One of the most intriguing discoveries of experimental economics is that (1) as we have seen, people invariably behave non-cooperatively in small and large group 'impersonal' market exchange institutions; (2) many (up to half in single play; over 90% in repeat play) cooperate in 'personal' exchange (two-person extensive form games); (3) yet in both economic environments all interactions are between anonymous players.

p. 528. - "Constructivist and ecological rationality in economics," 2002