This paper explores potential causes of the flattening of the Phillips curve and why they matter for monetary policy. I use a novel open economy nested-CES model to show that an increase in product market concentration (Herfindahl-Hirschman Index) and a higher degree of openness to international trade both lead to a flatter Phillips curve. Within the model, the central bank’s optimal policy choices depend separately on these two factors, not just on the resulting slope of the Phillips curve. My model predicts a 30% drop in the slope of the Phillips curve since the 1990s, where the major contribution comes from increases in product market concentration rather than increases in international trade openness. Through a series of policy experiments, I quantitatively show the suboptimality of the choices made by policymakers who are misinformed either about the slope of the Phillips curve or about its causes.