The global economy continues to struggle with the ongoing aftereffects of what many have come to call the Great Economic Crisis, a sharp downturn in the world economy that began in the United States in 2008 and had devastating worldwide ramifications. Economics is the social science that studies, describes, models, and makes projections about the production, distribution, and consumption of goods and services. Liberal economic theory is a theory of political economics first clearly articulated during the Enlightenment in Adam Smith's Inquiry into the Nature and Causes of the Wealth of Nations. Development economics studies economic growth in less-developed countries. Microeconomics assumes that there is a host of buyers and sellers in the market, none of whom can effectively control the price structure in the market. The economics of sustainability insists on including resource depletion as a cost in economic models. Modern forms of Marxism can be divided into two separate movements: Marxism-Leninism and social democracy.