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Motivated by Roemer s Kantian equilibrium (John Roemer, 2010, 2015, In Press) we performed some experiments on cooperation in extremely simple treatments (Bosch-Domènech and Silvestre, 2017, and Appendix below). Here we comment on some lessons that we have learned from them, and offer some thoughts on Roemer s theory of cooperation based on social situations that can be easily compared to those in our experiments.

We report on a simple experiment that addresses three factors in the frequency of cooperation: (i) framing, (ii) the number of players and (iii) the perceived risk of cooperating. We work with a (two-player) Prisoner s Dilemma and with a three-player, two-strategy Public Good Game. These themes have been separately studied by a large number of experimental papers, using diverse methodologies. Our experiment targets them in a common, clear-cut framework. We find three strong effects (a) Framing; (b) The number of players (there is less cooperation in the three-person games that in the two-person ones); (c) The neutralization of risk (about 50% of participants cooperate when risk is neutralized). Both (a) and (c) go in the expected direction but, in all three cases, the strength of the effect is surprising.