Essays
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Below‐Replacement Fertility - Philip Morgan, S.
When a human population has underlying birth rates too low to sustain its current population size, it has below‐replacement fertility. If mortality rates are low, then replacement‐level fertility is slightly above two births per woman. Currently, over 50% of the global population lives in a country with below‐replacement fertility; below‐replacement fertility is especially widespread in developed countries and is emerging in many developing ones. But there is substantial variation in degree—some countries having very low fertility (below 1.5 births) and other countries (such as the United States) have levels at or near the replacement level. Because the level of fertility intended (or desired) approximates two births per woman in most countries, explanations for fertility levels below replacement levels focus on why people fail to have the number of children they intend. An important factor is fertility timing. Postponement of fertility to older ages reduces birth rates in current periods (lowering period fertility rates), but it also exposes persons to events and experiences that may lead them to forego childbearing or additional births. Below‐replacement fertility produces important macro‐level effects (e.g., a population with older persons and a declining population size). Average fertility below two births also impacts families and individuals' life courses and activities. -
Demography and Cultural Evolution - Shennan, Stephen
Trying to explain the increase in cultural complexity over the long term of human history has long been an interest of anthropology and of historical social sciences more generally. In recent years, interest has grown rapidly in the idea that a key factor in accounting for it might be the size of the human population itself and the extent of interaction between people, because of the effect these have on the innovation rates in populations and on the success with which innovations are transmitted. An important driver of this growth of interest has been the emergence of the new interdisciplinary field of cultural evolution, which makes extensive use of mathematical techniques, especially methods derived from population genetics. The result has been the development of a range of analytical and computer simulation models that make various predictions about the way in which population size influences cultural change, and in particular the growth of cumulative culture, including the processes that have led from the very simple forms of culture possessed by other great apes to those characteristic of Homo sapiens. The aim of this review is to distinguish them, so that future work can focus on evaluating their strengths and weaknesses and the circumstances in which they are useful. -
Limits to Human Longevity - Preston, Samuel H.
Longevity has increased sharply in the past century and it is likely to continue increasing. Historical trends in maximum life expectancy at birth show major improvements since 1760. Life expectancy at age 80 has also improved with an accelerating pace in recent years suggesting we are not approaching a biological limit to the length of life. Anticipating the near future of longevity typically relies on extrapolating either longevity itself or age‐specific death rates. The principal alternative to extrapolative methods attempts to model factors affecting mortality and to project those factors into the future. In the more distant future, rather than targeting specific diseases, much research would attempt to arrest the aging process itself either through gene therapy or through medicines that replicate the genes' activities. Stem cell technologies may make it possible to create new body organs to replace defective ones. Although discoveries in laboratories will play an important role in determining the future of longevity, many puzzles remain to be worked out in translating individual behaviors into population‐level indexes. Quasi‐experimental designs may provide a useful approach to investigate systemic determinants of mortality, with implications for the future of longevity. In addition to projections of longevity for national populations, there would also be projections for major groups within populations. Future projections of longevity are likely also to involve much more consideration of the epidemiology of diseases and their interactions. Finally, an attractive approach to longevity is to base projections on birth cohorts instead of, or in addition to, period‐specific data.