As someone who was unfamiliar with the Donor Game which was the metric they used, here's how the authors described it for others who are unaware: "A standard s

As someone who was unfamiliar with the Donor Game which was the metric they used, here's how the authors described it for others who are unaware: "A standard setup for studying indirect reci- procity is the following Donor Game. Each round, individuals are paired at random. One is assigned to be a donor, the other a recipient. The donor can either cooperate by providing some benefit at cost , or defect by doing nothing. If the benefit is larger than the cost, then the Donor Game represents a collective action problem: if everyone chooses to donate, then every individual in the community will increase their assets over the long run; however, any given individual can do better in the short run by free riding on the contributions of others and retaining donations for themselves. The donor receives some infor- mation about the recipient on which to base their decision. The (implicit or explicit) representation of recipient information by the donor is known as reputation. A strategy in this game requires a way of modelling reputation and a way of taking action on the basis of reputation. One influential model of reputation from the literature is known as the image score. Cooperation increases the donor’s image score, while defection decreases it. The strategy of cooperating if the recipient’s image score is above some threshold is stable against first-order free riders if > , where is the probability of knowing the recipient’s image score (Nowak and Sigmund, 1998; Wedekind and Milinski, 2000)."