Contemporary Mexican and, more broadly, Latin American immigration is without precedent in U.S. history. The experience and lessons of past immigration have little relevance to understanding its dynamics and consequences. Mexican immigration differs from past immigration and most other contemporary immigration due to a combination of six factors: contiguity, scale, illegality, regional concentration, persistence, and historical presence.

Contiguity | Americans' idea of immigration is often symbolized by the Statue of Liberty, Ellis Island, and, more recently perhaps, New York's John F. Kennedy Airport. In other words, immigrants arrive in the United States after crossing several thousand miles of ocean. U.S. attitudes toward immigrants and U.S. immigration policies are shaped by such images. These assumptions and policies, however, have little or no relevance for Mexican immigration. The United States is now confronted by a massive influx of people from a poor, contiguous country with more than one third the population of the United States. They come across a 2,000-mile border historically marked simply by a line in the ground and a shallow river.

This situation is unique for the United States and the world. No other First World country has such an extensive land frontier with a Third World country. The significance of the long Mexican-U.S. border is enhanced by the economic differences between the two countries. “The income gap between the United States and Mexico,” Stanford University historian David Kennedy has pointed out, “is the largest between any two contiguous countries in the world.” Contiguity enables Mexican immigrants to remain in intimate contact with their families, friends, and home localities in Mexico as no other immigrants have been able to do.

Scale | The causes of Mexican, as well as other, immigration are found in the demographic, economic, and political dynamics of the sending country and the economic, political, and social attractions of the United States. Contiguity, however, obviously encourages immigration. Mexican immigration increased steadily after 1965. About 640,000 Mexicans legally migrated to the United States in the 1970s; 1,656,000 in the 1980s; and 2,249,000 in the 1990s. In those three decades, Mexicans accounted for 14 percent, 23 percent, and 25 percent of total legal immigration. These percentages do not equal the rates of immigrants who came from Ireland between 1820 and 1860, or from Germany in the 1850s and 1860s. Yet they are high compared to the highly dispersed sources of immigrants before World War I, and compared to other contemporary immigrants. To them one must also add the huge numbers of Mexicans who each year enter the United States illegally. Since the 1960s, the numbers of foreign-born people in the United States have expanded immensely, with Asians and Latin Americans replacing Europeans and Canadians, and diversity of source dramatically giving way to the dominance of one source: Mexico.

Mexican immigrants constituted 27.6 percent of the total foreign-born U.S. population in 2000. The next largest contingents, Chinese and Filipinos, amounted to only 4.9 percent and 4.3 percent of the foreign-born population.

In the 1990s, Mexicans composed more than half of the new Latin American immigrants to the United States and, by 2000, Hispanics totaled about one half of all migrants entering the continental United States. Hispanics composed 12 percent of the total U.S. population in 2000. This group increased by almost 10 percent from 2000 to 2002 and has now become larger than blacks. It is estimated Hispanics may constitute up to 25 percent of the U.S. population by 2050. These changes are driven not just by immigration but also by fertility. In 2002, fertility rates in the United States were estimated at 1.8 for non-Hispanic whites, 2.1 for blacks, and 3.0 for Hispanics. “This is the characteristic shape of developing countries,” The Economist commented in 2002. “As the bulge of Latinos enters peak child-bearing age in a decade or two, the Latino share of America's population will soar.”

In the mid-19th century, English speakers from the British Isles dominated immigration into the United States. The pre-World War I immigration was highly diversified linguistically, including many speakers of Italian, Polish, Russian, Yiddish, English, German, Swedish, and other languages. But now, for the first time in U.S. history, half of those entering the United States speak a single non-English language.

Illegality | Illegal entry into the United States is overwhelmingly a post-1965 and Mexican phenomenon. For almost a century after the adoption of the U.S. Constitution, no national laws restricted or prohibited immigration, and only a few states imposed modest limits. During the following 90 years, illegal immigration was minimal and easily controlled. The 1965 immigration law, the increased availability of transportation, and the intensified forces promoting Mexican emigration drastically changed this situation. Apprehensions by the U.S. Border Patrol rose from 1.6 million in the 1960s to 8.3 million in the 1970s, 11.9 million in the 1980s, and 14.7 million in the 1990s. Estimates of the Mexicans who successfully enter illegally each year range from 105,000 (according to a binational Mexican-American commission) to 350,000 during the 1990s (according to the U.S. Immigration and Naturalization Service).