We conduct a large-scale ...eld experiment with 2,440 subjects in which we exogenously vary the price of contributing to the closest empirical counterpart of an in...nitely large public good, climate change mitigation. We ...nd that the price e¤ect is robust and negative, but quantitatively weak, with a price elasticity of 0:25. Socioeconomic variables such as education, situational variables such as meteorological conditions around the time of the experiment, and attitudinal variables that can

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... be linked to guilt and moral responsibility dominate the price e¤ect. The latter also explain better than price arbitrage the decision of subjects to declare to be ...eld price censored. The results provide an experimental window on the absolute and relative role of price e¤ects on public goods contributions in a large economy and inform current attempts to build a coherent theory of charitable giving.