 The whole world is aging, and the developed countries are leading the way. For most of human history, the elderly comprised only a tiny fraction of the population, never more than three or four percent in any country at any time. Today in the developed world, they comprise 15 percent, and that share is headed for 25 percent by the middle of the century. And that's just the average. In Japan and some of the fast-aging countries of continental Europe, the elder share of the population will be shooting past 35 percent. Now, there are two forces behind the aging of the population. The first force is falling fertility. People are having fewer babies, and this reduces the relative number of young in the population. Every developed country today has a fertility rate that's at or below the so-called 2.1 replacement rate needed to maintain a stable population from one generation to the next. And most developed countries are far below it. Overall, in the European Union, the fertility rate is about 1.5 or 1.6. In Mediterranean Europe, in Italy and in Spain and central and eastern Europe, the fertility rate is between 1.2 and 1.3, as it also is in Japan. So the first force is falling fertility. At the other end of the life cycle, the second force is rising longevity. Worldwide life expectancy has risen by about 25 years since the 1950s, which is a greater gain over the past half-century than humanity had achieved over the previous 5,000 years. The developed countries, life expectancy since the 1950s, has risen from the mid to upper 60s to the mid to upper 70s. In a couple of countries, including Japan and Italy, it's now past age 80. The aging of the population is about as certain as social science comes to assure prediction about the future. Not the precise numbers, of course, but the overall direction and magnitude of the trend. The decline in fertility is the result of deep-seated long-term social and economic forces from rising affluence to rising educational attainment, particularly the educational attainment of women, to the availability of effective contraception and abortion. Few people believe these trends are going to be reversed anytime soon, and perhaps more to the point, even if they were, even if fertility doubled from one year to the next, it would have a negligible effect on the relevant indicators, on the ratio of working-age adults to elders or the elder share of the population for the next 20 to 25 years, and not a major effect for the next 30. As for longevity, the official projections that I cited and which demographers around the world use, the UN projection scenario, actually assumes, despite the potential of biomedicine to increase human longevity, actually assumes a dramatic slowdown in the historical rate of improvement. So if anything, the risk there, if risk is the right word to use about the possibility of longer life spans, but the risk there is on the upside, not the downside.