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Morality Games 299

Why don’t we consider it murder to let someone die that we could have easily
saved? For example, we sometimes treat ourselves to a nice meal at a fancy restau-
rant rather than donating the cost of that meal to a charity that fights deadly diseases.
This extreme example illustrates a general phenomenon: that people have a ten-
dency to assess harmful commissions (actions such as killing someone) as worse, or
more morally reprehensible, than equally harmful omissions (inactions such as let-
ting someone die). Examples of this distinction abound, in ethics (we assess with-
holding the truth as less wrong than lying (Spranca, Minsk, & Baron, 1991)), in law
(it is legal to turn off a patient’s life support and let the patient die, as long as one
has the consent of the patient’s family; however, it is illegal to assist the patient in
committing suicide even with the family’s consent), and in international relations.
For example, consider the Struma, a boat carrying Jewish refugees fleeing Nazi
persecution in 1942. En route to Palestine, the ship’s engine failed, and it was towed
to a nearby port in Turkey. At the behest of the British authorities then in control of
Palestine, passengers were denied permission to disembark and find their way to
Palestine by land. For weeks, the ship sat at port. Passengers were brought only
minimal supplies, and their requests for safe haven were repeatedly denied by the
British and others. Finally, the ship was towed to known hostile waters in the Black
Sea, where it was torpedoed by a Russian submarine almost immediately, killing
791 of 792 passengers. Crucially, though, the British did not torpedo the ship them-
selves or otherwise execute passengers—an act of commission that they and their
superiors would undoubtedly have found morally reprehensible.

Why do we distinguish between transgressions of omission and commission? To
address this question, we present a simple game theory model based on the insight
by DeScioli, Bruening, and Kurzban (2011). The intuition can be summarized in
four steps:

1. We note that moral condemnation motivates us to punish transgressors. Such
punishment is potentially costly, e.g., due to the risk of retaliation. We expect
people to learn or evolve to morally condemn only when such costs are worth
paying.
2. Moral condemnation can be less costly when others also condemn, perhaps
because the risk of retaliation is diffused, because some sanctions do not work
unless universally enforced or, worse, because others may sanction individuals
they believe wrongly sanctioned. This can be modeled using any game with
multiple Nash equilibria, including the Repeated Prisoner’s Dilemma and the
Side-Taking Game. The Coordination Game is the simplest game with multiple
equilibria, so we present this game to convey the basic intuition. In the
Coordination Game, there are two players who each simultaneously choose
between two actions, say punish and don’t punish. The key assumption is that
each player prefers to do what she expects the other to do, which can be captured
by assuming each receives a if they both punish, d if neither punish, b