Collecting—and drawing precise conclusions from—border crossing statistics is notoriously difficult. But there has been a decline in apprehensions of undocumented immigrants at the southern border in the years since America's border security buildup. Advocates of stricter security point to that statistic as evidence that border controls work. And last year, the number of deaths at the southern border was estimated by the U.S. government at 307, a 15-year record low.
Still, no barrier will ever be high enough or secure enough to completely halt attempts to cross the border illegally. And the more obstacles that stand in the way, the more perilous it will be to cross.
"A wall can slow someone down. It can compel them to change the route they take. But when people want to cross, whatever the motivation is, they will find a way to cross," said Marc Pierini, the former European Union ambassador to Turkey and a visiting scholar at Carnegie Europe.
Available statistics also point to a rise in smuggling as enforcement along the border has tightened, a trend that might intensify if the U.S. builds a permanent border wall.
Data from the Mexican Migration Project, a yearly survey of Mexicans on both sides of the border carried out by Princeton University and the University of Guadalajara, shows a sharp uptick in the share of undocumented immigrants from Mexico that made use of smugglers over the period from 1975 to 2006.
"Many workers from developing regions are willing to … risk their lives to access opportunities in the most affluent countries. Since they cannot do this legally, they often employ organized criminals to assist them, and become more likely to do so as immigration controls tighten," a 2010 report from the United Nations Office on Drugs and Crime concluded.
A report from the Homeland Security Department's Office of Immigration Statistics policy directorate in 2010 suggests that the price that illegal immigrants are willing to pay smugglers has also risen as border enforcement intensified.
The amount of money smugglers can now charge has increased to roughly $3,000 to $6,000, according to 2015 data from the Mexican Migration Field Research and Training Program at the University of California at San Diego.
"The harder it is to get across the border, the more people are willing to pay. Those costs have risen almost in lockstep with the amount of money that the United States spends to secure the border," said John Skrentny, director of the Center for Comparative Immigration Studies at UCSD.
Immigration experts also warn that the more lucrative illegal smuggling becomes, the more incentive there is for organized criminal networks to get involved.
"The conventional wisdom is that smuggling networks have become more professionalized, moving from mom-and-pop organizations to more serious criminal enterprises that have increasingly become entangled with drug smugglers," said Marc Rosenblum, the deputy director of the U.S. Immigration Policy Program at the nonpartisan Migration Policy Institute.