Phil Zuckerman's chapter in the new Cambridge Companion to Atheism makes illuminating reading. Zuckerman is a sociologist of religion at Pitzer College in California, and he takes as his task an objective survey of rates and patterns of non-belief around the world.
One correlation stands out. Those nations with the highest rates of non-belief are also the nations with the highest quality of life, as measured by food, housing, health care, literacy, low rate of homicide, low infant mortality, and gender equality. Low suicide rate is the only indicator of societal health in which religious nations fare better than secular nations. The growing popularity of suicide bombing by religious extremists may change this statistic.
A few exceptions to the general pattern: The United States and Ireland. Ireland's economic boom is so recent that I doubt if the data is meaningful; from personal experience of the youngest Irish generations, I'd say that nation is well on its way to typically European levels of secularism. The American fascination with God stands out as a particular anomaly; the U. S. has one of lowest rates of unbelief in the world, right down there with Albania and Kyrgyzstan.
Why the correlation between economic development and atheism? If God exists, he certainly has an inscrutable way of rewarding his followers.
Of course, Marx and Freud were not the first to suggest that belief in God serves to comfort humans in the face of pain, suffering and death. Some writers of antiquity offered the same explanation for belief. Zuckerman's data would seem to confirm that theism fades when life becomes secure. The opposite interpretation -- that secularism is a driving engine of economic development -- might also have merit.
And what about the U. S.? Why the anomaly? I would suggest that American prosperity has been driven by European secular values, imported at the time of the nation's founding and fed by a steady influx of European scientific genius. Rates of belief among America's scientific elite are virtually the inverse of that of the general population, and more typical of other developed nations. Another explanatory factor might be this country's great system of secular public education, which the courts have so far successfully defended against religious incursions.