For years scientists have been warning about the dangers of overpopulation in the third world and the economic and environmental disasters it could bring. Now researchers caution that Europe faces the reverse problem: the aging and shrinking of its population as women have children later in life and couples give birth to fewer children. Watson Institute for International Studies researcher Brian C. O’Neill and colleagues from the International Institute for Applied Systems Analysis in Austria say that such “negative momentum” can create its own problems, including straining social security and health systems, stifling economic growth, and encouraging intergenerational strife.

“Europe has just entered a critical phase of its demographic evolution,” O’Neill and his colleagues wrote in Science this spring. While Europe’s population is expected to rise over the next fifteen years, the researchers say that for each decade birth rates remain at their current level the European Union’s population will decline by 25 million to 40 million— barring dramatic increases in immigration or life expectancy. Twenty years of such low fertility rates would translate to a population decline of 88 million by 2100.

One solution may be to encourage women to have children at an earlier age—exactly the opposite message being given to women in developing countries where overpopulation is the problem. “Childbearing could come to be considered a ‘social act’ rather than a purely private decision,” the researchers write.