There’s a stark demographic divide in the world. On one side, countries in Europe, North America, Latin America and East Asia are seeing declining mortality and fertility rates, such that nations like Germany and Japan are struggling to maintain their population numbers (at least without immigration). On the other, countries in Africa and parts of South Asia and the Middle East are seeing slight falls in mortality but ongoing high fertility. According to current projections, 40 countries could double their populations over the next 35 years, while three–Niger, South Sudan, and Zambia–could triple them.

A report from the Population Institute says this massive growth increases their vulnerability to a series of problems. “Rapid population growth for these countries is a challenge multiplier. Their populations are demographically vulnerable and more likely to suffer from hunger, poverty, water scarcity, environmental degradation, and political turmoil,” it says.

The Washington, D.C. group identifies the 20 countries most at risk based on both their demographic projections and their vulnerability across those five areas. The new state of South Sudan comes off worst (ranked number one), followed by Somalia and Niger. Afghanistan is 8th, Iraq is 13th, and Haiti is 20th. The population of Yemen (9th), for example, is projected to rise almost 50%, while it faces extreme water shortages. Iraq is forecast to see its population rise 125% while it’s already overusing its “renewable resource base” by 500%. Many of the 20 countries face increasing food shortages as a result of climate change.

The macro solution to all these challenges, of course, is to encourage greater use of contraception. “If contraception can be made more widely available and if we can dismantle the cultural and informational barriers that inhibit women in the developing world from freely exercising their reproductive choice, fertility rates will decline faster than anticipated,” the report says.

The experience of other countries shows that family planning works: Bangladesh, Brazil, Iran, Jamaica, Malaysia, Mexico, Qatar, South Africa, Thailand, Tunisia and Vietnam all now have lower fertility rates and therefore are less vulnerable to the problems the report identifies. As a result they’ve achieved, or have potential to achieve, a “demographic dividend” whereby lower fertility rates generally boosts growth.

Of course, countries like Germany and Japan do face demographic challenges of their own. Today, with fewer workers generating tax receipts and more elderly dependents, they have an aging population that is less conducive to economic prosperity. But then you can’t really compare the two problems. “Aging countries are not vulnerable in the same way as Niger, Yemen, and Somalia,” the Population Institute says.