Across the Western world, many people have left or are leaving religions and adopting secular lives. Yet, despite the growing number of secular people in the West—on the global scale—religion is growing. The contemporary resilience of religion is in large part due to the high fertility of many religious people. Indeed, even in Western societies, religious people tend to have more children than their secular counterparts. While many people are turning away from religion, secular communities are not growing as fast as high fertility religious communities.

The high fertility of many religious communities represents a paradox. In modern societies, children born to large families face obstacles to flourishing because parents have limited resources, and each child draws more of those resources. For example, children with a larger number of siblings are smaller in stature, face greater mortality risks, and have lower socio-economic success in adulthood. Yet, despite the higher fertility of religious communities, children born to religious parents appear buffered from the costs of high fertility. Conclusive tests, however, have yet to compare secular and religious children of similar family sizes in developmental trajectories. This study will take advantage of the unique nature of the ALSPAC study to compare religious and secular individuals on fertility, and compare children in their physiological and health trajectories as a result of their family size and parent’s religious involvement. We will also test the hypothesis that aid from co-religionists helps to offset the costs of the large families of religious individuals.