The minimal group experiments by Tajfel demonstrated that random assignment to groups (e.g., red team vs. blue team) immediately leads to dislike and negative stereotyping of the out-group.

“it's the the classic minimal group experiments of Zion in the in the 60s which I I love and I teach on this all the time because it's it's relevant for all these tribal biases and so and what he did was like you take the random people off the street and you go like okay you're on the red team you're on the blue team you're on the red team you're on the blue team okay uh in five minutes you're going to have to compete against the other team and immediately the people on the red team are like I don't like the people on The Blue Team they're stupid and they're ugly and you don't know anything about them right but you end up immediately forming a tribe”