Mercantilism was one of the earliest schools of economic thought. It was developed in the 16th century and gained popularity in the17th century. Those who believed in mercantilism thought that a nation's access to wealth was determined by the accumulation of gold and silver. The nations that did not have direct access to gold and silver needed to export by a greater margin than they imported. Mercantilism hinged on the assumption that governments would take total control over foreign trade.
The classical school of economics originated with "The Wealth of Nations," a publication written by Adam Smith in 1776. Smith's argument was explained by three keys, which include production of nation's wealth and invisible hand. Smith believed that a nation's wealth was produced by land, labor and capital. He also believed that society benefited most when all the individuals pursued their own interests. Later, David Ricardo, Thomas Robert Malthus and John Stuart Mill made small tweaks to the classical school thought.
Marxism was a popular school of thought at the end of the 19th century, when Karl Marx wrote that he believed capitalism would eventually destroy itself. Marxism teaches that labor or the workers owned production since they actually produced the items and offered the services that benefited the society. Marx believed that workers would become disgruntled with the distribution of profit and income, and they would rise up against the capitalist system.
During the Great Depression, Americans were looking for an answer to the devastating economic issues of the time. U.S. President Franklin Delano Roosevelt looked to John Maynard Keynes to help the American economy rebound. Unlike the classical economic theory, Keynes believed that the government must intervene by increasing total spending. Keynes' argument was that the government must adjust the total spending when the economy shifts from extreme to extreme in order to stabilize the economy.