Wow, just a fascinating article on this topic.‚ A paper has been released, Religion, religiousness and fertility in the U.S. and in Europe where “the authors observe that the United States is much more religious than Europe as a whole, and the average American woman is much more fertile than the average European woman. From this many intellectuals have adduced that these two characters exhibit a causal relationship so that the greater fertility of American women can be attributed to their greater religiosity.”
The author of the article cites the above paper, noting the decline of Europe culturally may result from the godlessness that exists in their culture.