The third section of Freeman’s article uses the rationale of “complementarity” to argue that immigration is good, as it complements trade. The post World War II period has been characterized by roughly an 80 percent decline in barriers to trade, and many economists argue that this increased openness has made the world economy more productive and richer. If trade and immigration are complements, then immigration should be liberalized as well. The theory of international trade suggests that trade in goods and services tend to substitute for factor flows of capital and labor. As Freeman states: “Restrict immigration, and trade should increase. Restrict trade, and immigration should increase” (p. 160). According to classical trade theory, even if immigrants (and capital flows) were entirely prohibited into the United States, as long as free trade is allowed, the relative earnings of unskilled and uneducated workers would still be diminished – just as if immigration were fully allowed!