Historical demographers have generally agreed on two interrelated features of the urban populations in early--modern Germany. First in accordance with the law of natural decrease, extremely high mortality rates in cities (urban areas with more than 10,000 inhabitants) meant that their populations could not be sustained by reproduction alone. Population density, inadequate sanitary conditions, and poor housing made cities too vulnerable to disease and death. Second, when city populations did increase, it was as a result of in-migration sufficient to overcome the population losses caused by the high mortality. But because the parish registers of urban communities with fewer than 10,000 inhabitants show that the annual number of births usually equaled or exceeded that of deaths, historical demographers assume that the law of natural decrease did not apply to small towns.