An estimated 11 million immigrants live and work in the United States illegally. Their fate is one of the big policy questions facing the country. The story of how that population grew so large is a long one that's mostly about Mexico, and full of unintended consequences.
Prior to the 1920s, the U.S. had few restrictions on immigration, save for a few notable exclusions.
"Basically, people could show up," says Jeffrey Passel, of the Pew Research Center.
For workers in Mexico, crossing into the U.S. made a lot of economic sense, then and now.