Memo to Donald Trump: Border fences aren't effective at keeping people out. But they do keep unauthorized immigrants who are already here from leaving. And that's one big reason there are 11 million unauthorized immigrants, most of whom have been here for a decade or longer, settled in the United States today.
The US may not have tried a literal wall at the US/Mexico border before, but it's been stepping up the presence of Border Patrol agents — a human wall — for 20 years:
Princeton sociologist Doug Massey, America's leading scholar on immigration policy, explains in Foreign Policy that this didn't exactly have the intended effect. Before the border buildup, unauthorized immigrants — mostly adult men — had come to the US for months at a time to work, then returned to their home countries (they were predominantly Mexican) to see their families. Once it became riskier to cross the US/Mexico border, however, they stopped taking the risk on a regular basis — instead crossing once and staying in the United States, often moving relatively far from the border.
And, as Massey explains, as they settled down, they turned the unauthorized population from a group of migrant workers into a group of settled families:
In addition, as male migrants spent more time north of the border, they were increasingly joined by their wives and children. And then they started making babies. At present, almost 80 percent of the 5.1 million children of unauthorized immigrants were born in the United States and are U.S. citizens. In the end, the militarization of the border transformed what had been a circular flow of workers going overwhelmingly to just three states — California, Texas, and Illinois — into a much larger settled population of families living across all 50 U.S. states — not a good outcome for a policy whose goal was the limitation and control of immigration.
Right now the unauthorized immigrant population is rooted in the US — a majority of unauthorized immigrants have lived here for more than a decade. That's part of what makes the debate over what to do with them so difficult, and why it's impossible (though Donald Trump might not understand this) to deport all unauthorized immigrants without either splitting up millions of families or expelling millions of US-born children. And in large part, it's a direct consequence of policymakers thinking that the answer to unauthorized immigration was to build a (physical or human) border wall.