A random world is a fair world.

A preference for fairness or equity in the distribution of resources influences many human decisions (1). The origin of this preference is a topic that has consumed philosophers (2), social scientists (3), and biologists (4) for centuries. However, although we feel a sense of fairness deeply and intuitively, it has so far been difficult to explain from first principles how such a feeling might have evolved. How could natural selection allow for the survival of “fair” individuals who sometimes give things away to equalize resources when they must compete with selfinterested individuals who keep everything for themselves? In PNAS, Rand et al. (5) provide a unique and compelling solution to this puzzle: it’s all because of dumb luck. To study fairness, authors use the so-called “ultimatum game” (6). In this game, one person (the proposer) offers a specific division of a sum of money, and the other (the responder) decides whether to accept this offer. If the responder accepts, they each receive the amount of money as proposed. If the responder rejects the offer, they each receive nothing. If both players are rational and self-interested and they play the game only once, then the responder should accept any nonzero offer (something is better than nothing!). Knowing that, the proposer should offer slightly more than zero to the responder and keep the rest for himself. However, this result is not what we observe, anywhere. Dozens of studies in both large (7) and small-scale (8) societies show that proposers tend to make “fair” offers, in the range of 30–50% for the responder. Furthermore, responders tend to demand such behavior, rejecting offers when they fall below 20–35%. Past efforts to explain the origin of these preferences have used deterministic game theory, which assumes that individuals with higher expected payoffs will always come to dominate the population (9). These models cannot explain fair offers or rejection of nonzero offers without making additional assumptions. For example, if we assume individuals have information about others’ past behavior, then they can make strategies contingent on the reputation of their opponents, and this will benefit individuals with a reputation for rejecting low offers. However, how did the individuals get this information? And how do they avoid being exploited by individuals who can fake such a reputation? The additional assumptions are complex and hard to justify. Instead, Rand et al. (5) return to first principles and use a different approach. Rather than assuming that evolution is deterministic, they assume it is stochastic. In reality, evolution sometimes favors the lucky, especially when the relationship between payoffs and survival is weak. This theory means a variety of strategies can endure and the winning strategy must do well in such an environment. Intuitively, if some of the responders are rejecting nonzero offers—not because it is the best strategy but because it happens to survive sometimes—then proposers need to make fairer offers.