Incluye 40 referencias bibliográficas en las páginas 1056-1060

We investigate the role of anonymity, reciprocity, and conformity for voluntary contributions, based on a natural fieldexperiment conducted at a national park in Costa Rica. Contributions made in public in front of the solicitor are 25% higher thancontributions made in private. Giving subjects a small gift before requesting a contribution increases the likelihood of a positivecontribution. At the same time, the conditional contribution decreases. The total effect of giving a gift is positive but small, andtaking the cost of the gift into account, it is far from profitable. When the subjects are told that the typical contribution of others is$2 (a small contribution), the probability of a contribution increases and the conditional contribution decreases, compared withproviding no reference information. Providing a high reference level ($10) increases the conditional contributions. Overall, the totaleffects have the expected signs, although the magnitudes are smaller than what one might have expected based on existing evidencefrom laboratory experiments.