Solving global challenges such as climate change requires that people cooperate across group boundaries. Yet, people cooperate more with those from their own group than those from different groups, a phenomenon known as ingroup favoritism. What are the psychological roots of ingroup favoritism? Why do people cooperate more with ingroup members than outgroup members? Previous studies suggested two possibilities: reputational concern and expected cooperation.

Reputational Concern

People often decide with whom to interact and how to behave towards others based on their reputations. Typically, people avoid and punish those with a bad reputation and reward and cooperate with those with a good reputation.

Is it equally important that your neighbors and others living in a different country have a positive impression of you? Because you often interact with your neighbors, it is indeed more important that they see you positively. People cooperate more with ingroup members for this reason; they want to establish a positive reputation in the eyes of ingroup members. People experience heightened concern about their reputation when paired with an ingroup member rather than an outgroup member, resulting in ingroup favoritism.

Expected Cooperation

When you cooperate with others and they also cooperate, you can enjoy the best collective outcomes. However, when you cooperate but others do not, your cooperation can be exploited for others' personal gain. For instance, when you and your friend work together to carry heavy boxes, the job becomes easier for both. But if your friend slacks off, your job, but not theirs, becomes harder. So, you might cooperate with others only when you can expect them to also cooperate. By default, people assume that ingroup members are more cooperative than outgroup members, leading to ingroup favoritism.

Reputation, Expectation, or Both?

In our recent work, we pitted these two explanations against each other and investigated whether increased ingroup cooperation is explained by more reputational concern or the expectation that ingroup members are more cooperative than outgroup members.

University students participated in an experiment in which they were paired with a partner online. Each of them received money, which they could keep or send to their partner.  They had to decide how much of the money they would send to their partner, knowing each cent they sent would be doubled before it was given to their partner. Thus, at the end of the study, participants would collectively have the most money if they gave all of their money to their partner (doubling it), and their partner gave all of their money to the participant (also doubling it).

Participants completed this task several times with different hypothetical partners, some of whom were ingroup members and others who were outgroup members. We measured how much money participants sent to their partners as a measurement of cooperation. We also asked participants about their feelings and expectations while making their decisions.

Participants did not have greater reputational concern when interacting with an ingroup member online. However, they expected partners to be more cooperative when the partner belonged to the same group as the participant rather than a different group. The different levels of expected cooperation from ingroup and outgroup partners were associated with ingroup favoritism—sending more money to partners.

Overall, our findings suggest that expected cooperation rather than reputational concern accounts for ingroup favoritism.

Extending Cooperation towards Outgroup Members

Expecting ingroup members to be more cooperative than outgroup members creates a self-fulfilling prophecy: because of their more favorable expectations of ingroup members, people favor their ingroup, making the expectation come true.

How, then, can you foster cooperation with outgroup members?  You could try to change people's expectations that outgroup members will be less cooperative.  Or, you could encourage people to be more cooperative with outgroup members, and be the starting point of changing expectations of how cooperative outgroup members can be.


For Further Reading

Imada, H., Mifune, N., & Shimizu, H. (2024). Psychological mechanisms underlying ingroup favouritism in cooperation: Revisiting the reputation management and expectation hypotheses. Group Processes & Intergroup Relations, 13684302241239860. https://doi.org/10.1177/13684302241239860

Balliet, D., Wu, J., & De Dreu, C. K. W. (2014). Ingroup favoritism in cooperation: A meta-analysis. Psychological Bulletin, 140(6), 1556–1581. https://doi.org/10.1037/a0037737


Hirotaka Imada is a lecturer at the Department of Psychology at Royal Holloway, University of London. He studies when and why individuals cooperate with and attack outgroup members.