Demography has very probably been a factor in religious history all along. The Concise Etymological Dictionary of the English Language (a favorite book of mine) tells us that our word “proletarian” derives from the Latin proles, ”offspring”: A proletarius was “a citizen of the lowest class, useful only by producing children.” One may modify this rather unsentimental description by saying that children were just about the only cherished possession of people in this class. Insofar as many intense religious movements, at least initially, appealed to poor people, this naturally gave a demographic benefit to religion.
The historian Philip Jenkins has been influential in his description of the massive demographic shift of contemporary Christianity from Europe and North America to the developing societies. Todd Johnson and his associates have labored for years to produce the Atlas of Global Christianity (2009), with a wealth of demographic data. A useful summary of the demography/religion connection is provided by a book just published, Shall the Religious Inherit the Earth? by Eric Kaufmann (who teaches politics at Birkbeck College, University of London). Despite the jaunty title of his book, Kaufmann is a careful scholar, not given to wild speculation. But he answers the question in the title with a cautious yes.
Social scientists frequently make predictions. Most of these turn out to be wrong. When demographers make predictions, they have one advantage: barring some catastrophic intervening events, such as epidemics, natural disasters, genocide, the demographic future is already present. To see this future, one just has to visit maternity wards and primary schools. Kaufmann makes ample use of this advantage.