Malthusianism is the idea that population growth is potentially exponential while the growth of the food supply is linear. It derives from the political and economic thought of the Reverend Thomas Robert Malthus, as laid out in his 1798 writings, An Essay on the Principle of Population. Malthus believed there were two types of "checks" that in all times and places kept population growth in line with the growth of the food supply: "preventive checks", such as moral restraints (abstinence and delaying marriage until finances become balanced), and restricting marriage against persons suffering poverty or perceived as defective, and "positive checks", which lead to premature death such as disease, starvation and war, resulting in what is called a Malthusian catastrophe. The catastrophe would return the population to a lower, more "sustainable", level. Malthusianism has been linked to a variety of political and social movements, but almost always refers to advocates of population control.
Neo-Malthusianism is the advocacy of human population planning to ensure resources and environmental integrities for current and future human populations as well as for other species. In Britain the term 'Malthusian' can also refer more specifically to arguments made in favour of preventive birth control, hence organizations such as the Malthusian League. Neo-Malthusians differ from Malthus's theories mainly in their support for the use of contraception. Malthus, a devout Christian, believed that "self-control" (abstinence) was preferable to artificial birth control. In some editions of his essay, Malthus allowed that abstinence was unlikely to be effective on a wide scale, thus advocating the use of artificial means of birth control as a solution to population "pressure". Modern "neo-Malthusians" are generally more concerned than Malthus with environmental degradation and catastrophic famine than with poverty.
Malthusianism has attracted criticism from diverse schools of thought, including Marxists and socialists, libertarians and free market enthusiasts, social conservatives, feminists and human rights advocates, characterising it as excessively pessimistic, misanthropic or inhuman. Many critics believe Malthusianism has been discredited since the publication of Principle of Population, often citing advances in agricultural techniques and modern reductions in human fertility. Many modern proponents believe that the basic concept of population growth eventually outstripping resources is still fundamentally valid, and "positive checks" are still likely in humanity's future if there is no action to curb population growth.
Malthus was not the first to outline the problems he perceived. The original essay was part of an ongoing intellectual discussion at the end of the 18th century regarding the origins of poverty. Principle of Population was specifically written as a rebuttal to thinkers like William Godwin and the Marquis de Condorcet, and Malthus's own father who believed in the perfectibility of humanity. Malthus believed humanity's ability to reproduce too rapidly doomed efforts at perfection and caused various other problems.
His criticism of the working class's tendency to reproduce rapidly, and his belief that this, rather than exploitation by capitalists, led to their poverty, brought widespread criticism of his theory.
Malthusians perceived ideas of charity to the poor, typified by Tory paternalism, were futile, as these would only result in increased numbers of the poor; these theories played into Whig economic ideas exemplified by the Poor Law Amendment Act of 1834. The Act was described by opponents as "a Malthusian bill designed to force the poor to emigrate, to work for lower wages, to live on a coarser sort of food", which initiated the construction of workhouses despite riots and arson.
Malthus revised his theories in later editions of An Essay on the Principles of Population, taking a more optimistic tone, although there is some scholarly debate on the extent of his revisions. According to Dan Ritschel of the Center for History Education at the University of Maryland,
The great Malthusian dread was that "indiscriminate charity" would lead to exponential growth in the population in poverty, increased charges to the public purse to support this growing army of the dependent, and, eventually, the catastrophe of national bankruptcy. Though Malthusianism has since come to be identified with the issue of general over-population, the original Malthusian concern was more specifically with the fear of over-population by the dependent poor.
One of the earliest critics was David Ricardo. Malthus immediately and correctly recognised it to be an attack on his theory of wages. Ricardo and Malthus debated this in a lengthy personal correspondence.
Another one of the 19th century critics of Malthusian theory was Karl Marx who referred to it as "nothing more than a schoolboyish, superficial plagiary of De Foe, Sir James Steuart, Townsend, Franklin, Wallace" (in Capital, see Marx's footnote on Malthus from Capital – reference below). Marx and Engels described Malthus as a "lackey of the bourgeoisie". Socialists and communists believed that Malthusian theories "blamed the poor" for their own exploitation by the capitalist classes, and could be used to suppress the proletariat to an even greater degree, whether through attempts to reduce fertility or by justifying the generally poor conditions of labour in the 19th century.
