Political polarization has ruptured the fabric of U.S. society. Across 5 pre-registered studies comprising 13 behavioral experiments and a diverse set of close to 8,000 participants, the focus of this paper is to examine various layers of (non-)strategic decision-making that are plausibly affected by existing polarization. Through the lens of one’s feelings of hate and love for Donald J. Trump, I document the behavioral-, belief-, and norm-based mechanisms through which perceptions of interpersonal closeness, altruism, and cooperativeness are affected, both within and between political factions. I find strong heterogeneous effects: ingroup-love occurs in the perceptional domain (how close one feels towards others), whereas outgroup-hate occurs in the behavioral domain (how one helps/harms/cooperates
with others). The rich setting also allows me to examine the mechanisms: the observed intergroup conflict can be attributed to one’s grim expectations about the cooperativeness of the opposing faction, rather than one’s actual unwillingness to cooperate. A final set of experiments reveals that two popular behavioral interventions (defaults and norm-nudging) alone are insuffcient to eradicate the detrimental behavioral impact of polarization.