Along the dusty Mexican border in California and Texas, new chain link fences are being built to keep out unwanted immigrants. In Sacramento, the California legislature looks with suspicion at the bilingual education it once pioneered.In Los Angeles, there is renewed political pressure to deny non-emergency medical services to county patients unable to provide evidence of citizenship.

These reactions reflect a persistent and growing belief that Mexican immigrants, especially illegal indocumentados, are a drain on tax-supported services, with the high-fertility potential of turning South California into a Spanish-speaking version of Quebec. Often the Mexican immigrants are targets of contradictory complaints, as with one voiced by a newspaper letter writer who referred to them as "lazy undesirables who clog the welfare rolls and take jobs held by Americans."

The facts give no comfort to this view. Indeed, they suggest just the opposite - that such immigration is a large plus for this country, a development we should be cheering, not deploring.

At least half a dozen studies on the impact of illegal Mexican immigration provide convincing evidence that these migrants pay heavy taxes (for which they receive no benefits), make scant use of welfare and other social services, and contribute far more to the United States than they take from it. As social scientist Wayne Cornelius expressed it in a recent paper to a conference on Mexican immigration in San Diego:

"More generally, it could be argued that Mexican migrants represent something of a windfall for the United States, in the sense that they are young, highly productive workers, whose health care, education and other costs of rearing have been borne by Mexico, and whose maintenance during periods of unemployment and retirement is also normally provided by their relatives in Mexico. The significance of this windfall becomes more apparent when one considers that as of 1977 the cost of preparing a U.S.-born man or woman for integration into the U.S. labor force was about $44,000."

Cornelius, who recently moved from the Massachusetts Institute of Technology to establish a Center for Mexican-American Studies at the University of California at San Diego, has interviewed thousands of Mexican migrants both in their home country and in California and Illinois. His research has confirmed that a large majority of immigrants remain by choice permanent residents of Mexico while working seasonally in the United States.

Ironically, Cornelius says, the tightening of border controls by the Carter administration "may turn what is now a largely floating population of Mexican migrants into a population which is more permanently anchored in the United States."

Subsidizing the system

These migrants pay into the Social Security trust fund millions of dollars which they will never collect, as well as state income, sales and even property taxes for which they receive relatively few benefits. In the words of Douglas S. Massey of Princeton's Office of Population Research: "Far from ripping off the system, illegal aliens are more likely to be subsidizing it."

Culturally, Mexican migrants are reluctant to accept welfare, an attitude reinforced in the United States by fear of deportation if they apply for any social benefit.

In San Diego County, by far the largest entry point for Mexican migrants, a 1977-78 screening of welfare, Medi-Cal and food stamp recipients found only 317 illegal immigrants in a caseload of 285,000. A study by the Orange County Task Force in 1978, though weighted toward long-term illegals presumably more likely to use such services, found only 9 percent of them had received public medical care, only 2.8 percent had collected welfare payments and only 1.6 percent had received food stamps.

Despite this record, Mexican immigrants are routinely blamed by southwestern politicians and editorial writers for public hospital deficits, common here as elsewhere. Partly, this is because the taxes paid by the illegals are widely distributed while the property tax-related costs of the hospitals are concentrated in a few counties. It is also because many critics fail to recognize that illegals frequently are covered by hospitalization insurance despite their immigration status.

One study found that 44 per cent of illegal immigrants had hospital insurance paid by themselves or by their employers. Another, by Gilbert Cardenas, discovered that Mexican illegals in San Antonio receiving health care usually were persons who had married U.S. citizens and whose children had been born in San Antonio.

The Orange County Task Force estimated that illegal migrants in the county paid a minimum of $83 million in taxes annually while receiving medical services costing $2.7 million a year. A 1977 study by the Human Resources Agency of San Diego County discovered that the cost of all services for illegal migrants, including education, health care and welfare assistance, totaled $2 million a year. These same migrants contributed $48.8 million annually in taxes.

It is this sort of data which prompts Mexican sociologist Jorge A. Bustamante to suggest that the proper term for describing illegals would be "undocumented taxpayers." The jobs issue

Another complaint against illegals is that they supposedly take jobs away from American workers or at least depress the labor market because, as U.S. Secretary of Labor Ray Marshall has put it, they work "hard and scared."

Undoubtedly there are instances, as in the current United Farm Workers strike in California's Salinas Valley, where illegal immigrants are placed in direct competition with American workers. But academic studies usually have concluded that fears of economic competition from illegals are greatly overstated.

Arguing against the view of the "docile and desperate" migrant, Cornelius contends:

"The available evidence does not support this characterization, which seems more apt to describe the poor, defenseless "wetback" or bracero farm laborer of an earlier era than the more sophisticated, better-educated, urban-based illegal migrant of today. The latter breed of Mexican illegal will not accept any job in the United States. The demand for their labor is sufficiently high in most urban labor markets that they can and do change employers when they feel they are being exploited and abused."

