On Friday morning, President Trump declared a national emergency at the southern border, in order to access billions of dollars in funding for the construction of a wall. In doing so, Trump defied the authority of Congress, which had settled on a bill to fund the government that included limited funding for border fencing. Another major issue for congressional negotiators was the number of immigrants held in detention. While Trump has been ramping up detention numbers, including of people seeking asylum, Democrats have been pushing back. Ultimately, lawmakers agreed to fund just more than forty-five thousand beds, which would decrease the number of detained immigrants by about seventeen per cent. The debate over how many people the U.S. should detain has skirted a larger question: Why does America detain so many illegal immigrants and asylum seekers in the first place?

To discuss this question, and to get some historical perspective on U.S. relations with Mexico, I spoke by phone earlier this week with Ana Raquel Minian, an associate professor of history at Stanford and the author of the book “Undocumented Lives: The Untold Story of Mexican Migration.” During our conversation, which has been edited and condensed for clarity, we discussed what the U.S. can do to create a more stable Mexico, the factors that cause net migration rates to go up and down over time, and why the U.S. did not always find it necessary to lock up people seeking asylum.

What is it about this moment in U.S.-Mexico relations regarding immigration that you think is interesting or unique?

Since 2008, there’s actually been net negative migration. So, what we see now is a lot of anti-Mexican rhetoric, for example, when Trump ran his campaign, we heard him say that Mexicans were coming in and they were probably criminals and rapists. But of course what he did not mention was that more Mexicans are leaving the country than coming in.

Mexican migration had grown steadily and increasingly since the end of the bracero program, especially undocumented migration. That was a guest-worker program that started in 1942, in which Mexican workers could come, work legally in the United States for short periods of time, and then return to Mexico. It continued until 1964. Undocumented folks were used to coming in the bracero program, and once the program ended, and they could no longer continue to come legally to the United States, they simply did so without papers. And migration continued to grow until 2008. So, in terms of what’s unique about this historical moment, in terms of Mexican migration, it’s that the rhetoric continues to be very anti-Mexican even though migration is actually in decline from Mexico.

And I assume the main reason for the decline is something to do with the economy in the United States and in Mexico?

Yes, well, a lot of things happened in 2008. The recession harmed Mexicans greatly and many just decided to start heading home. The other thing is that Mexican migration, up until 1986, used to be circular. It was primarily men who would come. They would stay here for a while, they would make some money, and then they would return home to be with their families. And then when they needed money again they would come again. Now, in 1986, what happens is there’s a new law [the Immigration Reform and Control Act] that legalizes a lot of people, but another thing that it does is it fortifies the U.S.-Mexico border, making it harder to engage in the circular migration of coming and going.

So if workers can’t continue to come and go between the two countries, and things in Mexico haven’t changed, they simply come to the United States and settle here permanently and they bring their families. Now, many cannot bring their families, so by 2008 many have lived in the United States for many, many years without seeing their mothers, for example, and the recession hit, and many thought that it’s no longer worth it to live in the United States, and they start heading back home to be with their family members. Another thing that occurs around that period is that the drug cartels make the border region much more dangerous and some people are scared of crossing. It’s the combination of those factors that encourages people to rethink their choices and to go back to Mexico instead of migrating in such high numbers.

Another factor that’s extremely important is that, in the nineteen-seventies, Mexico started a campaign to lower population size under the theme of “The small family lives better, la familia pequeña vive mejor.” Up until the mid-nineteen-seventies, or the early nineteen-seventies, the Mexican government had encouraged population growth, but in 1973 it passed a new population law which says, What we need to do is reduce this population growth, it’s a problem. And, in fact, population growth in Mexico has declined steadily, so if you think about the repercussions of this law, there has been a decline in fertility which eventually means there’s just fewer people who want to migrate.

That brings us to today, and one of the things that your work is focussed on is the fact that the U.S. government did not always detain immigrants who cross the border illegally. When did that shift, and why did it shift?

Right, so immigration detention has a very long history. It starts in the late nineteenth century. Immigrants were generally put in detention. For example, what happened at Ellis Island. Now, by 1954, there have been changing views of immigrants, there has been a declining rate of immigration, and so the Immigration and Naturalization Service says, There are better ways to handle these people. They don’t necessarily need to be kept in detention while we decide what to do with them. And so they release them, unless they pose a threat to U.S. society, or unless they believe that they can escape, that they’re not going to return when called upon to do so. They let them go out, and now the Supreme Court says that, basically, the qualities of an enlightened civilization mean that we’re no longer detaining people.

But in 1980 Fidel Castro decides to allow all Cubans who want to leave the island to do so, and a hundred and twenty-four thousand people leave Cuba to come to the United States—they leave through the port of Mariel, so this is known as the Mariel boatlift. And once they’re on their way Castro says, You know what? Among those who are heading to the United States, we sent some criminals and homosexuals and people who had been in mental institutions. And, of course, Americans go crazy. They become very, very scared as to what’s going to happen.

When folks from Mariel arrive, many are sent to [military] bases throughout the country, where someone needs to sponsor them, but those who were believed to be criminals are sent to prison in the United States. Additionally, during these years, Haitians start coming in increasing numbers as well. And the United States is also very opposed to Haitians—you know, Haitians are seen as particularly dangerous. One of the ways in which they want to deal with Haitians is to interdict them at sea, to prevent them from even entering the country. But another means to disincline them to come is to put them in detention. So, between the folks who come in Mariel and the Haitians, then detention starts to grow all over again.