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 members and on an intrinsic “moral ideal.” We show that the assumption of weakly morally motivated agents leads to the decline of the average contribution over time. The model is compatible with persistence of overcontributions, variability of contributions (across and within individuals), the “restart effect” and the observation that the decay in contributions is slower in longer games. Furthermore, it offers a rationale for conditional cooperation.