Over the course of President Donald Trump’s first year in office, his administration’s top immigration priority has shifted subtly. He’s talking less about deporting “bad hombres” and talking more — a lot more — about how “chain migration” is bad for the United States.

“Ending chain migration” has become one of the four pillars that the White House demands be part of any bill to protect the 690,000 young unauthorized immigrants affected by Trump’s decision to end the Deferred Action for Childhood Arrivals (DACA) program. The White House’s own proposal suggests doing this by preventing all US citizens from sponsoring parents, siblings or adult children to settle in the US — a radical change from current law.

This system of Chain Migration de-skills the labor force, puts downward pressure on wages, increases the deficit, and undermines national security. More: https://t.co/oNvVX4FXb1 pic.twitter.com/A3VJN27Eza — The White House (@WhiteHouse) December 17, 2017

“Chain migration” — which is loosely used as a synonym for all immigration to the United States that happens based on family ties (when a US citizen or, in some cases, a green card holder petitions for a relative to join them) — has become a conservative boogeyman, and an excuse to cut down on legal immigration. It’s long been a target of immigration restrictionists whose concerns about immigration are less about people “respecting the law” than about the government exercising stricter control over who enters the country.

Under the Trump administration, those restrictionists have more political power than they’ve had in a generation — and they’re using it to prosecute an aggressive case against the family-based system as it stands.

In response, Democrats and progressives have argued that the term “chain migration” is unhelpful at best and racist at worst. Sen. Chris Murphy (D-CT) has called the term an invention of white supremacists, echoing a growing liberal meme.

Reminder: "chain migration" is a made up term by the hardline anti-immigration crowd. Its purpose is to dehumanize immigrants. If you're using that word, you're declaring a side. — Chris Murphy (@ChrisMurphyCT) January 27, 2018

But attention to the term “chain migration” — which wasn’t actually “made up” by white supremacists — is a proxy for a much deeper disagreement about what the US currently prioritizes in immigration, and what it should: whether family reunification is an inherent good and a right that all citizens ought to have, or whether the government ought to be selecting immigrants as individuals. It’s about who is allowed to be a part of America — and whether changes to the country’s makeup are a healthy demographic development or a sign of uncontrolled invasion.

“Chain migration” is the technical name for a commonsense idea: People are more likely to move where their relatives are

The dynamic underlying “chain migration” is so simple that it sounds like common sense: People are more likely to move to where people they know live, and each new immigrant makes people they know more likely to move there in turn.

But as obvious as the reality is on the ground, it wasn’t always incorporated into theoretical models of migration (particularly economic models). Economists tended to think about the decision to migrate as a simple calculus of how much money someone was making at home versus how much he could be making abroad, rather than understanding that the decision was more complicated — and that family and social relationships played a role.

Princeton demographer Doug Massey, one of the leading scholars on immigration to the US at the end of the 20th century (and the beginning of the 21st), was one of the scholars who tried to correct this oversimplified view. As he put it in an essay for the Inter-American Parliamentary Group on Population and Development in the early 1990s:

The first migrants who leave for a new destination have no social ties to draw upon, and for them migration is costly, particularly if it involves entering another country without documents. After the first migrants have left, however, the costs of migration are substantially lower for their friends and relatives living in the community of origin. Because of the nature of kinship and friendship structures, each new migrant creates a set of people with social ties to the destination area.

These immigrants would also end up behaving differently once they arrived in their new countries. If they were just there for economic reasons, they’d have an incentive to move back once they’d made enough money, or circulate back and forth. But immigrants who move for social reasons are moving to a new community — a new place they’ll stay. That’s an upside if you think it’s important for immigrants to become American — and a downside if you think the US should be much pickier about who gets to move here for good than it is about who gets to work here.

One upshot of chain migration: Any policies that made it easier for immigrants to bring their relatives would allow migration chains to form, thus expanding immigration into the country. “Family reunification systems,” Massey wrote, “work at crosspurposes with the limitation of immigration.”

Massey and the other demographers of “chain migration” weren’t presenting it as a negative. But their words were easily adopted by people who did. The Massey essay quoted above ended up being reprinted in an issue of The Social Contract — the journal founded by immigration restrictionist mogul John Tanton, who also founded two of three most visible restrictionist organizations in American politics (the think tank the Center for Immigration Studies and the advocacy group FAIR). The third group, NumbersUSA, was founded by a former Washington editor for the journal.

