This explains how foreign-born workers compare to native-born Americans, from a survey at the Bureau of Labor Statistics:
Labor Force Characteristics of Foreign-Born Workers Summary
* In terms of educational attainment, 28 percent of the foreign-born labor force 25 years old and over had not completed high school in 2005, compared with about 7 percent of the native-born labor force. About equal proportions of both the foreign and native born had a bachelor’s degree and higher (31 and 33 percent, respectively). (See table 1.)
* On a regional basis, the foreign-born share of the labor force ranged from about a quarter in the West to 7 percent in the Midwest.
(The foreign born include legally-admitted immigrants, refugees, temporary residents such as students and temporary workers, and undocumented immigrants.)
* About 49 percent of the foreign-born labor force was Hispanic, and 22 percent was Asian, compared with about 7 and 1 percent, respectively, of the native-born labor force.
* 28 percent of the foreign-born labor force 25 years old and over had not completed high school in 2005,compared with about 7 percent of the native-born labor force. About equal proportions of both the foreign and native born had a bachelor’s degree and higher (31 and 33 percent, respectively).
* In 2005, foreign-born workers made up about 15 percent of the U.S. civilian labor force, little different from a year earlier. About 22 million, or 67.7 percent, of the foreign born were in the labor force in 2005, about the same proportion as in 2004.
* Since 2000, the foreign born have accounted for 46 percent of the net gain in the total labor force.
* In 2005, the median usual weekly earnings of foreign-born fulltime wage and salary workers were $511, compared with $677 for the native born.
* As with the native born, the earnings of foreign-born workers increased with education.
I am not certain if these numbers include the work done in the undergound economy, and I doubt it, but regardless these are interesting statistcs for reference.