U.S. demand for labor, combined with lack of opportunity in home countries, is the primary force driving immigration, both authorized and unauthorized. But many experts say U.S. immigration laws and policies have also contributed to the rise in the undocumented population.
First, immigration policy experts note that very few legal avenues exist for the foreign-born to enter the United States to fill current and future demand for low-skilled workers. This fact has immediate and long-term implications for the U.S. economy: Over the next ten years, low-skilled jobs will constitute two-thirds of all new jobs created.[source]
Second, heightened border enforcement has unintentionally kept many undocumented Mexican workers in the United States. In the early 1980s, about half of all undocumented Mexicans returned home within 12 months of entry, but by 2000, the rate of return migration was only 25 percent.[source]
As Princeton University researcher Douglas S. Massey points out, "The United States is now locked into a perverse cycle whereby additional border enforcement further decreases the rate of return migration, which accelerates undocumented population growth, which brings calls for harsher enforcement."[source]