Based on the work of political economist Thomas Malthus, Malthusian Theory was originally referencing the possibility to be a forced return to subsistence-level conditions once population growth had outpaced agricultural production. Malthus believed that unless something was done about immigrants and overpopulation of the poor, the nations of 19th century Europe could fall to a radical coup by the starving, teeming masses. Later formulations consider economic growth limits as well. The term is also commonly used in discussions of oil depletion.