Ever wondered why many religious people seem to be especially passionate about traditions concerning sexual mores and family? Somewhat ironically, this turns out to be a product of the finer workings of biological and cultural evolution.After all, the gold standard of fitness in evolutionary biology is the number of offspring throughout the generations. And as has been discussed here on TVOL: regularly praying people tend to have far more children than their secular neighbors, even after controlling for other factors such as education, income, and urbanization. Nevertheless, after the empirical rejection of religion-as-harmful-virus-theories, most adaptationist theories of religiosity emphasized religion’s potential in enforcing cooperative morals. Among other things, this approach resonated with classic secularization theories pointing out that as secular institutions increase in strength they tend to replace religious ones, thereby leading to an inevitable decline in religious activities and experiences. As Ara Norenzayan recently put it in his groundbreaking book “Big Gods” (2013): “Secular societies climbed the ladder of religion, and then kicked it away.”But in the same book, the social psychologist acknowledged “the secularists’ Achilles’ heel”: religious demography. It’s no longer only about the religious having some more kids, it’s about future generations in general. While numerous religious groups such as the Old Order Amish, Hutterites, Mormons or Haredim have been able to retain high fertility rates throughout the centuries, historical research reaching back to Roman, Greek, and Indian classic antiquity has not turned out a single example of a non-praying population featuring at least two children per woman (that is, demographic replacement level) for just a century. It seems that secular institutions might replace cooperative functions more easily than reproductive ones.Now, Jason Weeden and Robert Kuzban set out to directly test the conflicting theories empirically in “What predicts religiosity? A multinational analysis of reproductive and cooperative behaviors”, Evolution and Human Behavior (2013).Using data from no less than 296,959 individuals in around 90 countries from the World Values Survey & European Values Study (survey waves 1989 to 2008), they tested the relative strength of cooperative versus reproductive morals in predicting individual differences in religiosity. Their results clearly strengthened the reproductive perspective (p. 6):“Our results provide virtually no support to the position that the primary current function of religiosity involves the expression of cooperative morals. Clearly, support for stricter cooperative morals is often statistically significantly associated with increased levels of religiosity. Indeed, consistent with the findings of Atkinson and Bourrat (2011), we find a modest correlation (r = .090) between religiosity and cooperative morals. But, as shown in the analyses here, this small relationship often does not survive when controlling for the substantially larger relationship between religiosity and reproductive morals.”In fact, the relationship between religion and reproduction turned out to be even stronger among the wealthy and partially secularized milieus of our planet (p. 5):“Around the world, people in poorer regions tend simultaneously to have high average levels of restrictive reproductive morals, high levels of religiosity, and a weak individual-level relationship between religiosity and reproductive morals. Wealthier regions, in contrast, tend simultaneously to contain wide variance in reproductive morals, wide variance in religiosity, and a strong individual-level relationship between religiosity and reproductive morals.”Nevertheless, Weeden and Kurzban are careful enough to point out that their study explored individual differences – not the ways they were transmitted. And here, of course, social networks and groups could be of paramount importance in shaping and educating moral traditions and in forming and offering family-supporting institutions such as kindergartens, schools, hospitals and the like. Religious groups emphasizing the cooperative but not tapping the demographic potentials of religiosity – such as the communal, all-celibate Shakers – would tend to decline inevitably, whereas highly fertile groups such as the Old Order Amish or the Haredim would transmit their moral teachings to more and more offspring. Future studies and debates might decide whether cooperative and reproductive theories of religion are competing or complementary theories. As evolutionary studies of human beings repeatedly produce multi-layered results (for example, linking biological roots and sociocultural products in the evolution of speech or music), I would place my bet on future complementary models in the evolutionary studies of religion.