What theorem in international trade predicts that protectionism benefits the scarce factor of production?

What theorem in international trade predicts that protectionism benefits the scarce factor of production? The Stolper-Samuelson theorem does, under the model’s assumptions. It links changes in goods prices to changes in the real returns earned by labor, capital, land, or another factor.

In the standard two-good, two-factor Heckscher-Ohlin setting, a tariff raises the domestic relative price of the imported good. The factor used intensively to produce that good gains a higher real return. Because the imported good is typically associated with the country’s scarce factor, protection can raise that factor’s real income while reducing the return to the abundant factor.

Wolfgang Stolper and Paul Samuelson derived the theorem in 1941. The result helped explain why trade policy can create domestic winners and losers: freer trade tends to benefit a country’s abundant factor, while protection may aid its scarce factor.

The claim is not a universal prediction about every tariff. It relies on assumptions such as perfect competition, constant returns, and a simplified factor structure. The theorem is also distinct from factor price equalization, which concerns the tendency of factor returns to converge across trading countries.