The largest wave of immigration in history from a single country to the United States has come to a standstill. After four decades that brought 12 million current immigrants—more than half of whom came illegally—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.

The standstill appears to be the result of many factors, including 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 changing economic conditions in Mexico.