The study investigates why people tend to favor their own group over others, even when group differences don't matter. The researchers propose that this favoritism might stem from a difficulty in recognizing individuals from other groups (outgroup homogeneity bias). They created a model using prisoner's dilemma game and found that when people struggle to distinguish outgroup members, they show more favoritism to their ingroup to promote cooperation. This bias can be reduced by increasing the benefits of working together, having more diverse populations, and enforcing stricter social rules.