The Social Contract was a forum for concerns about the threat of mass immigration (particularly mass nonwhite immigration) to the United States. (The Southern Poverty Law Center has a rundown of some of the journal’s more incendiary content.) Massey, on the other hand is a longtime supporter of reforms that would make it easier for immigrants to come to America.

An article by a supporter of expansive immigration policy could be reprinted, with few apparent edits, in a journal for his intellectual opponents only because the debate over chain migration is fundamentally not about whether it happens, but whether it’s okay. Defenders of chain migration tend to argue that it’s important for immigrants to put down roots in the US, and that having a family here is part of what that means.

Opponents, on the other hand, see family-based immigration as the government ceding some control for who gets to come here, so that it’s not selecting individuals in a vacuum — which leads rapidly to fears of the US government losing control of the immigration system entirely.

The actual policy behind “chain migration”

It’s not clear whether President Trump understands how family-based immigration actually works — and when it can lead to “chains” of relatives. Trump has claimed that the man who ran over several pedestrians in New York in November brought 23 (sometimes he says 24) relatives to the US in the seven years he’d lived here — a claim that chain migration opponent Mark Krikorian of the Center for Immigration studies said was impossible. And the White House’s “chain migration” diagram makes it looks like each generation of adults brings in children, which brings their children — which isn’t how chain migration works.

To better understand what policies, exactly, opponents of “chain migration” are worried about, check out this chart from the restrictionist advocacy group NumbersUSA — which is a more detailed representation of the same fear of overwhelming, uncontrollable waves of migration.

Let’s walk through the scenario in that chart. It depicts an immigrant who’s come to the US on an employment-based green card (in black) and is able to bring over his spouse and children immediately. He can also — after he becomes a citizen (something the NumbersUSA chart doesn’t clarify) — petition for his parents and siblings to come to the US on green cards (all in gray).

The siblings all bring over their spouses and children immediately, and the spouses (in orange, maroon, navy, and teal) can (upon naturalization) petition to bring over their own parents and siblings. The original immigrant’s parents (eventually) petition for their own siblings to come to the US, and the siblings then petition to bring over their married adult children — whose spouses can then (eventually)petition for their own parents and siblings, etc., etc.

Meanwhile, the original immigrant’s spouse, once she becomes a citizen, can petition for her parents (in pink) and her siblings (in blue, purple, red, and green). Those siblings bring over their spouses, who subsequently petition for their own parents and siblings, etc., etc.

There are a ton of assumptions in this model about the way immigrants behave — why is everyone in families of four or five? Does no one really want to stay in her home country? Is there no such thing as a bachelor in any of these families? — but the visa categories under US law make it a hypothetical possibility. But the thing is, US policymakers know that it’s a hypothetical possibility. And there are safeguards built into the system that restrict family-based immigration far more than the diagram would have you believe.

In practice, bringing over a family member takes years — which makes it very hard to build a chain

No one is automatically allowed to immigrate to the US. Anyone applying for residency in the country has to go through a standard vetting process — including a criminal and terrorism background check, and an evaluation of whether they’re likely to become a “public charge” in the US (i.e., be unable to support themselves for income and rely on social programs).

Trump’s National Security Strategy claims that “chain migration” is a problem for national security, but there’s nothing inherent to the way someone is allowed to immigrate to the US that makes it harder for the US to catch would-be terrorists — that is, if anything, a failure of the screening process.

The bigger obstacle, though, isn’t qualifying to immigrate — it’s that the number of hypothetically qualified family-based immigrants greatly exceeds the number of slots available for immigrants each year. The US doesn’t set caps on the number of spouses, minor children, or parents of US citizens who can come to the US each year — but, again, those categories in themselves don’t create chains.

The categories that do create chains are strictly capped: 23,400 married children of US citizens (plus their own spouses and minor children) are allowed to immigrate each year, and 67,500 adult siblings of US citizens (plus spouses and minor children). Furthermore, because the total number of immigrants coming from a particular country each year is capped, would-be immigrants from Mexico, China, India, and the Philippines end up facing even longer wait times.

