How does your political color “dye” your social expectation in trust games?

Trust is a key component that shapes inter-personal relationships and is known to vary with social contexts. Previous evidence has shown the power of ascribed identity (e.g., ethnicity, gender) with respect to trust behaviors of human beings. However, few studies have investigated the neural mechanisms underlying how acquired identity (e.g., political party) may influence one’s trust-related decision making. To address this issue, we enrolled 58 healthy adults who share different political identities, defined by their presidential choices in the 2012 Taiwan presidential election (i.e., Kuomintang (KMT) vs. Dmocratic Progressive Party (DPP) supporters), to participate in a repeated binary trust game experiment while undergoing functional magnetic resonance imaging (fMRI) scan. For each trial of the game, participants (investor) could choose to invest in (“trust”) their partner or not (“keep”) in the first round and their partner would reply with either a “reciprocate” or “defect” feedback decision.  Participants were informed that they would play the game with partners with the same, a different or no political identity. At the behavioral level, participants showed significantly higher probability of trust decisions when a partner shared the same political identity, suggesting that political identities indeed modulate their cooperative decisions. At the neural level, we found that identities defined by political preferences have different neurophysiological effects on decision outcomes. When playing with partners with the same political identity, functional contrasts between trials in which a partner defected participants’ trust and trials in which a partner reciprocated participants’ trust showed significant hemodynamic signal changes in brain regions including anterior insula (emotional processing), the temporoparietal junction (mentalizing), and the dorsolateral prefrontal cortex (self-regulatory control and/or working memory). In contrast, when playing with partners with a different political identity, participants exhibited greater activation in the striatum (reward learning) in response to trials in which a partner reciprocated as compared with trials in which a partner defected. More interestingly, increased activation in the anterior insula significantly correlated to closer perceived social distances between participants and their partners. In summary, these findings provide the first evidence on the neural foundations for the modulation effects of political identities upon trust behaviors, and indicate that studies of decision making should account for the role of social identity in altering behavior and brain response.