  Political polarization has become a growing concern in democratic societies,
as it drives tribal alignments and erodes civic deliberation among citizens.
Given its prevalence across different countries, previous research has sought
to understand under which conditions people tend to endorse extreme opinions.
However, in polarized contexts, citizens not only adopt more extreme views but
also become correlated across issues that are, a priori, seemingly unrelated.
This phenomenon, known as "ideological sorting", has been receiving greater
attention in recent years but the micro-level mechanisms underlying its
emergence remain poorly understood. Here, we study the conditions under which a
social dynamic system is expected to become ideologically sorted as a function
of the mechanisms of interaction between its individuals. To this end, we
developed and analyzed a multidimensional agent-based model that incorporates
two mechanisms: homophily (where people tend to interact with those holding
similar opinions) and pairwise-coherence favoritism (where people tend to
interact with ingroups holding politically coherent opinions). We numerically
integrated the model's master equations that perfectly describe the system's
dynamics and found that ideological sorting only emerges in models that include
pairwise-coherence favoritism. We then compared the model's outcomes with
empirical data from 24,035 opinions across 67 topics and found that
pairwise-coherence favoritism is significantly present in datasets that measure
political attitudes but absent across topics not considered related to
politics. Overall, this work combines theoretical approaches from system
dynamics with model-based analyses of empirical data to uncover a potential
mechanism underlying the pervasiveness of ideological sorting.
