BEFORE WORLD WAR II, dreams of economic improvement were nurtured more by movement to the urban industrial economy than by upward movement within it.

Between 1900 and 1930 nearly 20 million immigrants arrived in the United States, while the percentage of the population living in urban places rose from 40 percent in 1900 to 56 percent in 1930.

Within the urban economy, however, progress was modest and life often demanding. Between 1901 and 1934-6, the real material living standards of urban working families increased by an average of only 1 percent a year. As millions of immigrants poured into semi-skilled factory jobs, the wage gap between skilled and unskilled workers widened. Factory working conditions were arduous and hazardous. But the hopes fueled by geographic mobility provided reservoirs of patience. "The work is very heavy, but I don't mind it," an East European brick-factory worker wrote his wife just after the turn of the century. "Let it be heavy, but let it last without interruption."

Patience wore thin during the Great Depression as unemployment climbed above one-fifth of the labor force. Strikes, sit-ins, protest marches -- all signaled the threat of political rebellion. "This outburst . . . was almost spontaneous and spread like the plague," one state governor wrote about a textile strike. "It only confirms my general feeling that the spirit of revolt is widespread."

But the spirit ebbed as the nation entered the affluent postwar years. Upward mobility was one of the most important factors in restoring political stability in the postwar years. The American Dream was sustained not so much by immigration (which slowed dramatically after the war) or by movement off the farms (which began to ebb during the 1960s), but by increasingly rapid and palpable economic progress for those already working in the nonfarm wage labor force. Real hourly compensation for nonfarm private employes increased by 4 percent a year from the late 1940s through the mid-'60s. Median family income, adjusted for inflation, increased by slightly more than 4 percent annually over the same period.

Between 1950 and 1970, economists Frank S. Levy and Richard C. Michel have estimated, the average man of 30 could look forward to an increase in his real annual income over the 10 years between ages 30 and 40 -- the period of prime career advancement -- of substantially more than 50 percent.

Equally important, families had every reason to expect that their children's future prospects would be at least as bright. New white-collar occupations were opening up; the university system was mushrooming. No longer as likely to wish a career in self-employment for their children, parents turned increasingly to the promise of stable and remunerative professional work as the next inter-generational step up the ladder to status and economic security. "The American Dream for my father is to see his kids get a college education, something he never had," a student of lower-middle-class origins commented in a study on mobility in the late 1960s.

Nearly everyone's boat rose with the tide. Inequality of earnings did not widen substantially and wealth inequalities actually narrowed. Both women and minorities were able to improve their earnings substantially, although the gap between minority workers and white men did not narrow.

All that is now history.