The classical economists were mainly interested in trade theory from a normative
viewpoint and the theory of comparative advantage was a response to the political
debate over free trade. Ricardo and other classical economists did not pay much
attention to explaining what determines the comparative advantage a country may have.
However, by the early twentieth century when the free trade battle had been won, economists
became more interested in explaining trade patterns. The key issue turned out to
be the assumption of constant costs, which is one of the basic building-blocks of
Ricardo’s model.