Malthusianism is a school of ideas derived from the political/economic thought of the Reverend Thomas Robert Malthus, as laid out in his 1798 writings, An Essay on the Principle of Population, which describes how unchecked population growth is exponential while the growth of the food supply was expected to be arithmetical. Malthus believed there were two types of "checks" that could then reduce the population, returning it to a more sustainable level. He believed there were "preventive checks" such as moral restraints (abstinence, delayed marriage until finances become balanced), and restricting marriage against persons suffering poverty and/or defects. Malthus believed in "positive checks", which lead to 'premature' death: disease, starvation, war, resulting in what is called a Malthusian catastrophe. The catastrophe would return population to a lower, more "sustainable", level. The term has been applied in different ways over the last two hundred years, and has been linked to a variety of other political and social movements, but almost always refers to advocates of population control.