Wealthy countries worried about their shrinking birth rates may have had their prayers answered. If they get just a little richer, birth rates should head up again, US researchers reported.

They studied 24 countries over 30 years, looking at fertility rates and a measure of education, income and lifespan called the human development index.

"Although development continues to promote fertility decline at low and medium human development index levels, our analyses show that at advanced human development index levels, further development can reverse the declining trend in fertility," Hans-Peter Kohler of the University of Pennsylvania and colleagues wrote in the journal Nature.

The United States and the Netherlands are both approaching this point, Kohler's team said, and immigration can further boost fertility.

"Low fertility means fewer babies, and eventually a smaller workforce that would have to pay higher per capita costs of infrastructure and social support systems," Shripad Tuljapurkar of the Stanford Center for Population Research at Stanford University in California wrote in a commentary.

"A consequence of low fertility and long lives is an aging population with its attendant social and economic effects population. Thus, for many rich countries, population decline is a serious concern."

But the report shows that policymakers can plan for a rosier future.

"Whereas a decade ago Europe, North America and Japan were assumed to face very rapid population aging and in many cases significant population declines, our findings provide a different outlook for the 21st century," Kohler's team wrote.

"As long as the most developed countries focus on increasing the well-being of their citizens, and adequate institutions are in place, the analyses in this paper suggest that increases in development are likely to reverse fertility declines -- even if we cannot expect fertility to rise again above replacement levels," they added.