In, The Evolution Of Cooperation, written in 1984. Robert Axelrod suggested there are three necessary conditions for people to cooperate with each other.
1) A likelihood of meeting in the future.
2) An ability to identify each other.
3) A record of past behavior.
For all I can say, Mr. Axelrod was as spot in 1984 as he would have been today. Even though Axelrod’s view on cooperation was written many year ago, the rules very much so still carry merit. People are friendly and “cooperative” amongst strangers simply because (most of the time) Axelrod’s rules are met. For example, everyone’s nice to the cashier at the grocery store because 1) Odds are that I’m going to see that cashier again. 2) I can identify that person as a cashier and therefore have an idea of what to expect from that person. 3) I can make the safe assumption that because that person was hired from said grocer, he/she can’t be all that bad of a person nor have all that terrible of past behavior. Pretty cool!