Procompetitive gains from trade and comparative advantage

Having made the concepts of pro or anticompetitive output changes precise, necessary and sufficient conditions for trade gains are derived under conditions of increasing returns to scale and imperfect competition in a general equilibrium model with any (finite) number of goods and factors. These conditions yield generalizations of the theorems of comparative advantage in goods and implicit factor trade (the factor content version of the H-O theorem). Other generalizations can be obtained if one assumes quasihomothetic preferences and that welfare effects are dominated by price rather than income effects.