Dependency theory first emerged in the 1950s, advocated by Raul Prebisch whose research found that the wealth of poor nations tended to decrease when the wealth of rich nations increased. The theory quickly divided into diverse schools. Some, most notably Andre Gunder Frank and Walter Rodney adapted it to Marxism. "Standard" dependency theory differs sharply from Marxism, however, arguing against internationalism and any hope of progress in less developed nations towards industrialization and a liberating revolution. Former Brazilian President Fernando Henrique Cardoso wrote extensively on dependency theory while in political exile. The American sociologist Immanuel Wallerstein refined the Marxist aspect of the theory, and called it the "world system." World system theory adds another layer to what dependency theorists describe as the structure of the world: the semi-periphery. The semi-periphery is composed of countries such as Brazil and South Africa that can't simply be categorized as part of the core or the periphery (i.e. they have developed urban areas but also large areas of rural poverty). World systems theory also states that the dynamic of surplus extraction that occurs between nations also occurs within them, between their elite and poor classes. According to this structure, which includes a core, a periphery, and a semi-periphery, not every nation can develop simultaneously (which directly challenges the linear model of modernization which suggests that all countries are on the trajectory of development). WST instead argues that development and underdevelopment are created simultaneously.