One proponent of Malthusianism was the novelist Harriet Martineau whose circle of acquaintances included Charles Darwin, and the ideas of Malthus were a significant influence on the inception of Darwin's theory of evolution. Darwin was impressed by the idea that population growth would eventually lead to more organisms than could possibly survive in any given environment, leading him to theorize that organisms with a relative advantage in the struggle for survival and reproduction would be able to pass their characteristics on to further generations. Proponents of Malthusianism were in turn influenced by Darwin's ideas, both schools coming to influence the field of eugenics. Henry Fairfield Osborn, Jr. advocated "humane birth selection through humane birth control" in order to avoid a Malthusian catastrophe by eliminating the "unfit".
Malthusianism became a less common intellectual tradition as the 19th century advanced, mostly as a result of technological increases, the opening of new territory to agriculture, and increasing international trade. Although a "conservationist" movement in the United States concerned itself with resource depletion and natural protection in the first half of the twentieth century, Desrochers and Hoffbauer write, "It is probably fair to say ... that it was not until the publication of Osborn’s and Vogt’s books  that a Malthusian revival took hold of a significant segment of the American population".
Malthusian theory is a recurrent theme in many social science venues. John Maynard Keynes, in Economic Consequences of the Peace, opens his polemic with a Malthusian portrayal of the political economy of Europe as unstable due to Malthusian population pressure on food supplies. Many models of resource depletion and scarcity are Malthusian in character: the rate of energy consumption will outstrip the ability to find and produce new energy sources, and so lead to a crisis.
In France, terms such as "politique malthusienne" ("Malthusian politics") refer to population control strategies. The concept of restriction of the population associated with Malthus morphed, in later political-economic theory, into the notion of restriction of production. In the French sense, a "Malthusian economy" is one in which protectionism and the formation of cartels is not only tolerated but encouraged.
"Neo-Malthusianism" is a concern that overpopulation as well as overconsumption may increase resource depletion and/or environmental degradation will lead to ecological collapse or other hazards.
The rapid increase in the global population of the past century exemplifies Malthus's predicted population patterns; it also appears to describe socio-demographic dynamics of complex pre-industrial societies. These findings are the basis for neo-Malthusian modern mathematical models of long-term historical dynamics.
There was a general "neo-Malthusian" revival in the 1950s, 1960s, and 1970s after the publication of two influential books in 1948 (Fairfield Osborn's Our Plundered Planet and William Vogt's Road to Survival). During that time the population of the world rose dramatically. Many in environmental movements began to sound the alarm regarding the potential dangers of population growth. The Club of Rome published a book entitled The Limits to Growth in 1972. The report and the organisation soon became central to the neo-Malthusian revival. Paul R. Ehrlich has been one of the most prominent neo-Malthusians since the publication of The Population Bomb in 1968. Leading ecological economist Herman Daly has acknowledged the influence of Malthus on his concept of a steady-state economy.:xvi Other prominent Malthusians include the Paddock brothers, authors of Famine 1975! America's Decision: Who Will Survive?
The neo-Malthusian revival has drawn criticism from writers who claim the Malthusian warnings were overstated or premature because the green revolution has brought substantial increases in food production and will be able to keep up with continued population growth. Julian Simon, a cornucopian, has written that contrary to neo-Malthusian theory, Earth's "carrying capacity" is essentially limitless.[how?] Responding to Simon, Al Bartlett reiterates the potential of population growth as an exponential (or as expressed by Malthus, "geometrical") curve to outstrip both natural resources and human ingenuity. Bartlett writes and lectures particularly on energy supplies, and describes the "inability to understand the exponential function" as the "greatest shortcoming of the human race".
Prominent neo-Malthusians such as Paul Ehrlich maintain that ultimately, population growth on Earth is still too high, and will eventually lead to a serious crisis. The 2007–2008 world food price crisis inspired further Malthusian arguments regarding the prospects for global food supply.
From approximately 2004 to 2011, concerns about "peak oil" and other forms of resource depletion became widespread in the United States, and motivated a large if short-lived subculture of neo-Malthusian "peakists."