GDP, Technical Change, and the Measurement of Net Income: the Weitzman Model Revisited

We show how technical change, measured as a shift in the GDP function, is combined with net income to track welfare change. This provides a bridge between the productivity literature and the welfare-related literature that tends to reason in terms of net product functions: although the relevant income measure is net of depreciation, productivity is measured based on gross output. We show that net product, net income, net expenditure and productivity change are complements, not substitutes. We also examine whether holding gains and losses should be part of depreciation and conclude that in a general equilibrium setting, either productivity change or holding gains should be part of an extended Weitzman-type net income measure, but not both.