When people talk about the “visa backlog,” this is what they mean: In January 2018, for example, the US government will start processing applications for F4 visas (the siblings of US citizens) who first petitioned to let them immigrate on June 22, 2004, or earlier. That is, unless the sibling lives in India (in which case the petition had to be filed by December 2003 to get processed in January 2018), Mexico (November 1997), or the Philippines (September 1994).

Understanding that an F4 visa is a 13- to 23-year process throws that NumbersUSA diagram into a different light. How implausible it is depends on your assumptions about how close together generations are, and how young the immigrants are when they come to the United States. But if you start by understanding that the first members of the orange, maroon, navy, teal, blue, purple, red, and green chains don’t enter the US until 18 years after the original immigrant (signified by black) does — and that the first immigrants in the yellow section of the chart don’t enter the country until 23 years later — it should give you a sense of how long it will take in to fill in the rest of the chain.

In practice, this ultimately looks like a lot of people coming to the US in late middle age. That’s backed up by the data: A study from Jessica Vaughan of the Center for Immigration Studies — which is critical of “chain migration” — found that the average age of immigrants to the US has risen over the past few decades, and that family-based immigration was a substantial cause.

But even then, the NumbersUSA scenario assumes that all the immigrants can afford to sponsor a family member to immigrate to the US. A US citizen (or green card holder seeking to bring an unmarried child or parent) has to prove to the government that they can provide financial support if their relative needs it, rather than relying on the government for aid.

In practice, this means that every immigrant needs to have someone vouching for them whose household income is 125 percent of the poverty line — and the “household” includes the relative who’s trying to come to the US. In other words, a single adult could sponsor his parent to immigrate if he made at least $20,300 — 125 percent of the federal poverty line for a two-person household — but if he had a spouse and two children, he’d have to be making 125 percent of the poverty line for a five-person household. And that includes any other immigrants who the household is sponsoring at the same time.

So an immigrant with a wife and two children who wanted to sponsor his parents and four siblings to immigrate as soon as he became a citizen would have to be making $56,875 — around the median income in the US. And if his spouse were trying to do the same thing with her parents and four siblings, as in the NumbersUSA chart, they’d have to be making $83,000 — which would place them in the 66th percentile of US household income.

That’s not impossible. But it certainly calls into question the stereotype of family-based migration as a way for “low-skilled,” low-earning immigrants to bring their low-skilled, low-earning relatives into the US.

There are ways for citizens to get other people to agree to help support a potential immigrant relative. But at the same time, the US government has discretion to reject an application, even if the citizen meets the income threshold, if they suspect that in practice the immigrant won’t be supported in the US. (Another factor in determining “public charge” is age — which is interesting, given the data about family-based immigrants being older.)

Add all of these factors together, and it becomes clear that an immigrant won’t be able to bring that many relatives to the US over the course of his or her lifetime. Vaughan’s study found that as of 2015, immigrants who came to the US from 1981 to 2000 had sponsored an average of 1.77 relatives to come join them. The most recent immigrants in the study — those who came to the US in the late 1990s — had sponsored the most relatives: 3.46. But both of those numbers include the minor children they brought with them at the time: In other words, they were hardly starting 3.46 new “chains.”

If anything, in fact, the family-based system is so overloaded that it ends up creating unrealistic hopes in people that they’ll be able to immigrate to the US. If your sibling moves to the US on a work visa, for example, you might start to hope that he’ll eventually be able to bring you along — but if you try to plan your life around that, you’ll end up waiting for two decades.

There are hints all this panic over “chain migration” is really about fear of cultural change

All of this is relevant to a conversation about whether to further restrict, or eliminate, the F3 and F4 visas for married children and adult siblings of US citizens. And indeed, that’s the most common policy demand being made by Republicans who are seeking to end or reduce “chain migration.”

But the most stalwart opponents of “chain migration,” the ones who use it to refer to all family-based immigration, period, are talking not just about the mechanics of the chain but about a bigger normative question: whether allowing immigrants to come as family units, or allowing people to immigrate based on family relationships, gives the US too little control over who gets to come.

