Negishi welfare weights in integrated assessment models: the mathematics of global inequality

In a global climate policy debate fraught with differing understandings of right and wrong, the importance of making transparent the ethical assumptions used in climate-economics models cannot be overestimated. Negishi weighting is a key ethical assumption in climate-economics models, but it is virtually unknown to most model users. Negishi weights freeze the current distribution of income between world regions; without this constraint, IAMs that maximize global welfare would recommend an equalization of income across regions as part of their policy advice. With Negishi weights in place, these models instead recommend a course of action that would be optimal only in a world in which global income redistribution cannot and will not take place. This article describes the Negishi procedure and its origin in theoretical and applied welfare economics, and discusses the policy implications of the presentation and use of Negishi-weighted model results, as well as some alternatives to Negishi weighting in climate-economics models.