Population density
An Essay on the Principles of Population's main theme is of course: the population and all that it factors into. Through theoretical scenarios, Malthus comprehensively explores the many key, unchanging concepts of population. Malthus concludes that population density and makeup affects essentially every facet of life, from wages to nutrition to family structure.
Iron Law of Wages
One of Malthus's most famous and seminal theories was proposed in this essay - the Iron Law of Wages, which states that when population size increases, wages decrease because of an increased availability of workforce, and when population size decreases, wages increase because of increased demand for workers and less ability to find new ones to replace fired ones. This law has been applied to many settings throughout history, from the Black Plague in the medieval era to the Great Depression of 1929, and is a economic observation that is undeniably accurate.
Population & Agriculture
Malthus foresees a grim future in The Principles of Population: one where the human population is so great and dense that it surpasses all ability to sustain itself. He predicts that while population would double every 25 years, for technology is constantly increasing and most couples have multiple children, food production would only grow in a constant line, causing mass starvation and famine unless births were controlled. This theme is studied extremely throughout the essay, as Malthus believes it is great cause for concern.
