Morgan, S. Philip (2015). Below Replacement Fertility.. Scott, Robert A. & Kosslyn, Stephen M. (Eds.) (pp. 1-17). Hoboken, N.J.: John Wiley & Sons, Inc..
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.
Morgan, S. Philip