The New Keynesian Phillips Curve and the cyclicality of marginal cost

Several authors have argued that if the labor share of income is used as the proxy for real marginal cost, then the sticky-price version of the New Keynesian Phillips Curve does a good job of approximating US inflation dynamics. However, this paper argues that the labor share is an inappropriate measure of real marginal cost for two reasons: it is countercyclical whereas theory predicts marginal cost should be procyclical, and it employs a counterfactual assumption about the behavior of labor over the business cycle. Relaxing this assumption to a more realistic one leads to a measure of marginal cost that is markedly procyclical. Testing this improved measure of marginal cost then produces results that are contradictory to the entire underlying model of the NKPC. Thus I conclude that the NKPC fails to give a sound explanation of inflation dynamics.