Conclusion

This review of past trends and future prospects of population growth, population aging, and urbanization has highlighted the crucial changes that world population dynamics have undergone over the course of the 20th century and the fact that the demographic transition, being a unique event in human history, still remains to play itself out to its full extent. Today, when most developing countries are already fairly advanced in the transition to low fertility and low mortality, the issue of whether the developing world will embark on the demographic transition has been largely put to rest. Nevertheless, some issues that had not been expected twenty or thirty years ago when the transition to low fertility in the developing world was beginning have now taken center stage and are leading to a reassessment of future prospects of population change.

The first of these developments is the persistence of low fertility in a large number of countries, especially in those that underwent the demographic transition early, since such persistence suggests that when couples master the means to control their fertility and live in societies where the risks of dying before old age are low, the number of children they will decide to have may be below the number needed to ensure the long-term replacement of the population. If this behavior becomes the norm rather than the exception in the countries that are today in the intermediate stages of the transition to low fertility, the prospects for a sustained and deep reduction of the world population over the coming century increase. However, this issue is only beginning to be addressed in a systematic way and the possibility of different outcomes cannot be ruled out, particularly given the heterogeneity that is still evident among the populations of the world. Furthermore, the persistence of high fertility in a significant number of countries and evidence suggesting a slowdown in the transition to low fertility in some of the most populous countries where fertility levels are still at an intermediate stage suggest that the universal adoption of low fertility norms is not so readily at hand.

The second development involves reversals of the transition to lower mortality, especially those stemming from a resurgence of infectious diseases as major causes of death and from the persistence of high rates of death caused by chronic and degenerative diseases such as those affecting the cardiovascular system. In addition, in a growing number of countries mortality due to violence and accidents has been increasing and although it is generally not sufficient to cause a sustained reversal of mortality trends, it may contribute to dampen further mortality reductions in many countries. These factors, among which the uncertainty about future levels of HIV prevalence is perhaps the most prominent, suggest that future assumptions about sustained mortality reductions in the majority of countries may turn out to be too optimistic. On the other hand, past experience has shown that projections of mortality decline have tended to err in the other direction, underestimating the reductions of mortality that have occurred in countries with advanced health systems where new medical interventions and behavioral change have contributed to reduce mortality in adult ages more rapidly than expected.

These considerations imply that the results of current projections can at best be taken as indicative of possible future developments. Nevertheless, as documented in this chapter, they already cover a wide range of outcomes and provide fairly solid basis for some key conclusions. The first is that the world population will continue to increase during the best part of the next fifty years and the potential for continued growth is large. The second is that population aging will continue and with it the rapid increase of the elderly population. The third is that the population of the future will be more urban and that growth of the rural population will be low or negative almost everywhere. The fourth is that the differences in demographic dynamics between the more-developed and the less-developed countries will persist for at least the next fifty years, and there will be considerable variation in the experiences of countries. A homogeneous world in which all populations have similarly low levels of fertility and mortality and are equally highly urbanized is still far away in the future.