关键词: 外部性, 合约, 公共选择, 帕累托最优, 退出权

Abstract: This paper posed a new definition of externality: the influence on the contractor by other people who did not involve in conclusion of the contract.The internal contract was the contract mentioned above,and the external contract was realized by the internal contractor and the aforesaid "other people" who were not under control of the internal contract,but have influence on the internal contractor.The internal contract and external contract formed a dual contract,which was universal.The dual contract could be explained by public choice,cooperative gambling,bargaining and general equilibrium.

Key words: externality, contract, public choice, Pareto optimality, withdrawal right