The famous metaphor for the baby boom was the pig in the python, a population bulge that is digested over time. That demographic accommodation was easy in the years after 1946, when the baby boom began: The U.S. had a burgeoning economy, so plenty of jobs awaited the boomers as they came into working age.

The bulge this time is in the developing world, where the jobs aren’t. As world population grows from 7 billion today to more than 10 billion in 2100, it will do so for the most part in places where digestion is by no means assured.

“The world is not one world,” says Patrick Gerland of the U.N. Population Division. “You have at least two or three kinds of trends happening at the same time in different regions of the world that are going a bit in the opposite direction.”

These trends arise from distinct demographic stages that are measured in births and deaths. Fertility and mortality rates determine what demographers call a country’s dependency ratio—that is, the number of children (0 to 14) and seniors (65+) divided by the working-age population.

According to Gerland, there is a golden moment in the course of every nation’s economic development, 20 or 30 years when the dependency ratio is low, a lot of people are in their working prime, and relatively few are living on the country’s reserves.

The endpoint for that demographic cycle is the moment when birth rates begin to approach or dip below the replacement level (an average of 2.1 children per woman) and when people begin to live longer. If they persist, these demographic shifts lead to an aging population and a nation that has fewer income-producing workers than people who depend on them.

For most developed nations, this moment has long since passed. According to U.N. data, the ratio of dependents to workers in Germany, for example, is now hovering around 50 percent, and it is expected to climb to 80 percent by 2050.

On the other hand, many countries in Asia and some in Latin America (China and Brazil, for example) are now experiencing dependency ratios below 50 percent. According to Gerland, Africa will be next.

“The question is to what extent all these opportunities will be translated into real development,” he says.

Clearly, if a nation is to prosper from workforce expansion, it must have conditions and policies in place that encourage job creation. An economic review for the Federal Reserve Bank of Kansas City found, for example, that countries in Latin America experienced a favorable change in their dependency ratios in the last half of the 20th century without showing proportionate economic growth. During the same time, Brazil and Japan both saw their dependency ratios fall by about 10 percentage points, but while Japan’s GDP rose by 5.4 percent a year, Brazil’s increased by an average of 2.9 percent.

In the absence of supportive policies, says Gerland, a booming population of potential workers is less likely to be a nation’s best hope for economic growth than the spark that ignites popular protest. “Unless you have opportunities in those countries, you get a very large cohort of people who are frustrated because so many of them are unemployed,” he says. “And eventually, they will either want to migrate elsewhere or you get social unrest.”

Next: The Continental Divide Between Workers and Jobs