The issue of how the U.S. should control migration on the border with Mexico is politically sensitive and emotionally charged. It's formed a major focus for the Obama administration and a key question for the Republican candidates. But the latest data from both sides of the border suggest that, in fact, Mexicans are no longer migrating to the U.S. in the way they once were. This year's net migration will be close to zero. They are either not crossing the border, or if they do, many end up returning to Mexico. Dr. Juan Carlos Calleros, a researcher at the Center of Immigration Studies in Mexico, discusses what the numbers mean.  Gustavo Arellano, managing editor with OC Weekly and author of the syndicated column Ask a Mexican, thinks tougher border crossing controls are the real reason for the migration decrease.