In recent decades, most Western countries have experienced a religious decline, reflected in shrinking membership in religious organizations. Explanations of this decline have primarily emphasized individual-level predictors of joining or leaving religious organizations or broad macro-level processes such as modernization. Missing from this picture is the meso-level: the social context and networks in which individuals are embedded in. Attending to this level is crucial to understand not only how individuals make decisions in response to their social environment, but also how these interdependent behaviors aggregate into broader patterns of organizational growth and decline dynamics. To address this gap, this dissertation examines how joining and leaving a religious organization are shaped by the behavior of others, that is, by social influence, and how these micro-level processes generate organizational growth or decline. This is investigated in three essays, all using the Church of Sweden as the empirical case and analyzing full-population Swedish register data by combining conventional statistical approaches with machine-learning techniques and computational simulation methods. 

Essay I, co-authored with Peter Hedstrom, analyzes how exposure to leavers shapes membership dynamics in the Church of Sweden. Using Swedish full-population register data from 2002 to 2023 and a dynamic matched-sample design, the analysis shows that exposure to leavers increases the likelihood of leaving the Church of Sweden. However, this effect is moderated by how deeply socially embedded an individual is in the Church, being particularly strong among individuals with few social ties to other members. To explore how these dynamics shape collective membership trajectories, an empirically calibrated agent-based simulation model is used. The simulation results suggest that social influence has amplified the overall decline in membership of the Church of Sweden, but also that the decline would have been even steeper in the absence of religious embeddedness. The essay demonstrates how, and under what conditions, social influence shapes individual decisions to leave and what implications this has for the broader organizational decline of the Church of Sweden. 

Essay II analyzes how exposure to religious pluralism through social ties shapes parents’ decisions to baptize their children in the Church of Sweden. Using Swedish full-population register data on all parents with children born between 2002 and 2022, network-based measures of religious pluralism are constructed, and an inverse probability weighting design is used to estimate the effect of exposure to religious pluralism. The results show that exposure to religious pluralism decreases the likelihood of baptism, with particularly strong effects under religious cross-pressure (when parents differ in religious affiliation) and especially when the father has another affiliation. The essays highlight how social networks shape the intergenerational transmission of religious behavior, providing a meso-level perspective on the link between religious pluralism and religious participation. 

Essay III examines how individuals’ social ties to other church members shape their susceptibility to the Church of Sweden’s membership fee. Using full-population register data from 2001 to 2023, the analysis shows that higher fees increase the risk of leaving the Church of Sweden, but that this effect is weaker for individuals with many social ties to other church members. To explore how these micro-level dynamics shape collective membership trajectories, an empirically calibrated agent-based simulation model is used. The simulations show that while church fees contributed to membership decline, religious embeddedness dampens this effect overall while simultaneously reinforcing spatial differences in affiliation. Together, these findings demonstrate that social networks shape when and for whom costs affect religious participation, and that this micro-level heterogeneity has large consequences for the collective patterns of religious participation. 

Together, the three essays show that social influence is crucial for understanding the membership dynamics of religious organizations. In doing so, the dissertation demonstrates how micro-level interactions and their social interdependencies aggregate into the collective patterns of religious decline and secularization.