Societal Sorting as a Systemic Risk of Recommenders

Political scientists distinguish between polarization (loosely, people moving further apart along a single dimension) and sorting (an increase in the probabilistic dependence between multiple dimensions of individual difference). Among other harms, sorting can increase the risk of conflict escalation by reinforcing us-and-them group identities and reducing the prevalence of cross-cutting affiliations. In this paper, we (i) review normative arguments for high or low sortedness, (ii) summarize the mechanisms by which sortedness can change, and (iii) show that under a simple model of social media recommender-driven preference change, personalized engagement-based ranking creates a systematic tendency towards sorting, while ranking by diverse engagement (sometimes called “bridging-based ranking”) mitigates this tendency. We conclude by considering the implications for those conducting systemic risk assessments of very large online platforms under the EU Digital Services Act.