The Department of Homeland Security released a report this week showing that apprehensions of undocumented immigrants at the U.S.-Mexico border are at their lowest level since 1973, leaving many observers contemplating the factors responsible for this decline. Is it the recession-plagued U.S. economy or beefed-up enforcement efforts? New data from a research team led by Wayne Cornelius, Director of the Center for Comparative Immigration Studies at the University of California, San Diego, sheds light on the decline in apprehensions and reveals the surprising, unintended consequences of border enforcement.

According to 4,012 interviews conducted with migrants and prospective migrants in Mexico, California, and Oklahoma between 2005 and 2009, the drying up of the U.S. job market, coupled with the high personal and financial costs of migration, are slowing the pace of undocumented migration from Mexico. At the same time, undocumented immigrants already here are staying put due to the high costs and physical risks of being smuggled back into the United States, fear of losing their U.S. jobs, and lack of economic opportunities in Mexico. The data reveals that, ironically, enhanced border security has created “reduced circularity in migration”—essentially locking migrants in the United States as the prospect of going home and returning later has become increasingly expensive and dangerous.

Cornelius’ research finds that undocumented migration from Mexico has diminished mainly because there are fewer jobs available in the United States.

• In 2008, only 8% of the “economically active” residents of Tunkás, Yucatán, in Mexico were thinking of migrating to the United States—down from 34% in 2006.

• In 2009, 90% of interviewed Yucatecos said it was more difficult to find a job in the United States now than it was a year ago.

While migrants may be caught on their first attempt at crossing the border, they have an almost 100% chance of eventual success.

• During 2007-2009, 46% of interviewed Yucatecos were apprehended at least once while crossing the border, but 100% eventually succeeded.

Ironically, border enforcement is more successful at keeping undocumented immigrants in the United States than in persuading them to not come in the first place.

• The most profound impact that border enforcement has had on migrants in the past decade has been “reduced circularity in migration (less return migration).”

• In the last 5 years, the average Yucateco migrant living in the United States has returned to his or her hometown only once.

• 37% of interviewed Yucatecos said “that they have relatives who have stayed in the U.S. for fear of not being able to get back in if they return to Mexico.”

Border enforcement has inadvertently increased the costs of using human smugglers (coyotes).

• According to interviewed Yucateco migrants, the cost of hiring a coyote has increased from an average of $861 before 2001 to $2,858 in 2007-2009.

• 91% of interviewed Yucateco migrants interviewed in 2009 had hired a coyote to assist their most recent crossing into the United States.