Why were cities once unable to sustain their populations without newcomers?

Until the mid-19th century, cities were considered demographic sinkholes, meaning they were incapable of sustaining their own populations through natural increase. This inability to maintain population levels was primarily driven by two factors:1

• High Mortality Rates: Cities served as centers for the intense spread of infection. Because human viral diseases often require high population densities to propagate—levels far exceeding those found in sedentary agricultural or nomadic societies—urban environments became breeding grounds for epidemics.1
• Resource Instability: Cities relied on the importation of food and raw materials from surrounding hinterlands. Disruptions in transportation or political stability frequently led to erratic patterns of scarcity, famine, and disease, which further elevated death rates.1

Consequently, urban growth during this period was heavily dependent on a constant influx of newcomers from rural areas to offset these high mortality rates. It was only with later advancements in public health, sanitation, and disease control that cities began to overcome these systemic demographic challenges.12