Disentangling the relationships between religion and fertility

In this issue of Religion, Brain & Behavior, scholars from multiple disciplines offer comments on Philip Jenkins’ Fertility and Faith (Jenkins, 2020). The debate, and Jenkins’ response to the commentaries, contribute to moving scholarship forward in an often-neglected area in the scientific study of religion. The book tackles an extensive literature, synthesizing work on several topics: drivers of secularization, drivers of fertility decline, the relationship between religion and fertility, and whether these relationships are consistent across countries and religious communities. Through this synthesis, Jenkins argues that religiosity and fertility are tightly linked, rising and falling in tandem through time and across the world. The commentary authors highlight several questions that remain unsolved by Jenkins’ synthesis. Voas (2022), for example, notes that Jenkins does not favor one mechanism over another, suggesting that changes in both religiosity and fertility may affect change in feedback loops. Potentially, Jenkins’ non-preference for a single mechanism is because he does not employ a strong theoretical framework to explain the relationship between religiosity and fertility, as Lynch and co-authors argue (2022). This is complicated by the range of data available to study this question. Globally, analyses of religiosity and fertility have to deal with the problem of scales of analysis, balancing studies at individual-versus country-level data, as Peri-Rotem (2022) highlights. Focusing on global trends to the exclusion of individual-level data, as well as using a lens which prioritizes a Western and present-centered viewpoint, has the potential to erase local and temporal variation in the relationship between family formation and religion both in Western and global settings (Brown, 2022; Shaver et al., 2022; Walters & Sear, 2022). Our own work employs evolutionary theory and in-depth fieldwork to investigate the dynamics between religion and fertility. In the remainder of this editorial, we describe our efforts as part of the Evolutionary Demography of Religion project, which was designed to further understand the links between religiosity, fertility, and child success. Three of the commentaries on Fertility and Faith were contributed by members of the Evolutionary Demography of Religion team, and here we give a general description of this project. In doing so, we hope to explain how the project will further advance our understanding of the complex relationship between religion and fertility. The project’s central hypothesis is that religious systems promote collaboration between individuals, thus facilitating greater access to social support systems among more religious women and/or families (e.g., help with childcare). The support provided to religious women and families can help offset the costs of reproduction, resulting in higher fertility compared to their secular counterparts (Shaver, 2017). Moreover, differences in fertility between religious groups are expected to vary as a result of religious groups’ ability to overcome cooperative dilemmas that themselves vary across socioecological contexts. To evaluate these hypotheses, our mixedmethods project combines anthropological and demographic methods, conducting surveys and focus group discussions across five study locations: India, Bangladesh, Malawi, The Gambia, and the United States. Our research is being conducted in partnership with local institutions including the International Center for Diarrheal Disease Research, Bangladesh (icdrr,b), the Society for Health and Demographic Surveillance in India, the West Kiang HDSS based at