In reciprocal altruism, one must be able to detect cheats who take without giving. Evolutionary psychologists Leda Cosmides and John Tooby reasoned that if humans evolved as reciprocal altruists, we must have moral capacities for detecting cheats. They have run many experiments demonstrating that the human mind is highly attuned to detecting situations where individuals take benefits without fulfilling a social requirement. Many of the situations described in their experiments do concern genuine reciprocity, in which mutual benefits are exchanged between two individuals.
However, some of their examples of detecting cheats seem more concerned with the reliability of sexual status indicators than with the maintenance of reciprocity. Consider their (fictional) example of "Big Kiku," which is often cited. A tribal chief called Big Kiku establishes the rule that an individual must have a special tattoo in order to eat cassava root, a local delicacy. When asked to identify various possible ways in which this rule could be violated by cheats, participants in the Cosmides experiments could easily see that individuals without tattoos might be cheats, and individuals eating cassava root might be too. Yet the participants' ability to reason correctly about this problem does not necessarily depend on their understanding reciprocal altruism in Trivers's sense. If I get a tattoo, it is not giving you some benefit that you reciprocate by allowing me to eat cassava root. The tattoo is simply a costly, painful signal of tribal sexual status which entities me to enjoy the associated status display of eating cassava root.
The Cosmides experiments, often replicated and extended by other psychologists, are one of the best examples of empirical evolutionary psychology. They have revealed a specific human adaptation for detecting cheats that is distinct from general intelligence, social intelligence, or the comprehension of arbitrary social rules. But these experiments also reveal that reciprocity is not the only context in which we look for cheats. People seem to regard any status display as a benefit, and look for people who cheat by producing the display without deserving the status. They use the same mental adaptations to look for cheats who undermine fitness indicators (by pretending to a status they do not deserve) and cheats who violate reciprocity arrangements (failing to return a benefit to one who gave you a benefit).
Because the same cheater-detection module is apparently used to detect both status cheats and reciprocity cheats, evidence for cheat detection is not necessarily evidence of the importance of reciprocity in human evolution. The conventions of rank, privilege, and status are distinct from the conventions of reciprocity that yield mutual benefit. This was one of Karl Marx's key insights. A society could be based on status signals without reciprocity (a simple dominance hierarchy), or on reciprocity without status signals (an egalitarian Utopia). In either, the ability to detect cheats would be useful. Our outrage against cheats is directed at those who display deceptive fitness indicators, not just those who fail to return a kindness.
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