The missing link is another factor that can be explained by particular, necessary demand forces. In U.S. cities in the time period from 1970 to 1978, growth of the central business district at around 20% did not dictate conditions for gentrification, while growth at or above 33% yielded appreciably larger gentrification activity. Succinctly, central business district growth will activate gentrification in the presence of a surplus in the inner city housing market. The 1970s brought the more "widespread" second wave of gentrification, and was sometimes linked to the development of artist communities like SoHo in New York.