In acausal trade, two agents each benefit by predicting what the other wants and doing it, even though they might have no way of communicating or affecting each other, nor even any direct evidence that the other exists.

Background: Superrationality and the one-shot Prisoner's Dilemma

This concept emerged out of the much-debated question of how to achieve cooperation on a one-shot Prisoner's Dilemma, where, by design, the two players are not allowed to communicate. On the one hand, a player who is considering the causal consequences of a decision ("Causal Decision Theory") finds that defection always produces a better result. On the other hand, if the other player symmetrically reasons this way, the result is a Defect/Defect equilibrium, which is bad for both agents. If they could somehow converge on Cooperate, they would each individually do better. The question is what variation on decision theory would allow this beneficial equilibrium.

Douglas Hofstadter (see references) coined the term "superrationality" to express this state of convergence. He illustrated it with a game in which twenty players, who do not know each other's identities, each get an offer. If exactly one player asks for the prize of a billion dollars, they get it, but if none or multiple players ask, no one gets it. Players cannot communicate, but each might reason that the others are reasoning similarly. The "correct" decision--the decision which maximizes expected utility for each player, if all players symmetrically make the same decision--is to randomize a one-in-20 chance of asking for the prize.

Gary Drescher (see references) developed the concept further, introducing an ethical system called "acausal subjunctive morality." Drescher's approach relies on the agents being identical or at least similar, so that each agent can reasonably guess what the other will do based on facts about its own behavior, or even its own "source code." If it cooperates, it can use this correlation to infer that the other will probably also cooperate.

Acausal trade goes one step beyond this. The agents do not need to be identical, nor similar, nor have the same utility function. Moreover, they do not need to know what the other agents are like, nor even if they exist. In acausal trade, an agent may have to surmise the probability that other agents, with their utility function and proclivities, exist.

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