For most of the past century an inverse correlation between human fertility and economic development has been found.They related countries' fertility rates to their human development index (HDI), a figure with a maximum value of 1.0, which assesses life expectancy, average income and education level.Over 20 countries now have an HDI of more than 0.9, and in a majority of these the fertility rate has started to increase, and in some is approaching two children per woman.But a recent study by researchers from Pennsylvania University suggests that this pattern may be changing.