Central American migrants, who are part of a caravan trying to reach the United States, walk on a street as they continue their journey in Tapachula, Mexico, on Monday. (Reuters)

President Trump vowed Monday to reduce aid to Guatemala, Honduras and El Salvador if they couldn’t stop their citizens from fleeing their violence and poverty to travel to the United States.

The tweet came in response to ongoing reports on the migrant “caravan,” in which thousands are walking together toward the U.S. border, including people the United States has previously deported who are trying to rejoin their families. But would cutting aid and doubling down on the Trump administration’s current immigration policies deter such efforts to enter the United States? And how did immigration become the political hot button it is today? Here’s a roundup of what TMC has published in the past several years about these questions.

What policies discourage immigration — and what policies increase it?

Crafting effective policies requires, at a minimum, an understanding of what’s prompting the problem. Rachel Schwartz looked deep into the data to find out why so many children are fleeing Central America for the United States in the first place and found that it’s slightly different for each of the countries that Trump is threatening. Meanwhile, Christian Ambrosius and David Leblang explained their findings that U.S. deportation policy is boomeranging, likely encouraging more people to join the migrant caravan. As they explained:

Deportations return criminals to their home countries. In some cases, those deported criminals help develop and extend criminal networks used to traffic drugs, weapons, and people. This, in turn, increases the frequency of violent crime in those countries — which sends more people fleeing those countries and migrating to the United States.

And while it may intuitively seem as if cutting foreign aid would have similar effects, decreasing the quality of life and pushing people to leave for wealthier countries, the evidence is mixed. Charles Martin-Shields, Steffen Angenendt and Benjamin Schraven explained that development aid and humanitarian assistance can — over time — slow migration from poor countries, especially when people are leaving their homes because of a lack of food or shelter, while immediate cuts aren’t likely to make any immediate differences either way.

But Sarah Berneo and David Leblang warn that shifting foreign aid away from public health or food security in poor countries in order to help countries where many are fleeing to prevent citizens from migrating can backfire, as small immediate increases in income can sometimes encourage those potential migrants to spend that money leaving. Margaret Peters, meanwhile, explained that Trump can’t reduce trade and immigration at the same time because free trade actually discourages migration.

TMC authors have also examined other policy efforts to discourage migration. Anna Oltman explained that the family separation policy isn’t likely to be effective, as the existence of the caravan suggests, writing that “deterrence does not seem to stop migration so much as redirect it” toward more dangerous routes. Michelle Brané and Margo Schlanger suggested that the policy is meant to be punitive rather than preventive.

Nor is the Trump administration merely “enforcing the law as written,” as Inés Valdez, Mat Coleman and Amna Akbar explained last year, writing, “Laws don’t enforce themselves; people make decisions about how to enforce them.”

The U.S. politics of immigration

Since he first declared that he was a candidate for president, Trump has been suggesting that most Latin American immigrants are criminals. He has sometimes blurred the line between all undocumented immigrants and members of violent gangs like MS-13. “These aren’t people. These are animals,” Trump said at a White House roundtable on immigration after one participant mentioned MS-13 in a question.

In response to some observers’ claims that such language could lead to violence, Aliza Luft and Daniel Solomon took a close look at the research into dehumanizing language and genocides and explained that while such language doesn’t cause violence, it can desensitize the public and ease the way for dehumanizing policies.

Of course, with a worldwide migrant crisis, immigration is exacerbating political tension around the world. But Republicans and Democrats are more polarized on immigration than parties in the U.K. or Australia, Glenn Kefford and Shaun Ratcliff explained here in August.

Why? It’s particularly puzzling, given that public opinion surveys find American nativism decreasing rather than increasing. Margaret E. Peters explained that the Republicans increasingly oppose immigration in part because most businesses — which Republicans traditionally support — “no longer care about immigration. Increased globalization has changed the amount and kind of labor that most U.S. businesses need.” That frees the party to respond to its anti-immigrant wing.

And yet the Republican Congress hasn’t been able to take much action on immigration, including the popular DACA, which would protect undocumented immigrants brought here as children — because, as TMC’s Sarah Binder explained, the GOP caucus, like the nation, is divided on the issue.