This theory implies that the very structure of the global market is responsible for the persistent inequality within the world system. This provides an interesting twist on Wallerstein's neo-Marxist interpretation of the international order which faults differences in power relations between 'core' and 'periphery' states as the chief cause for economic and political inequality (However, the Singer–Prebisch thesis also works with different bargaining positions of labour in developed and developing countries). As a result, the hypothesis enjoyed a high degree of popularity in the 1960s and 1970s with neo-Marxist developmental economists and even provided a justification for an expansion of the role of the commodity futures exchange as a tool for development.