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 of retirees supported by every working age person will increase significantly. However, if Mikko Myrskyla from the University of Pennsylvania and his colleagues was right, things might not be that bad. A study they just published in Nature shows that along with development, the demographic transition is reversed.

Dr. Myrskyla sees the world as it was in 1975 and as it is now (or, at least, as in 2005). He compared two things. One is the total fertility rate (the number of children to be born by a woman in a particular country during her lifetime if she experiences an age-specific fertility rate observed in that country during the calendar year concerned). The other is the human development index for the country. HDI, a measure used by the United Nations, has three components: life expectancy; average income per person; and education level. The maximum possible value is one.

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