In the past four decades, 12 million Mexican immigrants came to the U.S., just over half of whom were unauthorized.  Recently, though, this flow of immigration, which represented the largest flow between any two countries in the world, has reversed.  Whereas from 1995 to 2000 the net flow of immigration into the United States via Mexico was positive, from 2005 to 2010, that flow reversed.  The Pew Research Center released a study investigating immigration between the bordering nations, and pointed to “the weakened U.S. job and housing construction markets, heightened border enforcement, a rise in deportations, the growing dangers associated with illegal border crossings, the long-term decline in Mexico’s birth rates and broader economic conditions in Mexico.”  In 2011, about 6.1 million unauthorized Mexican immigrants lived in the U.S., down from a peak of almost 7 million in 2007.  Over the same time period, the number of authorized Mexican immigrants rose about 200,000.