the fact that defection remains a dominant strategy for self-interested players. In particular, cooperation can be increased by allowing people to communicate with each other before they make their choice, even though they cannot make binding agreements (Dawes 1980; Colman 1999, ch. 9); exposing the players to “moralizing” and a discussion of the social benefits of cooperation (Dawes 1980); encouraging people to believe that the other players will cooperate (Seabright 1993; Frank, Gilovich, and Regan 1993); and publicly disclosing the players’ contributions (Dawes 1980).

_Group size_ . Theory suggests that cooperation becomes more difficult as groups become bigger. Some field studies of common-pool resources find cooperation is indeed more likely among relatively small and cohesive groups (Ostrom, Gardner, and Walker 1994). One review of experimental studies of one-shot or finitely repeated public-good games also concludes that cooperation decreases as group size increases, at least when the group has fewer than eight members (Colman 1999, ch. 9). But another review of experimental studies concludes that group size has no effect on cooperation (Ledyard 1995).

_Framing_ . The framing of a social dilemma can also affect cooperation, even though it doesn’t change anyone’s payoffs. James Andreoni finds that people are more cooperative if their choice is framed as whether or not to create a positive externality than if it is framed as whether or not to create a negative externality (Andreoni 1995b). This would explain why there is apparently more cooperation in public-good games than in commonpool-resource games. Thus, if people frame their choice about greenhouse-gas emissions as “should I cut back emissions to avoid harming other people?” they may choose a higher level of emissions than if they frame the choice as “should I help people by choosing a low level of emissions?” Studies of decision making under uncertainty also find framing effects, including loss aversion and risk-seeking in the face of possible losses (Tversky and Kahneman 1981). So the effects of the framing of emissions choices may be complex.

_Reputational spillovers_ . People cooperate more when they can take advantage of their reputation for cooperating. Charities, for example, understand that they can raise more money if they ensure that donors can publicize their gifts (Harbaugh 1998). It has been shown that cooperation in a public-good game is an evolutionarily stable strategy when this game is alternated with an indirect-reciprocity game (Panchanathan and Boyd 2004). And it has been found in an experiment that the alternation of a public-good game and an indirect-reciprocity game does sustain high-levels of cooperation in the public-good game (Milinski _et al_ 2002).

Pursuing this idea, Milinski and his colleagues conducted a similar experiment framed in terms of climate change (Milinski _et al_ 2006). They tested people’s willingness to contribute to the costs of an advertisement that would provide information about climate change and tell readers how to reduce their emissions—in a set up in which contributions were sometimes made public and could affect behavior in an indirect-reciprocity game. Specifically, 156 students were divided into 26 groups of 6, each of which played a public11