Since formal rules can only partially reduce opportunistic behavior, third-party sanctioning to promote fairness is critical to achieving desirable social outcomes. Social norms may underpin such behavior, but they can also undermine it. We study one such norm —the “don’t be a toad” norm, as it is referred to in Colombia— that tells people to mind their own business and not snitch on others. In a set of fairness games where a third party can punish unfair behavior, but players can invoke the “don’t be a toad” norm, we find that the mere possibility of invoking this norm completely reverses the benefits of third-party sanctioning to achieve fair social outcomes. We establish this is an anti-social norm in a well-defined sense: most players consider it inappropriate, yet they expect the majority will invoke it. To understand this phenomenon we develop an evolutionary model of endogenous social norm transmission and demonstrate that a payoff advantage from adherence to the norm in social dilemmas, combined with sufficient heterogeneity in the disutility of those who view the norm as inappropriate, can generate the apparent paradox of an anti-social norm in the steady-state equilibrium. We provide further evidence that historical exposure to political violence, which increased the ostracization of snitches, raised sensitivity to this norm.
