Read: How will detained children find their parents?

This zero-tolerance policy sparked outrage across the country. But the message the administration was trying to send to immigrants was clear: Come at your own risk. In unveiling the policy in April, Attorney General Jeff Sessions said: “To those who wish to challenge the Trump administration’s commitment to public safety, national security, and the rule of law, I warn you: Illegally entering this country will not be rewarded, but will instead be met with the full prosecutorial powers of the Department of Justice.” In an interview with NPR shortly after, John Kelly, the White House chief of staff, cited deterrence as a reason for the policy. “They’re coming here for a reason. And I sympathize with the reason. But the laws are the laws. But a big name of the game is deterrence,” he said.

On Sunday, Trump himself made the case for deterrence in a 60 Minutes interview. “When you don’t do separation, when you allow the parents [and children] to stay together, okay, when you allow that, then what happens is people are gonna pour into our country,” he said.

It isn’t the first time the U.S. has relied on a deterrence policy. Former President Barack Obama’s administration tried to prevent people from crossing the border through a number of tactics short of separating parents from their children, like family detention and running ads in Central America discouraging people from making the dangerous journey. But as evidence has shown again and again, these policies don’t always succeed, especially when people are fleeing dangerous conditions in their homeland.

Simply stated, deterrence policies “don’t work,” Rebecca Hamlin, an associate professor of legal studies at the University of Massachusetts at Amherst, told me in 2016, following new initiatives by the Obama administration to address Central American migrants. “They might work when someone’s only motivation to migrate is economic, but they really don’t work—and this is consistently found all over the world—when it comes to people who are fleeing what they believe to be potentially a life-or-death situation.”

Read: Obama’s last attempt at immigration reform

The Center for American Progress, a left-leaning think tank, found in analyzing federal data that both family detention, as used during the Obama administration, and family separation, under Trump, are “ineffective deterrents.”

“We’ve certainly seen more than enough evidence of what conditions are like in places like Honduras and El Salvador that are pushing people experiencing high levels of gender-based violence or gang violence out of the countries,” said Philip Wolgin, the managing director for immigration policy at the Center for American Progress. “The analogy I’ve heard most often is: If the choice is sudden death or imminent certain death in your country, or going somewhere else with the possibility you might be detained or separated, you take the lesser of the two evils.”