The evolutionary approach to ethics has by and large focused almost exclusively on the question of altruism. Such single-mindedness is problematic for it neglects other important features of moral systems such as codes of behavior. These so-called moral codes or moral norms guide individual behavior in a number of ways, from hygiene to prohibiting certain activities such as the eating of pork of engaging in violent and reckless behavior. I devise a simple game-theoretic framework to explain the sustained existence of these often costly moral norms. These norms can be easily sustained through the combined mechanisms of reputation tracking and altruistic punishment. Reputations track both adherence to the norm and whether one punishes those in disrepute. Thus all are incentivized to punish both norm violators as well as well as those who fail to administer punishment, on threat of punishment. This maneuver allows us to avoid the infamous “second-order free-rider problem” known to plague costly punishment. I formally demonstrate the above arrangement is evolutionarily stable and, under the right parameter values, can support norms that are detrimental to both the group and individual. This theoretical result not only sheds light on an underappreciated facet of moral systems, but can also help explain the fact of moral diversity – i.e., that moral codes vary wildly from society to society. This formal model allows us to precisely articulate the conditions under which we'd expect there to be much moral diversity: when there are little to no selection at the level of the group.