  We experimentally study a game in which success requires a sufficient total
contribution by members of a group. There are significant uncertainties
surrounding the chance and the total effort required for success. A theoretical
model with max-min preferences towards ambiguity predicts higher contributions
under ambiguity than under risk. However, in a large representative sample of
the Spanish population (1,500 participants) we find that the ATE of ambiguity
on contributions is zero. The main significant interaction with the personal
characteristics of the participants is with risk attitudes, and it increases
contributions. This suggests that policymakers concerned with ambiguous
problems (like climate change) do not need to worry excessively about
ambiguity.
