Civilization is the advanced social development that is believed to owe its origin to human interaction in cities. But cities do not always have a civilizing effect. In history, the size of a population has corresponded to the availability of resources, particularly food and water. Ever since the Age of Agriculture, the stability of food supplies and the relative security of group settlements have decreased mortality rate and increased fertility. The consequent increased demand leads to exploitation of resources, which invariably exceeds the accessible supply.