Malthus' most well known work 'An Essay on the Principle of Population' was published in 1798, although he was the author of many pamphlets and other longer tracts including 'An Inquiry into the Nature and Progress of Rent' (1815) and 'Principles of Political Economy' (1820). The main tenets of his argument were radically opposed to current thinking at the time. He argued that increases in population would eventually diminish the ability of the world to feed itself and based this conclusion on the thesis that populations expand in such a way as to overtake the development of sufficient land for crops. Associated with Darwin, whose theory of natural selection was influenced by Malthus' analysis of population growth, Malthus was often misinterpreted, but his views became popular again in the 20th century with the advent of Keynesian economics.
He saw positive checks to population growth as being any causes that contributed to the shortening of human lifespans. He included in this category poor living and working conditions which might give rise to low resistance to disease, as well as more obvious factors such as disease itself, war, and famine. Some of the conclusions that can be drawn from Malthus's ideas thus have obvious political connotations and this partly accounts for the interest in his writings and possibly also the misrepresentation of some of his ideas by authors such as Cobbett, the famous early English radical. Some later writers modified his ideas, suggesting, for example, strong government action to ensure later marriages. Others did not accept the view that birth control should be forbidden after marriage, and one group in particular, called the Malthusian League, strongly argued the case for birth control, though this was contrary to the principles of conduct which Malthus himself advocated.