Still yet another theory, as introduced by Robert Brenner in a 1976 paper, is that the economic system of the High Middle Ages limited population growth. Feudal lords and landlords controlled most of Europe's land; they could charge high enough rents or demand a large enough percentage of peasants' profit that peasants on these lands were forced to survive at subsistence levels. With any surplus of food, labor, and income absorbed by the landowners, the peasants did not have enough capital to invest in their farms or enough incentive to increase the productivity of their land.