For years, economists and politicians have said the solution to surging emigration is prosperity at home. If Mexico and other Latin American nations that send millions of migrants to the U.S. could grow fast enough, the theory goes, their residents wouldn't head north for work. Last month, Mexican President Vicente Fox, looking to influence the U.S. Congress's latest efforts at an immigration overhaul, pledged his country would do its part, creating good manufacturing jobs for Mexican workers on their home soil. He pointed to more than 100,000 job openings in assembly plants established within a few miles of the U.S....