Under the world systems approach, there are three sub-systems recognized--core, semi-periphery, and periphery. The political characteristics for each are as follows in terms of centralization and effectiveness of political institutions: for the core system both are high, for semi-periphery both are mixed, and for periphery both are low. The three differ in terms of economic characteristics as well: the core has a diverse and flexible economy, the semi-periphery a mixed economy, and the periphery is focused on natural resources and lacks diversity and flexibility. Each subsystem has its own social characteristics. In terms of discretion of the labor force, the core has high levels of discretion, the semi-periphery mixed levels, and the periphery low levels of discretion. In terms of the extent of traditionalism in society, the core shows great fluidity, the semi-periphery mixed elements of traditionalism and modernity, and the periphery the influence of traditionalism. At different times from 1815 to the 1990s, the states that would be identified with each of these subsystems has changed as economic and political development has shifted states from one subsystem to another as the world system has developed. Hegemony is another term referring to certain states, and as explained by Wallerstein, hegemony means more than core status. Wallerstein defines hegemony as a situation in which the products of a core state are produced so efficiently that they are competitive even in other core states so that the given core state will be the primary beneficiary of a maximally free world market. Certain characteristics are necessary: 1) the state must be strong enough to prevent or minimize the erection of internal and external political barriers to the free flow of factors of production; and 2) the dominant economic forces preserve their advantage by encouraging certain intellectual and cultural movements and ideologies (Selected Data and D...