Attraction by ingroup coherence drives the emergence of ideological sorting
The increasing political polarization is driving hatred and segregation, and these pose a threat to democracy. While disagreement on policy issues is increasing and receiving great attention, people are also becoming more aligned across diverse and seemingly unrelated topics, leading to issue polarization and ideological sorting. Observational studies have described the rising ideological alignment and experimental work has addressed the relevant mechanisms involved in political interactions. However, how these two relevant phenomena relate to each other is still unclear. To address this question, we propose a multidimensional agent-based model that incorporates two main mechanisms: homophily, in which people who share similar opinions are more likely to interact, and ingroup-coherence favoritism, in which partisans are more attracted to coherent ingroups rather than incoherent or outgroup members. We developed and solved the model's master equations that perfectly describe the simulated model's dynamics. We found that the ideological alignment reported in political opinions in several experiments can only be explained by the presence of ingroup-coherence favoritism in pairwise interactions. By comparing the model's outcomes with more than 20,000 opinions from people living in different countries, the model can infer the weight of ingroup-coherence needed to match each other. We found a strong correspondence between the presence of this mechanism and the political content of the answers. This work helps us understand the role of ingroup-coherence favoritism in political opinion formation and sheds light on the importance of combining efforts from experimental approaches, theoretical modeling, and large-scale empirical studies.