Ricardian economics are the economic theories of David Ricardo, an English political economist born in 1772 who made a fortune as a stockbroker and loan broker. At the age of 27, he read An Inquiry into the Nature and Causes of Wealth of Nations by Adam Smith and was energised by the theories of economics.

His main economic ideas are contained in On the Principles of Political Economy and Taxation (1817). This set out a series of theories which would later become theoretical underpinnings of both Marx's Das Kapital and Marshallian economics, including the theory of economic rent, the labour theory of value and above all the theory of comparative advantage.

Ricardo wrote his first economic article ten years after reading Adam Smith and ultimately, the "bullion controversy" gave him fame in the economic community for his theory on inflation in 19th-century England. This theory became known as monetarism, the theory that excess currency leads to inflation. He also played a part in the emergence of classical economics, which meant he fought for free trade and free competition without government interference by enforcing laws or restrictions.

Another idea Ricardo is known for in his Essay on the Influence of a Low Price of Corn on the Profits of Stock is the Law of Diminishing Returns (Ricardo, Economic Essays, Henderson 826). The law of diminishing returns states that if you add more units to one of the factors of production and keep the rest constant, the quantity or output created by the extra units will eventually get smaller to a point where overall output will not rise ("diminishing returns").

For example, consider a simple farm that has two inputs: labor and land. Suppose the farm has 100 hectares of land and one worker (the labor input). This land-labor combination produces some level of output. If the amount of land is increased, and the amount of labor stays the same, the worker will have to give less attention to each acre of land (provided that nothing else changes). So, output may increase, though the additional (marginal) output from adding an acre of land may decrease.

If more and more land is added that must be tended by this one worker, there will eventually be so much land that output starts to decrease as the worker becomes overwhelmed (that is, less labor time, on average, is devoted to each acre). This is the typical stylized result of increasing one productive input while holding the others constant (versus increasing all inputs, generating economies of scale).

Ricardo was opposed to tariffs and other restrictions on international trade. Ricardo devised an idea that is well known as the theory of comparative advantage (Henderson 827, Fesfeld 325). According to the Washington Council on International Trade, comparative advantage is the ability to produce a good at a lower cost, relative to other goods, compared to another country. In the Principles of Economics, Ricardo states that comparative advantage is a specialization technique used to create more efficient production (52) and describes opportunity cost between producers (53). With perfect competition and undistorted markets, countries tend to export goods in which they have a comparative advantage.

For example, we should think of two countries that both make cards and pencils and use the same amount of time to make one unit of items (see table). Country one can make 4 pencils if they specialize just in pencils at the expense of one card, but this country can also make ¼ of a card at the expense of one pencil. The same logic goes for country two: if country two makes only pencils, it will make 2 pencils at the expense of 1 card.