Wolfgang F. Stolper, 89, an economist whose work included a theory used to explain the effect of international trade on wages, died Monday in Ann Arbor, Mich., during surgery to clear a blood clot.

In Part 1, I examined what economic theory has to say about the winners and losers from trade. The main conclusion is based on the Stolper-Samuelson Theorem: Because the United States is labor scarce ...