Free trade can make an entire class of workers poorer, even as the country gains

The Stolper-Samuelson theorem shows trade doesn't just shift jobs between industries: it raises real returns to whichever factor a country's exports use intensively, and lowers returns to the other factor. So a nation can grow richer from trade while one group, say unskilled labor, sees its real wages permanently squeezed.

— Wolfgang Stolper and Paul Samuelson, Protection and Real Wages — Review of Economic Studies, 1941