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 […]