Urbanisation is associated with economic development in complex ways. For urbanisation goes hand-in-hand with other demographic transformations, notably lower fertility, longer life expectancy and population ageing, and greater geographic mobility. Migration to urban areas continues everywhere and, in many cities, now plays a more important role in population development than birth and death rates. In the developing world, urbanisation has accelerated dramatically since the 1950s. Urbanisation in the modern sense of the world began in Europe, and until the middle of the twentieth century, most of the world's urban centres were located in the northern hemisphere. As a field of study, urban sociolinguistics cannot but put multilingualism in all its shapes and appearances centre stage. In one of the few books on sociolinguistic theory building, Chambers dismisses the status of co-existent languages and the choice of languages as "codes" in multilingual societies as "largely sociological in importance" and, hence, outside the core area of sociolinguistic theory.