In post-industrial societies, number of children is inversely related to each child’s success. In these same settings, religious people have more children.

Religious individuals are consistently found to be more cooperative than secular individuals (e.g., Sosis and Ruffle, 2003). Our preliminary work suggests that this cooperation extends to childcare, and that shared childcare among co-religionists may help to mitigate the costs of high fertility, positively affecting both fertility and child success (Shaver et al., 2019). That is, the cooperative fictive kin groups that religious communities create lower the costs of raising additional children and therefore enable larger family sizes. The levels of cooperation within religious communities, our preliminary work suggests, may be critical to explaining divergent fertility levels between religious and secular individuals, as well as variance in fertility between religious groups. Despite our supportive work to date, rigorous tests of core hypotheses remain ahead.

This project will be the first to systematically address the hypotheses that:

• Religious cooperation positively affects fertility (family size) at the individual level
• At the group-level, religious cooperation is positively associated with fertility, both within each religion, and between religious and secular groups
• Religious cooperation positively impacts child outcomes
• Relationships between religious commitments, fertility, and child outcomes become more pronounced as societies modernize.

Our team will conduct cross-cultural studies of 8,500 Christian, Hindu, Jewish, and Muslim participants from Bangladesh, the Gambia, India, Malawi, and the United States, societies that range dramatically in degree of modernization as well as majority/minority religious dynamics.

Complementarily, our team will test evolutionary predictions with the substantial available secondary data on religion and fertility, leveraging our preliminary work in the area as well as substantial expertise with secondary analyses among the research team.

Understanding the evolutionary dynamics surrounding religion’s influence on family size and child success is not just of interest to the scholarly community. Due to its relevance for economic and social development, health, and demographic projections, our research will be of significant interest to governments, NGOs, and public policy officials.

In conducting innovative research of concern to academic, government, and public audiences, this project will initiate a new and vibrant subfield of inquiry: the evolutionary demography of religion.