Why have citizens become increasingly polarized? The answer is that there is increasing identification with political parties —a process known as partisanship (Mason, 2018). This chapter will focus on the role that social identity plays in contemporary politics (Greene, 2002). These party identities influence political preferences, such that partisans are more likely to agree with policies that were endorsed by their political party, regardless of the policy content, and, in some cases, their own ideological beliefs (Cohen, 2003; Samuels & Zucco Jr, 2014). There are many social and structural factors that are related to partisanship, including polarization (Lupu, 2015), intergroup threat (e.g., Craig & Richeson, 2014), and media exposure (Tucker et al., 2018; Barberá, 2015). Our chapter will focus on the psychology and neuroscience of partisanship within these broader socio-political contexts. This will help reveal the roots of partisanship across political contexts.