Traditional rice harvesting in Southeast Asia is characterized by common ownership: everybody is allowed to participate in harvesting any farmer's crop and receive a certain share of the output. Work and income is shared. Under conditions of simple technology, relative scarcity of labour and subsistence oriented objectives (output maximization), the non-exclusive ownership of harvesting rights will allow rational resource allocators (land-owners, peasants, and harvesting workers) to maximize their welfare. But strong population pressure that reduces the retum to labour relative to the return to land has led to a dissipation of the harvesting rent and increased considerably the importance of the externalities associated with free harvesting. Similarly, the diffusion of new rice technology such as fertilizer-responsive high-yielding modern varieties has increased the incentives for a shift from the traditional crop-share contracts (bawon, hunusan) based on common property to more exclusive rights, which allow the farmer/landowner to obtain a bigger share of the increased harvest and reduce negative externalities. New harvesting institutions have evolved spontaneously which reflect changes in the relative scarcity of the factors of production and provide stronger incentives for the introduction and diffusion of agricultural innovations.