Malthusianism is the theory that population growth is potentially exponential while the growth of the food supply or other resources is linear, which eventually reduces living standards to the point of triggering a population decline. This event, called a Malthusian catastrophe (also known as a Malthusian trap, population trap, Malthusian check, Malthusian crisis, Malthusian spectre, or Malthusian crunch) occurs when population growth outpaces agricultural production, causing famine or war, resulting in poverty and depopulation. Such a catastrophe inevitably has the effect of forcing the population to "correct" back to a lower, more easily sustainable level (quite rapidly, due to the potential severity and unpredictable results of the mitigating factors involved, as compared to the relatively slow time scales and well-understood processes governing unchecked growth or growth affected by preventive checks). Malthusianism has been linked to a variety of political and social movements, but almost always refers to advocates of population control. These concepts derive 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 suggested that while technological advances could increase a society's supply of resources, such as food, and thereby improve the standard of living, the resource abundance would enable population growth, which would eventually bring the per capita supply of resources back to its original level. Some economists contend that since the industrial revolution, mankind has broken out of the trap. Others argue that the continuation of extreme poverty indicates that the Malthusian trap continues to operate. Others further argue that due to lack of food availability coupled with excessive pollution, developing countries show more evidence of the trap. A similar, more modern concept, is that of human overpopulation. 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" (i.e., abstinence) was preferable to artificial birth control. He also worried that the effect of contraceptive use would be too powerful in curbing growth, conflicting with the common 18th century perspective (to which Malthus himself adhered) that a steadily