Assuming current levels of immigration, a little more than half the growth in the U.S. population -- and the workforce -- between 1995 and 2025 will come from new immigrants and their children, say economists at the Federal Reserve Bank of Dallas. However, sizing up immigration's overall impact on the economy is not a straightforward process.

Partly that is because of the diversity of immigrants:

• Immigrants to the United States disproportionately include both some very low-skilled occupations -- waiters, housekeepers, agricultural and textile workers -- and some very high-skilled occupations -- physicians, chemists, engineers and physics professors.

• Also, immigrants are represented in occupations that require little education but much skill, such as tailors, dressmakers and jewelers.

• Relative to natives, more immigrants have less than a high school education and more have college degrees.

A National Research Council (NRC) study found immigration in the 1980s increased the supply of workers with less than a high school education by 15 percent. This competition reduced average wages of workers in this group by about 5 percent. However, less than 10 percent of U.S. workers are in this category.

Interestingly, those who face the greatest wage losses are prior waves of immigrants, because newly arrived immigrants are their close substitutes. Thus a 10 percent increase in the supply of immigrants reduces the immigrant wage by at least 2 percent to 4 percent.

The net economic effects of immigration also depend on the fiscal impact on local, state and federal governments -- which depends on the composition of new immigrant populations. For instance, welfare use among immigrants is mostly concentrated among refugees and the elderly; but working-age nonrefugee immigrants are less likely than U.S. natives to receive welfare.