 A variation on the idea of reciprocal altruism can be seen in Axelrod (1986). Axelrod presents a “norms game” in which agents probabilistically choose to comply with the norm, or deviate from it, and then other agents can probabilistically choose to punish any deviations at some cost to them. Agents can choose over time to be more or less “bold”, which determines the rate at which they attempt defections, and they can likewise choose to be more or less “vengeful”, which determines how often they punish. Axelrod noted that if the game is left like this, we find that the stable state is constant defection and no punishment. However, if we introduce a meta-norm—one that punishes people who fail to punish defectors—then we arrive at a stable norm in which there is no boldness, but very high levels of vengefulness. It is under these conditions that we find a norm emerge and remain stable. Axelrod’s model aims to illustrate that norms require meta-norms. That is, failure to retaliate against a defection must be seen as equivalent to a defection itself. What Axelrod does not analyze is whether there is some cost to being vigilant. Namely, watching both defectors and non-punishers may have a cost that, though nominal, might encourage some to abandon vigilance once there has been no punishment for some time.