People will return favors without thinking much about it. A vague sense of justice pervades how people behave in society. It’s not the justice of law, though—it’s a more abstract justice that only has one rule: if someone does something for me, I must do something for them. The values of these two actions need not balance, though usually action and counter-action are not vastly disproportionate.
You can harness this fallacious tendency when designing guild, party, and other team play systems. Encourage players to give gifts to other players—you may even want to make generosity an advantageous strategy by rewarding guilds and players who help out non-aligned players and newbies. When players receive gifts or favors from other players, the receiver will feel a subconscious (if not conscious) sense of being beholden to continue playing so that they can return the favor. This is the positive side of the “revenge” motivator in games. Even if players don’t remember who their benefactors are, they will have a quiet sense of responsibility towards your game world in general.
Harnessing reciprocity in design encourages players to make the game world more alive. Players will interact more if interaction yields positives for all sides. When interaction is a beneficial experience, players that would otherwise keep to themselves will willingly reach out and become part of the community. Breaking the tunnel-vision of solo play results in a significant increase in time spent in-game. When you have other people with whom you’ve developed these positive relationships, you are going to work to keep those relationships up more than you would if you were purely thinking rationally of the risk and reward of solo play.
The effect of reciprocity is dulled somewhat by anonymity, but it is a powerful force—it’s the intangible force that binds guilds and keeps people logging on every night.