For decades, demographers have reported that the more developed a country is in terms of wealth, health, and living standards, the lower its citizens' fertility rate — so much so that most rich European and North American nations cannot sustain their populations without immigration. (The United States is a notable exception.) Eco-activists tend to welcome such news, foreseeing an end to overpopulation. But many economists and sociologists worry, because low fertility rates entail population aging, which often brings on socio-economic problems.

Both camps will be interested in a recent study by Mikko Myrskylä and Hans-Peter Kohler of the University of Pennsylvania, and Francesco C. Billari of Bocconi University in Milan, Italy.

Crunching the latest (2005) numbers for 140 countries, the study still finds a negative correlation between national fertility rates and the United Nations' development index, but only up to a point. At the highest development levels — attained only in recent years — countries' fertility rates rise again. Some are approaching the replacement value of 2.1 children per woman, whereas others still have a long way to go.

The team can't yet pinpoint the reasons for the upturn. Perhaps general affluence, or government programs, help women reconcile childrearing with professional pursuits.

Economists will be relieved, but the environmental implications are less clear. Development in the poorest nations, which are driving global population growth, will undoubtedly lower fertility rates there. Yet it's individuals living in highly developed countries who consume and emit the most.

The findings were detailed in the journal Nature.