In a canonical model of collective action, individual contribution to collective action is negatively correlated with group size. Empirical evidence on the group size effect has been mixed, partly due to heterogeneities in group activities. In this paper, we first construct a simple general model of collective action with the free-riding problem, altruism, public goods, and positive externalities of social networks. We then empirically test the theoretical implications of group size effect on individual contribution to four different types of collective action, i.e., monetary or nonmonetary contribution to directly or indirectly productive activities. To achieve this, we collect and employ artefactual field experimental data such as public goods and dictator games conducted in southern Sri Lanka under a natural experimental situation where the majority of farmers were relocated to randomly selected communities based on the government lottery. This unique situation enables us to identify the causal effects of community size on collective action. We find that the levels of collective action can be explained by the social preferences of farmers; we show evidence on the free-riding by self-interested households with no land holdings. The pattern of collective action, however, differs significantly by the mode of activities; the collective action which is directly related to production is less likely to suffer from the free rider problem than from indirectly productive activities. Finally, the monetary contribution is less likely to cause the free riding than the non-monetary contribution.
Keywords: collective action, social preference, natural and artefactual field experiment, irrigation, South Asia