(May 2002) The United States and Mexico share a border more than 3,000 miles long, and Mexico is the single largest source of both legal and undocumented immigration into the United States. Data from the Census 2000 Supplementary Survey indicate that about 8.8 million people living in the United States in 2000 were born in Mexico. This represents just under 30 percent of the total U.S. foreign-born population. There are three states in which Mexican Americans make up the majority of the foreign-born population: Arizona, New Mexico, and Texas. All three states share a border with Mexico and are therefore easier to reach than destinations farther north or east.
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But in absolute numbers, California has the largest Mexican-born population at 3.7 million people. In fact, the Mexican-born population in California accounts for almost half of the entire U.S. Mexican-born population.
The U.S. population born in Mexico is a subset of a larger group of people who identified with the "Mexican American or Chicano" ethnic group in the 2000 Census. In 2000, over 20 million individuals, including many first- and second-generation U.S. residents, identified themselves as Mexican American.
U.S. Census Bureau: Census 2000 Supplementary Survey