That kind of population decline can't simply be remedied with immigration without causing major cultural upheavals and nationalist backlashes.

Some social conservatives are using the threat of rapid First World population decline to argue for restrictions on women's rights. But that gets it precisely backward. In developing countries, lower social status for women is associated with higher fertility, but once societies become highly industrialized and women taste a certain amount of freedom, the reverse is true.

Fertility is reaching dangerously low levels in countries where social attitudes and institutions haven't caught up with women's desire to combine work and family. When faced with men who are unwilling to share domestic burdens, inflexible workplaces and day-care shortages, many women respond by having fewer children or forgoing motherhood altogether.

But when societies make it possible for women to combine having children with pursuing their other ambitions, fertility rates are fine. It works differently in different cultures. In Scandinavia and France, working mothers are aided by lavish state support. Britain and the United States lack such generous benefits, but their flexible, even volatile labor markets lessen the importance of working uninterrupted at a single job, providing more on-ramps for mothers to return to the work force. As one group of Yale political scientists wrote, a mother's "job insecurity becomes less of a liability when everyone is insecure."

It's counterintuitive at first, but it turns out that the more opportunity women in developed countries have to work, the more likely they are to have children, because they can do so without giving up their other dreams.

"The evidence from Italy, and indeed from Spain, is that a traditional family structure now leads to very low birthrates," David Willetts, a British member of Parliament and a Tory renowned for his intellect, concluded in a report on Europe's pension woes. Modern family policy, he wrote, must be about enabling women's choices so that they needn't forgo childbearing in order to have satisfying careers and egalitarian marriages. "Feminism," he wrote, "is the new natalism."

In coming decades, Europeans will constitute a declining percentage of the world's people. When the 20th century began, the population of Europe was three times that of Africa. In 2050, the population of Africa will be three times that of Europe. One can accept and even welcome the prospect of this new world and still want to see the changes happen in a way that allows countries to adapt.

Give women freedom and support, and they will find reproductive equilibrium, so that when societies do shrink or grow, they do so in a manageable way. The lesson of these twin demographic dangers is clear: Take care of women, and they'll take care of the rest.

Goldberg is the author of "The Means of Reproduction: Sex, Power, and the Future of the World."