He achieves this limitation by lowering the supply of low-income housing. Turning over excess Underemployed Housing to industry, for demolition and construction of manufacturing plant, will permit the city, he believes to realize very significant long-term improvement in upward job-mobility.

Implementation of his program begins with tax incentives for owners of buildings to rebuild or renovate. This has the effect of withholding housing from the Underemployed Housing market, because Premium and Worker Housing deteriorate less slowly, while simultaneously prolonging the life-span of New Enterprise--which increases upward job mobility for all classes. Forrester states that the goal of this policy should be to remove 5 per cent of the Underemployed Housing per year while using the land cleared in this manner to build new industries. The new industries would raise the income levels of Underemployed neighborhoods, which would permit better housing. Meanwhile, because the excess of Underemployed Housing was gone, the city would not be flooded by a new wave of Underemployed from outside.

The steady stream of immigrants who came to take the place of those Underemployed who moved up the economic ladder, however, would have very real opportunities for advancement. Therefore, tearing down some Underemployed Housing and encouraging New Enterprise construction, Forrester believes, would prove the soundest and most beneficial program for the Underemployed.

Obviously this is not the type of policy favored by liberal intellectuals and politicians. Forrester has been called "Insensitive" to the plight of the poor--a charge that ignores the long term benefits his proposals would produce--and his model has been challenged on several grounds. Gregory Ingram, a leading critic, maintains that Forrester never explicitly defines criteria for evaluating programs. John F. Kain, professor of Government at Harvard, believes that Forrester overstates the role of housing availability in attracting immigrants. Others have charged that the model is structurally inexact because it does not include the suburbs.

Despite this, the Forrester model has converted a growing number of urban specialists. Dr. Donald F. Shaughnessy, a former Lindsay Aide and specialist in the fields of urban housing, transportation, and economic development, is one:

"From my experience, conventional methods of solving urban, problems have been tried and have failed--it's been money down the rat hole. The more we spend the worse it gets. Forrester's analysis and description of the urban system is the only one that makes sense."

Eugene Callender, black, and past president of the New York Urban Coalition--is another.

In an interview with IBM's Think Magazine last April, Callender expressed his reaction--and the reaction of many urban experts--to Forrester's analysis upon first approach. "I was hostile at first--extremely hostile," he said. "You've got to sleep on his ideas for several nights before you begin to appreciate them. You have to put aside the romantic ideals of the New Deal and the Fair Deal. Jay Forrester has done a tremendous service to the whole field of urban policy by substituting rigorous analysis for the old platitudes. It's traditional for us to shove the pieces around the board; Jay starts a new game instead. Grant him a few assumptions, and the game makes plenty of sense."

The old platitudes persist. But Forrester's fundamental contribution--that attempts to improve all aspects of urban life will be defeated by in-migration--is beginning to make their hegemony a little less certain