Overurbanization is a thesis originally developed by scholars of demography, geography, ecology, economics, political science, and sociology in thrergence of International Nongovernmental Organizations Amid Declining States. The term is intentionally comparative and has been used to differentiate between developed and developing countries. Several causes have been suggested, but the most common is rural-push and urban-pull factors in addition to population growth. The concept of overurbanization first emerged in the mid-20th century to describe cities whose rate of industrialization was growing more slowly than their rate of urbanization. According to sociologist Josef Gugler, the concept was "widely accepted in the 1950s and into the 1960s" and was split into two approaches, a diachronic and a synchronic approach. The synchronic approach, the main one taken in the 1950s, was proposed by sociologists Kingsley Davis and Hilda Golden who defined whether a country was overurbanized based on how its relationship between industrialization and urbanization compared to that of other countries during the same time period. Specifically, countries considered part of the Third World were compared to countries deemed part of the First World. Davis and Golden used data on "the percentage of economically active males not engaged in agriculture and the percentage of the population in cities of 100,000 or above in a large number of the countries in the world," in order to define the normal relationship between industrialization and urbanization. They then determine that countries whose rate of urbanization is significantly higher than normal in relation to their rate of industrialization are "overurbanized." The authors calculate an "expected" level of urbanization based on the rates of urbanization of other countries of the world at similar levels of industrialization (measured by percentage of males not engaged in agriculture). A few countries in particular that Davis and Golden measured as having higher levels of urbanization than expected were Egypt, Greece, and South Korea.