Individuals preferentially reciprocate connections with co-partisans versus counter-partisans online. However, the mechanisms underlying this partisan bias remain unclear. Do individuals prefer viewing politically congenial information, or 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 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 45% more likely to follow-back co-partisans, and 57% less likely to follow-back counter-partisans, compared to neutral accounts. A follow-up survey experiment (N= 990) provides further evidence for distinct roles of polarization and in-party preference in moderating follow-back decisions online.