‘Cooperation Emerges When Groups Are Small and Memories Are Long, Penn Study Finds’, https://news.upenn.edu/news/cooperation-emerges-when-groups-are-small-and-memories-are-long-penn-study-finds, THE UNIVERSITY OF PENNSYLVANIA (June 1, 2016).

Game theory

Researchers use game theory to demonstrate the complex set of traits that can promote the evolution of cooperation. (…) The more players in a game the less likely that cooperative strategies could win out. (…) As a group size increases, the prospects for sustained cooperation go down. (…)  The evolution of longer memories led to an increase in cooperation.