The final three categories concern Housing.

Premium Housing is directly related to the size of and growth rate of the Managerial-Professional class, as demand for such housing accompanies the growth of the class. Forrester defines Premium Housing as having a normal life-span of about 30 year--a life-span which increases of demand for Premium Housing is high and decreases if it is low.

Worker Housing is created both by Worker Housing construction and the aging of Premium Housing. Forrester gives it an approximate life-span of 50 years, subject to to variations caused by changes in demand.

Underemployed Housing is created by the obsolescence of Worker Housing, supplemented by Low-Cost Housing Programs if such exist. It is assumed to have a 50 year life-span, after which it becomes unfit for human habitation or is destroyed to make way for new constructions.

These nine variables--and their rates of growth or decline--lead, Forrester maintains, to characteristic patterns of urban development.

The growth of an urban system over a 250 year period runs as follows in Forrester's computer model. For the first 100 years, as the city expands to fill its geographical boundaries, Labor, Housing, and Industry grow exponentially. But when all available land has been developed, the introduction of New Enterprise falls--drastically--as does the construction of housing. There is simply no place to put anything new without tearing buildings down. And since most of the buildings are of fairly recent origin, even a modicum of demolition is economically unfeasible.

So what happens? The demand for existing Managers--let alone new ones--declines as New Enterprise becomes Mature Business, thereby restricting the upward mobility of skilled and semi-skilled workers. The fall-off in the introduction of New Enterprise in turn means few new jobs are created in the Labor class itself, which limits the upward mobility of those in the Underemployed class.

At this point, if other factors are ignored, the city has become about equally unattractive to an influx of Managers, Labor, or Underemployed. The relative levels of Labor and Underemployed should remain fixed at their 100-year level. But the Underemployed have a tremendous ally--time--on their side. As time passes, Premium Housing turns into Worker Housing and Worker Housing becomes Underemployed Housing. With this steady flow of deteriorated housing entering the Underemployed Housing market, the availability and-or cost of the Underemployed Housing goes down. Consequently, the city becomes more attractive to the Underemployed, relatively speaking, than to Managers or Labor. Underemployed living outside of the city notice the platitude of low-cost housing in the stagnating city, and migrate there. This migration continues until the rent and-or availability of Underemployed Housing is brought roughly into equilibrium with that of surrounding areas, or other aspects of city life become so unattractive that they keep prospective immigrants home.

Eventually, this Underemployed Housing decays to the point where it is economically sound to tear down much of it and replace it with New Enterprise. But this first step toward economic renewal--which is accompanied by the building of New Premium and Worker Housing to accommodate the newly employed managers and workers--contains the seeds of its own destruction. When the economic revival grows to the point where further substitution of New Enterprise for Underemployed Housing and Declining Industry is not economically justifiable, the deadly flow of Premium and Worker Housing to the Underemployed category resumes. Eventually, after the cycle has completed several ever-decreasing rotations, a condition of equilibrium is reached with the surrounding environment. It is an equilibrium characterized by very low levels of New Enterprise (and the upward job mobility which accompanies it for all classes) and by large numbers of the Underemployed. The city is stagnating indefinitely.

Bleak as the picture of the stagnating city is, however, it becomes worse with the introduction of public housing, revenue sharing, rent control, high welfare benefits, and other currently favored liberal programs. All these programs--by increasing the attractiveness of the city to the Underemployed-will, Forrester maintains, prove self-defeating in the end because they create overwhelming inflows of Underemployed. The essential problem is that an environment attractive to the Underemployed will draw in enough people to bring the attractiveness level down to that of surrounding areas. The city will have done little but enlarge the problems, enlarge the trap.

Forrester looks at the effects of four current or proposed "solutions" to various urban problems, over a time-span of fifty years. The first concerns a job-creation program, in which public works jobs are opened up to the underemployed and-or transportation is given them for jobs located outside the city. This program has two substantial effects in Forrester's model--raising the number of jobs relative to Workers, which increases the upward mobility of the Underemployed, and vastly swelling migration of the Underemployed to the city. Having plenty of excess Underemployed Housing around to absorb large numbers of immigrants, the city winds up with the same sort of problem in the end, but on a larger scale. In addition, because Underemployed require higher percapita tax expenditures for police and welfare, the local tax burden rises. This has the effect of further discouraging construction of New Enterprise, and depressing upward job mobility.

The second program examined is job-training. Forrester gives it the benefit of every doubt; he assumes it costs the city nothing, adds nothing to the tx rate, and proves one hundred per cent effective in turning the Underemployed into skilled workers. The program is assumed to reach 5 per cent of the underemployed each year. The positive results of this program, however--like the last--are almost entirely consumed by in-migration. Moreover, the increased number of those in the Labor class makes crowding in Worker Housing more severe, competition for jobs tighter, and the prospect of leaving the city more attractive for Labor. So, despite a small increase in upward mobility and New Enterprise, he concludes that even a perfect job-training program would be of questionable value to the city.

The third program evaluated is that of intergovernmental financial aid, or revenue sharing. Forrester assumes the new revenue goes to programs which have some positive effects on the life of the Underemployed--either through better schools, better health care, welfare, etc.--while at the same time not reducing the local tax burden. Largely because of better education, upward mobility for the Underemployed does rise. But the positive results of the program are again negated by the influx of underemployed to the city. Over a fifty year period, the ratio of Underemployed to available jobs rises by 9 per cent and the total population of Underemployed goes up eight per cent. Surprisingly, too--despite revenue sharing--the local tax ratio rises to assist in support of the newly arrived Underemployed.

The final, and most damaging program Forrester looks at is the Low-Cost Housing Program. Assumed to be funded by a larger governmental body, the computer model's housing program does not effect the tx rate for the city but does provide jobs for Labor and Underemployed through construction. The program is designed to build housing for five per cent of the Underemployed per year, but because of the counteractive effects of land shortages and labor shortages (labor rapidly leaves the city), it averages only 2.5 per cent per year. Even this rate of construction, though, proves very harmful to New Enterprise, which faces artificial competition with the low-cost housing projects over use of available land. Moreover, the massive in-migration of Underemployed places a heavier tax ratio on the city, further discouraging business activity, as well as creating new demand for low-cost housing. The tax base erodes while the need for taxes mounts, leaving the city in extremely poor shape."

So runs Forrester's analysis of conventional programs and their benefits. Must we resign ourselves to pessimism Forrester thinks not. As was clear from the examination of the previous four public programs, conventional approaches to solving urban problems are defeated by massive in-migration of the Underemployed. Forrester believes that if this in-migration can be limited, the prospects for increasing the upward mobility--and therefore the quality of life--of the Underemployed in the city can be greatly enhanced.