Originally posted by Nesster Scientific research (game theory) has shown that humans have a very strong reciprocity drive. In practice this resolves into a turn-taking with trust strategy as the ideal, though with a punitive component towards those who don't reciprocate. It has been shown that people will act counter to self interest in order to punish those they feel are 'freeloading' or otherwise not reciprocating.
I (vaguely
) seem to recall experiments where the general idea was akin to: "Ok. Here is $100. You and <some other person unknown to you> are going to have it, but we need you to agree on our proposal of how it split up between you two or else neither of you won't get any. ... ". It was found that if the other person was getting an 'unfair' share, say $80, people would tend to walk away from the $20.