We develop a model that accounts for the decay of the average contribution observed in experiments on voluntary contributions to a public good. The novel idea is that people's moral motivation is "weak"". Their judgment about the right contribution depends on observed contributions by group...

This paper provides a general framework that accounts for the decay of the average contribution observed in most experiments on voluntary contributions to a public good. Each player balances her material utility loss from contributing with her psychological utility loss of deviating from her...