题目： Global Gains from Reduction of Trade Costs
摘要：We develop a simple formula for computing the global welfare effect of reduction of bilateral trade costs, such as shipping costs or the costs of administrative barriers to trade. The formula is applicable to a broad class of perfect-competition and monopolistic competition models and settings, including perfect competition with single-stage and multi-stage production and Melitz.s (2003) model with general firm productivity distribution. We prove that the underlying mechanism is the envelope theorem. We then extend our analysis to models with non-constant markup. Finally, we carry out some empirical applications to show the user-friendliness of the formula.