Some of their findings, which have been written about in The Economist and The Christian Science Monitor, first appeared in 2002 as a paper for the National Bureau of Economic Research.
As the couple began their study, McCleary said, it was clear that the widely discussed secularization thesis -- the idea that a country becomes more secular as it becomes richer and more industrialized -- did not apply to the US, one of the most religious nations in the world.
And over the last 30 years, many East Asian countries, including Malaysia, Singapore and South Korea, have experienced both rapid economic growth and the spread of Christianity, Barro said.
"South Korea is a good example of that rapid growth and more religion," he said. There the number of converts from Confucianism and other Eastern religions to Christianity is growing rapidly, he explained.
Some of the lowest levels of religiosity were found in China and North Korea. The lowest levels of economic growth were in sub-Saharan African countries. The former East Germany (which includes Weber's birthplace) was one of the lowest in both religiosity and growth.
But one of the major challenges to such research is that countries that vary in their religious beliefs and practices also vary in ways that have nothing to do with religion, said Paola Sapienza, a professor of finance at Northwestern University.
"Are you really picking up religion or something that correlates with it, like certain laws or social and economic institutions?" she asked.
Last year, in the Journal of Monetary Economics, Sapienza and her colleague Luigi Zingales at the University of Chicago, and Luigi Guiso, of the University of Sassari in Rome, published a paper that did not compare countries but looked at the relationship between religious beliefs and the attitudes shown to foster economic growth.
"On average," they wrote, "religious beliefs are associated with good economic attitudes, where good is defined as conducive to higher per capita income."
But that study found that more religious people were also less tolerant of other races and nationalities and had more negative attitudes toward women. The study based its findings on World Values Surveys data collected at the University of Michigan.
Barro and McCleary also used data from the World Values Surveys, which Ronald Inglehart, a political scientist at the University of Michigan, has been taking for more than 20 years. His surveys of 78 countries show strong links between widespread public values and beliefs, or political culture, and motivation to work, sexual and religious norms and the presence or absence of democratic institutions.
"I find that belief factors play a major role in economic growth, but here is one of the world's leading economists saying so," Inglehart said, referring to Barro. "When Weber argued that big breakthroughs in economic growth were in Protestant countries, it was at a time when many cultures were shaped by Protestant institutions. His notion in the broadest sense is that belief factors play a role in economic factors."