The book concludes that land rent grows as population increases. It also presents the theory of comparative advantage, the theory that free trade between two or more countries can be mutually beneficial, even when one country has an absolute advantage over the other countries in all areas of production.

Ricardo famously analysed the impact of the adoption of machinery on the different classes of society, revising his earlier view that mechanization could be expected to be of benefit to each of the classes of the society.