We study spatially differentiated competition between charities by partnering with two foodbanks in two neighboring cities to conduct a field experiment with roughly 350 donation appeals. We induce spatial differentiation by varying the observability of charities' location such that each donor...

Altruists and envious people who meet in contests are symbionts. They do better than a population of narrowly rational individuals. If there are only altruists and envious individuals, a particular mixture of altruists and envious individuals is evolutionarily stable.

This paper presents a formal theory of reciprocity. Reciprocity means that people reward kind actions and punish unkind ones. The theory takes into account that people evaluate the kindness of an action not only by its consequences but also the intention underlying this action. The theory...