Are people who cooperate ‘rational altruists’?

In experimental games, as in natural situations, people are often observed acting to the benefit of others even at considerable cost to themselves. Such behavior is contrary to the assumption of selfish behavior, but it is not necessarily contrary to the assumption of rationality: People cooperating under these circumstances could be ‘rational altruists’ who base their decision on the magnitude of the external benefit, as well as on the costs to themselves. We test that proposition using a prisoners' dilemma experimental paradigm. If it is correct, we should observe higher levels of cooperation as the external benefit in such games increases. But we do not — whether external benefit is measured in terms of number of individuals benefiting from a cooperative choice or the total dollar benefit produced. Cooperation must be explained otherwise.