The theory that a unit of housing gores through a gradual decline in quality related to its age, which makes it available to successively lower income groups until it becomes unlivable and is replaced. In this fashion, each income group would move one notch up the housing spectrum, and the housing vacated at the lowest level would ultimately be demolished. Espousers of the theory argue that subsidized housing for poorer families is unnecessary, as the market, left to its own forces, should take care of everybody. The filtering theory assumes that there is a fairly high interchangeability among housing units and there there would not be enough housing demand at each price level ready to absorb the newly vacated housing and thereby keep up the prices. However, there has been a continuous demand for nearly all grades of housing in most countries for many years, and this checks most filtering.
Market forces tend to keep supply at a level closely matching demand.
When a surplus in any incomes category appears, housing construction generally stops and so does filtration. An analytical approach to the understanding of the impact of telecommunications on urban places. Futurists generally tend to take a relatively optimistic view of the future impacts of telecommunications on cities and urban life. The proliferation of electronic spaces and networks are often seen to have very positive effects both for the physical aspects of cities and for urban life more widely. The gradual reduction of regional contrasts at the world scale, resulting from increasing international cultural, economic and political changes.