the individuals involved do not have to be rational: The evolutionary
process allows successful strategies to thrive, even if the players do not
know why or how. Nor do they have to exchange messages or commit-
ments: They do not need words, because their deeds speak for them.
Likewise, there is no need to assume trust between the players: The use of
reciprocity can be enough to make defection unproductive. Altruism is not
needed: Successful strategies can elicit cooperation even from an egoist.
Finally, no central authority is needed: Cooperation based on reciprocity
can be self-policing