SSuspicion: Programs supposed to be helping the cities are destroying them.

Jay W. Forrester--Professor of Management at MIT and one of the nation's pre-eminent systems analysts--stunned the urban planning world with this thesis two years ago with the publication of a book called Urban Dynamics.

Public housing, revenue sharing, job-training, and job creation programs all prove ultimately self defeating, Forrester maintains. He says--and presents impressive supporting arguments--that they turn cities into traps for the underemployed.

Why? Forrester believes that the iron rule of a social system is that you can't improve a given aspect of it without making other aspects worse. Programs which try to make life in the cities more attractive wind up defeating themselves because they draw immigrants in until the city's overall attractiveness level is driven down to that of surrounding areas.

Behind this simple concept is a highly sophisticated computer model of the city synthesized by Forrester and put forward in Urban Dynamics. The model's behavior led him to believe--and has convinced many others--that policies which ease the short run plight of the underemployed almost invariably work out to the long-run detriment of all concerned.

Three years ago, Forrester began to put the model together. Collaborating with former Boston Mayor John Collins, who had an office next to his at MIT, Forrester designed a mathematical representation of a 'typical' city--one which had all the common characteristics of American cities but none of the differences. The typical city, he found, should be reduced to nine major components.

The first three components cover categories of business and industry. Forrester calls them New Enterprise, Mature Business, and Declining Industry.

New Enterprise--newly built industry--he defines as having an average life-span of twelve years before becoming Mature Business. Being young, companies in the New Enterprise stage tend to have high ratios of managers to workers.

Mature Business is simply the older and more stable condition that New Enterprise eventually reaches. Because its physical plant is less modern than that of Mew Enterprise, and generally less profitable, it tends to have a lower proportion of Managers per Workers in each firm. Mature Business is assumed to have a life-span of approximately twenty years before entering the Declining Industry phase.

Declining Industry represents the terminal stage of a commercial establishment. It is characterized by an extremely low ration of Managers to Labor and is assumed to have a life-span of 30 years before being abandoned or destroyed.

The life-span in all three classes varies as a function of the cities' economic activity at a given time--in periods of business expansion, the life-spans are shortened as firms rapidly become obsolescent: in periods of business contraction, the life-spans are lengthened. Forrester is careful to include other qualifications of this sort in the computer model.

The next three categories Forrester identifies refer to the employment characteristics of the city's inhabitants.

The first class, the Managerial-Professional, consists of the white-collar workers. If the size of the Managerial-Professional class is large, or growing rapidly, then the urban system has provided or is providing a high degree of upward mobility to those in the next class down, Labor. Large numbers of managers are created by the growing number of commercial enterprises during the city's period of economic stagnation, however, the Managerial class declines in size as fewer new firms come into the city and what previously had been New Enterprise becomes Mature Business. And if the managerial class is declining for want of jobs, upward mobility in the lower classes suffers greatly.

The second class, Labor, refers to the number of skilled and semi-skilled workers in a city. As the number of jobs available to the labor class rise, as it does in periods of rapid economic growth, arrivals of Underemployed workers into the Labor class also rise.

The third class of inhabitants--the Underemployed--refers to the unemployed, the underemployed, and part-time workers. It is composed primarily of unskilled workers who have migrated to the city in search, most often, of jobs: it does not include skilled workers who are temporarily out of work.