The Role of Gene-Culture Coevolution and Parasite Stress in Diversifying Christian Denominations.

We examine whether denominational diversity within Christianity reflects long-run gene-culture coevolutionary processes. Using comparative phylogenetic methods with countries as the unit of analysis, we show that the spatial distribution of adherents to the oldest denominations-Roman Catholicism (RC) and Eastern Orthodoxy (EO)-is strongly structured by shared population history. Countries that are more ancestrally similar are therefore also more similar in terms of proportions of RC and EO adherents, with signals of phylogenetic conservatism in each case. In contrast, later Protestant denominations exhibited substantial model uncertainty, with Ornstein-Uhlenbeck, Delta, Lambda, and white-noise models receiving varying but often comparable support. In a second analysis, Y-chromosomal haplogroups-markers of paternal population history and migration-strongly predicted country-level prevalence of RC, EO, and Lutheran adherents, with different haplogroups showing opposing associations. In the case of EO, historical parasite prevalence independently predicted adherent proportions, suggesting a role for evoked cultural adaptations associated with the behavioral immune system. Denominational differentiation within Christianity may therefore partially reflect differential population-level predispositions toward alternative modes of ecclesial authority, social organization, and cultural retention. Subsequent institutional separation and cultural boundary maintenance may then have reinforced these differences over time, contributing to the historical persistence and distribution of specific denominations. These claims are not intended to address the theological legitimacy or spiritual gravity of ecclesial division-most importantly, they do not negate substantively religious perspectives on these divisions or suggest that they result simply from biological and other mundane processes.