A mathematical model for the evolution of in-group favouritism is published in Scientific Reports this week. The study suggests it can be advantageous to love your friends, even when today’s allies might be tomorrow’s enemies.

In-group favouritism is a central feature of human behaviour and people often help members of their own group more than members of other groups. But in humans, group membership is dynamic and flexible. During the 2008 US presidential election, for example, supporters of Democratic primary candidates Barack Obama and Hilary Clinton formed two separate groups and showed strong bias against each other. After the transition from the primary to the general election, the two groups merged and the intra-Democrat bias disappeared.

Martin Nowak and colleagues used the framework of evolutionary set theory to determine the conditions under which preferential in-group cooperation evolves. In the model, people can move among groups and can employ different strategies depending on whether or not they are interacting with members of their own group. Discrimination based on group identity can be a powerful force for both good and ill in human society and so understanding the evolutionary dynamics of group identity and discrimination is an important goal.