ONE of the paradoxes of human biology is that the rich world has fewer children than the poor world. In most species, improved conditions are expected to increase reproductive efforts, not reduce them, but along with economic development, country after country has experienced what is known as a demographic transition: fertility (defined as the number of children borne by a woman over her lifetime) down from about eight to near one and a half. That number is so small that even with the decline in child mortality that usually accompanies development, it is not possible to sustain the population.

The collapse of this reproduction is very worrying because it comes along with an increase in life expectancy which shows that, by the middle of this century, populations in the most developed countries will not only shrink (unless they are supported by enormous historical levels). immigration) but also that the number …