The number of illegal immigrants living in the U.S. has dropped for the first time in two decades -- decreasing by 8 percent since 2007, a new Pew Hispanic Center study finds. The reasons range from the sour economy to Mexican violence and increased U.S. enforcement that has made it harder to sneak across the border. Much of the decline comes from a sharp drop-off in illegal immigrants attempting to cross the southern border of the U.S.
