For most of human history, high birthrates and high mortality rates tended to balance each other out. That began to change in the nineteenth century, when better sanitation and nutrition lengthened life spans. The world's population surged from about one billion in 1800 to seven billion today.
Although overpopulation plagues much of the developing world, many developed societies are now suffering from the opposite problem: birthrates so low that each generation is smaller than the previous one. Much of southern and eastern Europe, as well as Austria, Germany, Russia, and the developed nations of Southeast Asia, have alarmingly low fertility rates, with women having, on average, fewer than 1.5 children each. For example, the total fertility rate is 1.6 in Russia, 1.4 in Poland, and 1.2 in South Korea. In the United States, it is 2.05, which is about the replacement level.
At the same time as women are having fewer children in developed countries, life expectancies there have reached record highs. As a result, the dependency ratio -- the ratio of the working population to the nonworking population -- has become increasingly unfavorable, and it is projected to get even worse. In many countries, the age distribution will someday resemble an inverted pyramid, with a bulge of the elderly perched precariously on a narrow base of the young. With fewer working-age people to tax, governments will have to choose from among several unpleasant options: cutting benefits, raising the retirement age, or hiking taxes. Making matters worse is that economic growth gets harder to achieve as workers age and their ranks dwindle; aging societies will have a tough time succeeding in an era of rapid technological change, which requires flexible employees.