PHILADELPHIA – A team of researchers at the University of Pennsylvania and the Università Bocconi in Milan have released a study that challenges one of the most established and accepted standards in the social sciences: Human fertility levels tend to decline as countries advance towards high levels of social and economic development.

The researchers question the conventional wisdom by documenting new findings, potentially relevant to discussions of economic and social policy, of a reversal of fertility declines in highly developed countries once they reach a certain level of wealth.

The study, “Advances in Development Reverse Fertility Declines," by Hans-Peter Kohler and Mikko Myrskylä of Penn's Populations Studies Center and Francesco C. Billari of the Università Bocconi, is published in the current issue of the journal Nature.

Researchers looked at total fertility rate and the human development index, HDI, in 24 developed countries during a 30-year period. The data demonstrated that the well-established negative relationship between fertility and development has been reversing as the global population entered the 21st century. While social and economic development continues to promote fertility decline at low and medium levels of HDI, at advanced HDI levels further development can reverse the declining trend in fertility.

Now, HDI is positively associated with fertility among highly developed countries. This reversal of fertility decline as a result of continued economic and social development has the potential to slow the rates of population aging, thereby ameliorating the social and economic problems that have been associated with the emergence and persistence of very low fertility.

“This study provides some ‘light at the end of the tunnel’ for countries that were concerned about population aging and population decline as a result of very low fertility rates,” said Kohler, a professor of sociology in Penn’s School of Arts and Sciences. “This is a surprising new empirical finding that will almost certainly generate additional research to better understand the underlying mechanisms of fertility change and possible policy responses to low fertility.”

The study was supported by the Center for Advanced Studies at the Norwegian Academy of Science, the University of Pennsylvania and the Università Bocconi and through the Distinguished International Scholars Program at the University of Pennsylvania.