Psychological underpinnings of partisan bias in tie formation on social media

Individuals preferentially reciprocate connections with co-partisans versus counter-partisans online. However, the mechanisms underlying this partisan bias remain unclear. Do individuals simply prefer viewing politically congenial content, or do they additionally prefer socially connecting with co-partisans? Is this driven by preference for in-party ties or distaste for out-party ties? In a Twitter field experiment, we created bot accounts varying by partisanship and whether they identified as bots or humans. We randomly assigned Twitter (now called X) users (N=3,013) to be followed by one of these accounts. We found evidence for social motivation - users were much more likely to reciprocate links to co-partisan relative to counter-partisan accounts when the accounts identified as humans versus bots. We also found evidence for both in-party preference and out-party dispreference - users were as likely to follow-back co-partisan accounts as they were unlikely to follow-back counter-partisan accounts, compared to politically neutral accounts. A follow-up survey experiment (N=990) provides further evidence for distinct roles of issue polarization, out-party animosity, and in-party affinity in moderating follow-back decisions online.