The concentric zone theory, developed in the 1920s by Ernest Burgess and Robert Park, proposed that social areas extend outward from the central business district in concentric zones. Population density decreases as one moves to outer zones. Zone 1 is the central business district. Zone 2 is a zone of transition with dilapidated housing and high crime. Zone 3 is a working class area. Zone 4 is middle class. Zone 5 is an upper class commuter zone. Later critics argued it does not apply to modern decentralized cities and assumes flat, unchanging landscapes.