The ultimate impression of both the White House and NumbersUSA “chain migration” diagrams is to make it seem that admitting a single immigrant unleashes an uncontrollable tide of infinite future family-based immigration — that each immigrant is a one-person Trojan horse for hundreds more.

“As more and more immigrants are admitted to the United States, the population eligible to sponsor their relatives for green cards increases exponentially,” the restrictionist group FAIR says on its website. “This means that every time one immigrant is admitted, the door is opened to many more.”

This potent visual is why “chain migration” has been a longtime target of immigration restrictionists, even when the Republican Party as a whole was attempting to welcome legal immigrants. For people whose biggest fear regarding immigration is that immigrants will change the face of America — that they’ll trample the country’s “traditionally” white, Christian majority — there’s little more potent than the idea of immigrants bringing over huge families, replanting their communities whole in American soil.

This fear goes hand in hand with a fear that immigrants won’t assimilate. When immigration restrictionists cite the second quarter of the 20th century as a great time for the United States, they’re not (at least explicitly) praising the racist country quotas that governed immigration at the time. They’re (explicitly) praising the fact that, with overall immigration levels low, immigrants were forced to interact with and eventually integrate among US citizens. The more immigrants that come over — and especially the more that immigrants bring their families over — the less, in theory, that they and their descendants will have to interact with people from outside of their community. In turn, this gets into fears that parts of America could become alien to Americans — cultural, or literal, “no-go zones.”

The use of “chain migration” in the current debate over DACA, to refer to DACA recipients allowing their parents to become legal immigrants, complicates the matter even further. Because the parents of DACA recipients have, by definition, lived in the US as unauthorized immigrants, this isn’t really about bringing new people into the US — it’s about legalizing people who are already here (or bringing people back who have been deported, something US policy already makes pretty hard).

The insistence among some Republicans that “Dreamers” not be allowed to sponsor their parents, even after they become US citizens, is really about not wanting to “reward” unauthorized immigrants for living in the US without papers. They’re worried about losing “control” in a slightly different sense — worried that any “reward” for illegal behavior will incentivize a new wave of unauthorized migration to take advantage of potential rewards. This is pretty far afield from the way that “chain migration” is commonly understood — but that’s the word being used in the DACA debate anyway, not least because the president has helped turn it into a buzzword.

Because these memes, and the fears that they provoke, are all so tightly connected, “chain migration” is both an ideological concern about America selecting immigrants based on their merit, and a racist smokescreen for fears of demographic change. It can be hard to separate the two. And it’s certainly not in the interests of the opponents of “chain migration” to try.

There’s a reason that family-based immigration has lasted as long as it has

It’s a lot easier to get people to agree, in theory, that the US should be accepting immigrants on the basis of “merit” — i.e., without concern for whether they have relatives living here — than it is to get them to agree on exactly what should be done to reduce the importance of family-based immigration to the current system.

For one thing, many policymakers, including many Republicans, see allowing some family members to immigrate as an important factor in encouraging integration. Allowing immigrants to bring along their spouses and minor children, for example, makes it less likely that they’ll decide to return to their home countries — and it means their children will grow up American, in more ways than one.

There are also policymakers who see family unity as a value worth protecting for its own sake (an argument you’ll often hear among religious advocates). And there’s, of course, an ethnic component. Asian Americans, in particular, feel that they are still trying to make up ground after decades of racist exclusion from the immigration system — and family-based immigration has been the best way for them to make that ground up. Mexican Americans, too, feel that the current system has unfairly forced Mexican immigrant families to be separated while other families get to reunite with ease.

All of these objections have combined, so far, to make Democrats firmly opposed to any proposal that would restrict future family-based immigration. But as “chain migration” begins to eclipse other issues (like immigration enforcement in the interior of the US) as a top Republican priority, it’s not clear whether Democrats’ commitment to hypothetical legal immigrants of the future is going to win out over their commitment to legalizing unauthorized immigrants who are currently here.

CORRECTION: This article originally said that permanent residents (green-card holders) could petition for their parents to enter the US; only US citizens can sponsor their parents. This article also originally misstated the relationship between John Tanton and NumbersUSA, whose founder, Roy Beck, has attempted to distance himself from Tanton in recent years.