摘要：This paper explores a newly-available panel data set merging balance sheet and
international trade transaction data for Belgium. Both imports and exports
appear to be highly concentrated among few firms and seem to have become more so
over time. Focusing on manufacturing, we find that facts previously reported in
the literature for exports only actually apply to imports too. We note that the
number of trading firms diminishes as the number of export destinations or
import origins increases. The same is true if we consider the number of products
traded. With regard to productivity differentials, firms that both import and
export appear to be the most productive, followed, in descending order, by
importers only, exporters only and non-traders. These results point to the
presence of fixed costs; not only of exporting, but also of importing and to a
process of self-selection in both export and import markets. Also, the
productivity advantage of exporters reported in the literature may be overstated
because imports were not considered.
