Thomas Malthus's An Essay on the Principles of Population proposed one of the most important economic core beliefs that we hold true even today: the Iron Law of Wages, which states that when population rises and subsequently so does workforce, wages decrease. Likewise, the opposite holds true: when population decreases, wages will increase for the remaining workers.
This economic phenomenon Malthus observed in The Principles of Population has proven true over hundreds of years both before and after he wrote it. For example, during the medieval Black Plague, when thousands of peasants died and lords were unable to staff their fields, wages and privileges increased for the serfs who survived. As well, in the same book, Malthus also delved into another theory of population where he predicted that population would double every 25 years, but agricultural growth would remain stagnant, meaning mankind would not be able to produce enough food to maintain this swollen populace size. He advocated for controlling births to prevent this bleak outcome.
Although first published in 1798, An Essay on the Principle of Population is recognized as an extremely seminal work, influencing economic decisions even today. Charles Darwin even cited Malthus's book as one of the roots of his famed theory of natural selection.