Some Surprising Facts About the Concentration of Trade Across Commodities and Trading Partners

The present paper documents unexplained concentration in trade. B ernard et al. ( P roducer D ynamics: N ew E vidence from M icro D ata , 2009, U niversity of C hicago P ress) have documented concentration at the level of the firm and in exports. Taking a step forward, we document trade volume concentration at the level of nations and in both exports and imports. Firm level concentration has been relatively easy to explain in terms of models of heterogeneous‐firm models with entry costs in both the domestic and foreign markets. But as we shall see, the concentration at the level of the nation, especially in imports if not exports, turns out to be far more difficult to explain.