An Immigration and Customs Enforcement officer prepares an undocumented immigrant for a deportation flight. (John Moore/Getty Images)

The Trump administration’s deportation policies and rhetoric have been controversial. The family separation policy, the zero tolerance policy, the language tagging immigrants as “animals” all appear to be trying to reduce the number of illegal immigrants, asylum seekers and unaccompanied minors who come to the United States.

Yet the practice of deporting migrants, particularly those convicted of crimes, has long been a key component of U.S. immigration policy. Between 1996 and 2015 the U.S. deported almost 5.4 million people to their countries of origin; 40 percent — approximately 2.4 million — had committed a felony criminal offense.

Although few would criticize the practice of deporting criminals, our research finds that this component of border control policy generates a vicious cycle. 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.

Violence drives people away from several Latin American nations

Why are so many people from Latin America attempting to enter the United States? Although some want to be reunified with their families or hope to find better economic opportunities, the vast majority of unauthorized migrants and asylum seekers arriving at the U.S. border are escaping from widespread violence. Many flee Central America’s so-called Northern Triangle — Honduras, El Salvador and Guatemala — which are among the most violent places on Earth, with homicide rates approaching that of the world’s most deadly war zones. A large number of unaccompanied Central American minors arriving at the U.S. border since 2014 are trying to escape either being killed or forced into a gang.

Here is how U.S. immigration policy contributes to this

Given that violence is a primary reason so many migrants leave Latin America for the United States, our research explores what factors increase the violence in these countries. One factor has received little attention: The United States’ deportation of criminal offenders spreads violence to Latin America.

Across countries and over time, violent crime has many causes. Some factors include whether countries had a history of civil wars, their levels of inequality and the strength of their political systems. After accounting for all the factors that might explain different levels of violence in a country, we still find that violence — measured as the annual number of homicides per capita — increases significantly as a country receives more convicts deported from the United States.

Our statistical results are striking: On average, when a country receives 10 convicts for each of its 100,000 residents, roughly two more people are murdered per 100,000 inhabitants.

To illustrate, consider the case of Honduras. In 2012, Honduras received an extremely high rate of deportees: 162 per 100,000 residents. Honduras experienced a very high homicide rate that year, with 93 murders per 100,000 residents. Our empirical analysis suggests the presence of these deported criminals accounts for approximately one-third of the reported homicides.

Notably, we find this effect comes only when criminals are deported. Deporting non-criminals has no statistical effect on a country’s rate of violence.

The roots of Central American gang violence lie in the U.S.

To better understand the link between deportation and violence, let’s look at El Salvador. This small Central American country has a Salvadoran-born diaspora in the United States estimated at 1.2 million, or roughly a fifth the size of its resident population of 6.3 million. Two rival gangs, the MS-13 (Mara Salvatrucha) and the 18th Street gang, have turned the country into one of the most violent places on Earth. Both gangs originated in Los Angeles, home to a large Salvadoran community.

Because Salvadorans have been involved in crime, the United States has deported criminals to El Salvador at a high rate since the mid-1990s. By 2015, the cumulative number of U.S.-deported convicts in El Salvador reached 95,000, or roughly 1.5 percent of the country’s population.

In El Salvador, homicide rates rose with the inflow of convicts. Gang-related violence is strongly linked to patterns of emigration and deportations. The areas of El Salvador that suffer the most from gangs are those whose migrants settled in U.S. cities with high crime rates, such as Los Angeles and Washington, D.C. When the United States began to deport felons back to El Salvador in the mid-1990s, those deported returned home with both criminal connections and experiences.

What is the takeaway?

Deporting convicts increased homicide rates in migrants’ countries of origin. Criminal offenders returned to violent regions with limited opportunities, where governments are already having difficulties enforcing criminal laws. It’s hardly surprising, then, that convicts return to criminal and violent activities.

Once violence increases, more people flee. Deportation policies thus don’t merely affect deportees’ countries of origin. It also creates a vicious migration cycle by pushing more people away from their homes.

Christian Ambrosius (@cuauhtemaco) is a lecturer at the Institute for Latin American Studies and the School of Business and Economics at Freie Universität Berlin and visiting professor at the National Autonomous University (UNAM) in Mexico City.

David Leblang (@realDLeblang) is Ambassador Henry J. Taylor Professor of Politics and Professor of Public Policy at the University of Virginia and a senior fellow at the Miller Center of Public Affairs.