Eight percent of all babies born in the U.S. in 2008 belonged to illegal immigrant parents, according to a groundbreaking analysis of U.S. Census Bureau data by the Pew Hispanic Center. Under the 14th amendment to the Constitution, each child obtained U.S. citizenship at birth while one or both of the parents remained undocumented.

[Study: 8 Percent of U.S. Births to Illegal Immigrants, Devin Dwyer, ABC News, August 11, 2010]

There are real psychological and economic reasons for this phenomenon. Women tend to have children when they are partnered with men who are doing better than the women's fathers did economically. That is pretty easy for a Mexican man in the U.S. to accomplish if his partner is a Mexican woman. But men's wages in the US have been stagnant since the 1960's expansion of immigration—which means it is harder for American men to make a similar impression on American women.