Jenkins, distinguished professor of history at Baylor University in both title and in reality, announces at the start an intuitively obvious but powerful generalization: "High-fertility societies ... tend to be fervent, devout, and religiously enthusiastic. Conversely, the lower the fertility rate, and the smaller the family size, the greater the tendency to detach from organized or institutional religion." Or, more succinctly, "fertility and faith travel together."
The bulk of Jenkins' study then works out the sometimes counterintuitive implications of this thesis, for example, "What separates the winners and losers in the religious economy is not the soundness of their theology but their fertility rates." Or this: "religions have to evolve new means of presenting their views" if they wish to survive and succeed. Or "security and stability tend to reduce fertility" (and thereby faith).
Turning to Muslims, Jenkins focuses on what he calls "two-tier Islam," distinguishing between high-fertility countries like Yemen and Afghanistan, and low-fertility ones like Albania and Iran. The former is characterized by a "package of values" that includes communalism and communitarianism, traditional orientation, less gender equality, more sexual regimentation, more honor orientation, more aggression and instability, and unquestioning commitment to religious values. The latter is more Western in outlook, with high stability and a developed sense of individualism. So much do Iran's rulers despise this increasingly Western outlook that Mahmoud Ahmedinejad in 2010 described wanting to consume more rather than having children as "an act of genocide."
Inconsistently, while Jenkins notes that "even Saudi Arabia is now below [demographic] replacement," he also insists that "by no rational standard can Saudi Arabia ... be said to be moving in a secular direction." In fact, there is massive evidence of such a move. To cite one statistic, a 2012 WIN/Gallup survey found that "convinced atheists" make up 5 percent of the population in Saudi Arabia while "not religious" persons account for 19 percent.
Jenkins throws off many an acute observation. Here are three: Demographics means "Global South Christians are in many ways more akin to their Muslim neighbors than their European coreligionists." "The ideology of honor [is] a gauge of development rarely appreciated by policy makers." Iran's demographic decline "make[s] it less likely that the regime would succumb to any popular turbulence."
As with other of Jenkins' writings, this one offers much to chew on and occasionally to disagree with.