Thomas Robert Malthus, an 18th-century British cleric and scholar, believed that human population growth would always outstrip humanity's ability to feed itself. His ideas, referred to as Malthusianism, have heavily influenced the fields of biology and environmentalism.
Malthus' theories, introduced in his work "An Essay on the Principle of Population," stated that when humans had more resources, their normal response was to have more children, which led to a Malthusian catastrophe: famine or disease leading to a sudden die-off of humans to return the population to a sustainable level. Malthus believed this was divine providence at work; by ensuring humanity could not become unquestioningly prosperous, God ensured that humans would follow moral and ethical principles, such as sexual abstinence and moderation. Malthus used these ideas largely to criticize British laws established to provide a minimal subsistence to prevent the poor from starving.
Malthusianism has been hotly debated since it was introduced, with scholars pointing out that much of the theory is based on mathematical extrapolation rather than actual observation of humans. In addition, 20th-century trends have shown that prosperous countries in which modern birth control is in common use show no signs of Malthusian population explosions. Nevertheless, Malthus' ideas have been used in works such as Paul Erlich's 1970s ideas on ecological collapse.