The Global Decline in Human Fertility: The Post-Transition Trap Hypothesis.

Over the past half a century many countries have witnessed a rapid fall in total fertility rates, particularly in the world's most advanced economies including the industrial powerhouses of Eastern Asia and Europe. Such nations have now passed through the first and second demographic transitions and are currently exhibiting fertility rates well below the replacement threshold of 2.1, with no sign of recovery. This paper examines the factors responsible for driving these demographic transitions and considers their impact on both fertility and fecundity (our fundamental capacity to reproduce). I argue that because the first demographic transition was extremely rapid and largely driven by socioeconomic factors, it has had no lasting impact on the genetic/epigenetic underpinnings of human fecundity. However, the second demographic transition will be different. A series of conditions associated with low fertility societies, including relaxed selection pressure for high-fertility genotypes, the indiscriminate use of assisted reproductive technologies to treat human infertility, and environmental contamination with reproductive toxicants, may impact our genetic constitution in ways that compromise the future fecundity of our species. Since any fundamental change in the genetic foundations of human reproduction will be difficult to reverse, we should actively pursue methods to monitor human fecundity, as sub-replacement fertility levels become established across the globe.