Comparative advantage, economic theory, first developed by 19th-century British economist David Ricardo, that attributed the cause and benefits of international trade to the differences in the relative opportunity costs (costs in terms of other goods given up) of producing the same commodities among countries. The theory had been brewing for decades, beginning with early 20th-century British economist Arthur Cecil Pigou. He argued that transactions can have effects that don't show up in the price of a Trade cannot be explained neatly by one single theory, and more importantly, our understanding of international trade theories continues to evolve. Modern or Firm-Based Trade Theories In contrast to classical, country-based trade theories, the category of modern, firm-based theories emerged after World War II and was developed in large part by