Los Angeles — Around Eastern cities such as Washington and Boston, ''help wanted'' signs are commonplace in store windows. The problem is the scarcity of people willing to work for low hourly wages.
Southern California faces few such problems - and is not likely to for the rest of the century.
The Los Angeles area - the favorite postwar destination of footloose Americans - is the scene of what demographers are calling the ''fourth wave'' of immigration to the United States. It is drastically altering the ethnic makeup of the southern California work force and contributing much-needed workers to the lower rungs of the economic ladder.
The first two waves of immigration in American history were transatlantic, bringing whites from Europe. The third wave was the postwar migration of rural American blacks to Northern cities. The current wave, which began in the 1960s and has grown larger since 1975, consists chiefly of Mexicans and, to a growing extent, Asians.
By the year 2000, non-Hispanic whites are no longer likely to hold majority status in southern California.
Two recent studies - one by the Southern California Assocation of Governments (SCAG) and the other by the Urban Institute in Washington, D.C. - assess the impact of these shifts on the California economy.
Without new immigrants, says the Urban Institute report, California's natural population growth and newcomers from elsewhere in the country would fill only half the new jobs produced in the state by 1990.
But there will be new immigrants. Thomas Muller, principal research associate of the Urban Institute and author of the report, notes that during the last 15 years, Los Angeles County has taken in more immigrants in proportion to the county's population than any major American city in this century. And the pace of immigration has accelerated into the first half of the 1980s.
Mexicans account for about one-third of California's immigrants. The vast majority of them have arrived since 1970, and about half work as factory production workers.
Almost 60 percent of all recent immigrants in the state, Dr. Muller estimates from census data and other indicators, are undocumented, therefore illegal. The share of illegal Asians is also likely to increase, he adds, because of over-educated people in countries like India and the Philippines who face a lack of economic opportunity in their relatively undeveloped homelands. One result is that white may no longer predominate in the southern California ethnic rainbow.
SCAG, which is a research and planning agency for the six southern California counties, estimates that by the end of the century, non-Hispanic whites will make up 42 to 52 percent of the population. At the lower end of this estimate, non-Hispanic whites would roughly equal the Hispanic population.
Waning birthrates and an aging population have led demographers to predict a labor shortage around the turn of the century, especially for entry-level jobs. But the southern California labor force, according to the SCAG report, will actually grow 5 percent more by the year 2000 than the number of new jobs.
Much of this growth will come from new immigrants who are more than willing to enter the US economy at entry-level wages.
These immigrants have added to the prosperity of most Californians. Mexicans, especially, have held wage levels down in low-wage, low-skill jobs. According to Dr. Muller, this has meant that manufacturers have been able to expand in spite of growing competition from imports, and other businesses have enjoyed higher profits while prices stayed lower for consumers.
The only people who may suffer economically from immigration are those who compete for those low-wage, low-skill jobs, says Muller. Neither study deals with that situation.