After four decades that brought 12 million current immigrants--more than half of whom entered the country without papers--the net migration flow from Mexico to the United States has stopped--and may have reversed, according to a new analysis by the Pew Hispanic Center of multiple government data sets from both countries. "We don't know whether the wave will resume, but we do know that the current standstill is more than just a temporary pause," said Paul Taylor, director of the Pew Hispanic Center. "Net migration from Mexico has been at zero----and perhaps less----since 2007." "Heightened enforcement of immigration laws has made it more difficult, expensive and dangerous for Mexicans to try to enter the U.S. illegally," said Jeffrey Passel, senior demographer at the Pew Hispanic Center. "In addition, the sluggish U.S. economy has weakened the jobs magnet, and changing demographics in Mexico have reduced the pool of potential migrants."