Conventional theories have little to say about the level at which fertility will stabilize at the end of the transition although it is often assumed that replacement fertility of about 2.1 births per woman will prevail in the long run. However, fertility has dropped below the replacement level in virtually every population that has moved through the demographic transition. If future fertility remains at these low levels populations will decline in size and age rapidly.This paper attempts to contribute to the understanding of levels and trends of post-transitional fertility by examining the causes of discrepancies between reproductive preferences and observed fertility. Important examples of such deviations are found in many contemporary developed countries where desired family size is typically two children while fertility is well below replacement. A total of six factors are identified as the causes of these discrepancies. Of these factors, the fertility depressing impact of the rising age at childbearing is one of the most important. Fortunately, this factor reduces fertility only as long as the age at childbearing keeps rising. As a consequence, once the mean age stops rising—as it eventually must—fertility will rise closer to the desired level of two children, because the depressing effect is then removed. The current low levels of fertility in many developed countries may therefore not be permanent.
Bongaarts, John. 1998. "Fertility and reproductive preferences in post-transitional societies," Policy Research Division Working Paper no. 114. New York: Population Council. Version of record: https://www.jstor.org/stable/3115260