The new Keynesian Phillips curve and the cyclicality of marginal cost

Several authors argue that if the labor income share is used as the proxy for real marginal cost, then the New Keynesian Phillips Curve does a good job of approximating US inflation dynamics. However, this paper argues that the labor share is not an ideal measure of real marginal cost for two reasons: it is countercyclical whereas marginal cost is likely to be procyclical, and it assumes that labor can be costlessly adjusted at a fixed real wage rate. Relaxing this assumption to a more realistic one leads to a measure of marginal cost that does turn out to be procyclical, which when tested produces results that are contradictory to the entire underlying model of the NKPC. Indeed, this paper argues that having procyclical marginal cost precludes us from even having the correct coefficient signs in the NKPC, which highlights a major problem that exists in the model.