This attitude is particularly evident in California, which is now enjoying the longest sustained economic boom in its history - a boom that coincides with heavy immigration, both legal and illegal. During this period, as Vilma Martinez, president of the Mexican American Legal Defense and Education Fund, observes, the areas absorbing the greatest numbers of migrants also have had the lowest unemployment rates.

In his most recent research in California, Cornelius found that the pay scale of an illegal worker depended less on his citizenship than on his union status.Historically, labor unions in the Southwest were slow to organize Mexican migrants, and some imposed citizenship tests for membership. Now, following the example of Cesar Chavez and the United Farm Workers, such unions as the International Ladies Garment Workers and the Teamsters no longer ask prospective members whether they are legal immigrants.

While serious wage discrimination against illegals exists in such pockets as the Rio Grande Valley in Texas, nine different studies show that urban-based immigrants from Mexico, both legal and illegal, are paid more than the U.S. minimum wage.

The economic benefit to the United States of Mexican migration is best demonstrated by California agriculture, the most prosperous, diverse and specialized in the world. A few years ago, when Chavez was beginning to organize, his opponents complained that paying higher wage scales and granting the "industrial" benefits of unemployment insurance and health insurance to farm workers would make it impossible for California agriculture to compete.

Instead, these reforms have guaranteed California farmers a stable work force which is mostly Mexican or Mexican-American and which, because of the skills involved in such work as lettuce-harvesting, is largely irreplaceable by untrained Anglos.

Even in areas where the UFW has proved unable to win converts or contracts, California growers rely on Mexican or Mexican-American workers and usually pay them union-scale wages and benefits to keep the union out. Privately, some of these same growers acknowledge that they use foreign labor, mostly from Mexico, in roughly the same numbers as they did when the bracero progam was at its peak 20 years ago.

At that time, nearly every illegal immigrant from Mexico was a farm worker. While farm work is still the largest single occupational category, more than half of the new immigrants head directly for the big cities, especially Los Angeles, where their entry into the labor market is apt to be at the bottom of the scale as dishwashers or busboys. Because of the Southern California business boom, there is a high demand even in these jobs, which means higher wages and some freedom of choice for the urban migrants. Furthermore, some 15 percent of the illegals may work in skilled or semi-skilled construction jobs, according to one study, though they are more likely to be concentrated in unskilled heavy labor and domestic service and in janitorial, laundry, food processing, garment or shoe factory jobs. Welcomed and kicked out

Some economists believe that many of the service and domestic jobs performed by illegals simply wouldn't exist if the illegals weren't in the market. And some of the other jobs might not be there, either.

"Illegal aliens typically work in menial, low-paying positions shunned by citizens, who often work in supervisory and administative positions in the same firms," writers Princeton's Massey. "If illegal aliens were unavailable, it is argued, these firms would either leave the country or go out of business, taking the supervisory and administrative positions held by American citizens with them."

Cornelius contends that the migrant is willing to take the "menial, unstable, dead-end positions" because he is economically benefiting his family "and because the absence of a long-term career ladder is not a disadvantage to the migrant who considers himself only a sojourner in the United States."

Typically, the Mexican migrant makes no distinction between "legal" and "illegal" immigration - and with good reason. Unlike immigrants from Europe and Asia, Mexicans lived in the area they are now emigrating to before the Anglo-Americans came. Geography and climate in Mexico and the U.S. Southwest are similar, access is relatively easy and the two countries are physically indistinguishable along much of the border.

The prevailing view among Mexicans seems to be that the illegals have every right to be in the United States, as indeed they did through much of U.S. history. The Border Patrol was not even established until 1924, and entry without a visa did not become a crime until 1929.

From the beginning of the western frontier, Anglo-Americans tended to regard Mexico as a vast labor reservoir that could be tapped and turned off at the asking. Mexicans have occupied the position of Charlie Chaplin's little tramp, whom the millionaire welcomed to his swank home when he was drunk and then kicked out in the morning after he had sobered up. In prosperous, labor-short periods, such as during the two world wars, Mexican workers were recruited, subsidized and lavishly praised. But during three periods of slack labor markets (1920-21, 1930-35 and 1953-54), Mexican migrants were rounded up and deported, sometimes in actions so indiscriminate that legal U.S. citizens of Mexican origin were deported with them.

Mexican sociologist Bustamente maintains that the model of industry in the Southwest on both sides of the border reflects the reality of the large and growing Mexican labor pool. "On the U.S. side, there is a real concrete need for cheap labor," he says. "On the Mexican side, there is an increasing population, unfair distribution of income and a traditional pattern of migration to the United States."

This pattern has long been recognized as a phenomenon of the borderlands, from San Diego to Brownsville. Now the border reality is becoming both regional and national.

"The border is not a place where realities end," says Bustamente. "It is a place where realities permeate. And we have to understand them in order to live together."

One precondition of that understanding is to recognize that migration from Mexico to the United States is here to stay, no matter what kind of fence is built on the border. Another would be to acknowledge that this immigration, in countless ways, is of real and lasting benefit to the United States. CAPTION: Illustration, no caption, By Robert Soule for The Washington Post