states in order to remain wealthy. Dependency theory states that the poverty of the countries in the periphery is not because they are not integrated into the world system, but because of how they are integrated into the system. These poor nations provide natural resources, cheap labor, and consumer markets for the wealthy nations, without which, according to dependency theorists, they could not have the standard of living they enjoy. First world nations actively, but not necessarily consciously, perpetuate a state of dependency through various policies and initiatives. This state of dependency is multifaceted, involving economics, media control, politics, banking and finance, education, sport and all aspects of human resource development. Any attempt by the dependent nations to resist the influences of dependency could result in economic sanctions and/or military invasion and control. This is rare, however, and dependency is enforced far more by the wealthy nations setting the rules of international trade and commerce. 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. According to Brazilian social scientist, Theotonio