The third theory, the cooperation hypothesis, holds that conflicting groups need to cooperate by laying aside their individual interests and learning to work together for shared goals.

 

Cooperation is defined as any adaptation that has evolved, at least in part, to increase the reproductive success of the actor's social partners. Inclusive fitness theory reveals that cooperation can be favoured by natural selection owing to either direct fitness benefits (mutually beneficial cooperation) or indirect fitness benefits (altruistic cooperation). Direct fitness benefits can arise as a simple byproduct of cooperation, or else owing to the existence of enforcement mechanisms, which may be fixed or conditioned according to the individual's cooperative behaviour. Indirect fitness benefits can arise when cooperation occurs between genetically similar individuals, as a consequence of limited dispersal, kin discrimination or greenbeard mechanisms. These theoretical mechanisms are illustrated with empirical examples, from laboratory experiments to field studies.

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Sources:https://www.cliffsnotes.com/study-guides/sociology/race-and-ethnicity/prejudice-and-discrimination

https://www.researchgate.net/publication/228018300_Theory_of_Cooperation