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In modern societies, family size is inversely related with each child’s success (Kaplan 1996; Kaplan et al. 2002; Lawson and Mace 2009, 2011; Lawson et al. 2013), because parental investment must be allocated across a larger number of offspring (Stearns 1992). Contemporary labour markets are also associated with smaller, more scattered kin networks, less kin support for reproduction, and reduced fertility (Turke 1989; Sear and Coall 2011). Besides biological parents, kin including older siblings, fathers, and grandparents, are known to make substantial investments in children (Hewlett 2004; Kramer 2010; Sear and Mace 2008; Sear and Coall 2011). These contributions are known as alloparental investments and are critical to child survival and success. It has been hypothesised that dispersed kin networks are one mechanism which leads to reduced fertlity in modern environments (Turke 1989; Colleran 2020). In contemporary societies, religious families tend to have more children than their secular counterparts (McQuillan 2004; Kaufmann 2010; Frejka and Westoff 2008). This fertility differential has become more pronounced as societies have modernized and secularized (Shaver et al. 2019). Puzzlingly, children from larger religious families do not exhibit the expected negative outcomes experienced by their secular counterparts (Bartkowski et al. 2008; Ellison and Xu 2014). Alloparenting and other cooperation from co-religionists have been proposed as the mechanism through which religious individuals achieve higher quantities and qualities of offspring than secular peers (Shaver 2017). This hypothesis has been tested using secondary analyses of data from New Zealand (Shaver et al. 2019), the United Kingdom (Shaver et al. 2020), and late 19th century United States (Hacker and Roberts 2017). This pre-registered study will be the first to collect data to assess whether religious parents receive more alloparental support than their non-religious counterparts. We present a large number of hypotheses and models below. Although we anticipate investigating all of these hypotheses, we plan to do so across multiple publications.

This registration consists of a list of research questions and predictions for the collaborative project, “The Evolutionary Dynamics of Religion, Family Size, and Child Success,” funded by the Templeton Foundation (https://www.evolutionarydemographyofreligion.org). More information about the project is available on its OSF page: https://osf.io/b865v/. The primary objective of this project is to provide novel cross-cultural data to study the relationship between religion, family size, cooperation, and women and children’s wellbeing. Using newly collected demographic, ethnographic and physiological data from five countries - Bangladesh, the Gambia, India, Malawi, and the United States - we aim to answer ten research questions (RQs) and make a number of specific predictions for each. We also include a list of key indicators, a brief outline of the study design, and a data analysis plan. As this project involves a complex multi-site qualitative and quantitative data collection process, we anticipate that these documents may be updated after data collection is complete due to potential constraints during data collection.