The challenges of collective action are presented by leaders in many industrialized countries as a major obstacle to e®ective action on climate change. Notably, the argument goes, a fair international solution must appropriately constrain large greenhouse gas emitters like China. This paper asks whether citizen support for multilateral climate policies also depends on whether other countries are seen to reciprocate. We analyze results from population-based survey experiments in the US, Canada, Norway, and Sweden, asking subjects whether they think their country should commit internationally to emission reductions. Randomly assigned sub-samples were presented with statements suggesting that China may or may not choose to cooperate, or alternatively making no mention of China. We¯nd that reciprocity is important to respondents in the smaller Scandinavian countries but not in North America. These¯ndings suggest that country size is more important than national traditions of multilateral cooperation in predicting support for unilateral climate action.