Public goods game: a group of people play a public game which is based on voluntary contribution mechanism (VCM). VCM is the game in which subjects have a certain amount of endowment (20$ in this study) and decide to allocate the endowment between public account and private account. The contribution to public goods is multiply with marginal per capita return. Marginal per capita return is the ratio of benefit to cost if people decide to give 1$ from private account to public account. An outcome of each player is:
In which ei is the endowment of each subject
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