Altruistic punishment and the spread of information about non-cooperators through gossip appears to be important components of human cooperation (and Robin Dunbar has made the case that gossip may be at the root of human spoken communication). -- Howard
"What makes these violations of moral rules so instructive is how societies choose to deal with them. Ultimately, it all comes down to gossip. More than tool-making, art, or even language, gossip is a human universal that is a defining feature of our species (though this could change if we ever learn to translate the complex communication system in whales or dolphins). Gossip is intimately connected with the moral rules of a given society, and individuals gain or lose prestige in their group depending on how well they follow these rules. This formation of group opinion is something to be feared, particularly in small rural communities where ostracism or expulsion could mean death. "Public opinion, facilitated by gossiping, always guides the band's decision process," Boehm writes, "and fear of gossip all by itself serves as a preemptive social deterrent because most people are so sensitive about their reputations." A good reputation enhances the prestige of those individuals who engage in altruistic behavior, while marginalizing those with a bad reputation. Since prestige is intimately involved with how desirable a person is to the opposite sex, gossip serves as a positive selection pressure for enhancing traits associated with altruism. That is, being good can get you laid, and this will perpetuate your altruistic genes (or, at least, those genes that allow you to resist cheating other members of your group)."
Via Howard Rheingold