KELLY MCEVERS, HOST:

At tomorrow's State of the Union, President Trump is set to make his case for changing the rules around who gets to come to the United States. And a big part of that case is scaling back what the administration calls chain migration. Trump has even described it as horrible chain migration. It's a policy that allows U.S. citizens to sponsor not just their spouses and children, but also parents and adult siblings. The policy is more commonly known as family unification. And it was introduced more than 50 years ago by an immigration hard-liner, a member of Congress who wanted to keep certain immigrants out.

Here to tell us that story is NPR's Tom Gjelten, who wrote a book called "A Nation Of Nations: A Great American Immigration Story." Hey, Tom.

TOM GJELTEN, BYLINE: Hi, Kelly.

MCEVERS: All right, so this story starts in 1965. Just tell us who that member of Congress was and how he was responsible for this family migration system that President Trump wants to get rid of.

GJELTEN: His name was Michael Feighan. He was a conservative Democrat from Ohio. And he was one of many conservatives at the time who thought it made sense to prioritize immigrants from Europe over immigrants from other regions of the world. And prior to 1965 that was how visas were given out, on the basis of your national origin. Now, as you can imagine, that approach was seen as racist and discriminatory. And by the 1960s, there was this sense that it really should be abolished.

President Johnson called for a switch to a merit-based system. But Feighan and others essentially thought a change like that would open the doors to too many immigrants of color, so he came up with a compromise. He suggested a new policy that would give preference to people who already had relatives here. He figured that would maintain the U.S. population the way it was at the time - largely white and European. It was actually seen - his proposal was actually seen as a naturally operating national origin system.

MCEVERS: But it didn't work out that way, right?

GJELTEN: Now, this is a story about unintended consequences.

MCEVERS: Yeah.

GJELTEN: In 1960, 7 of 8 immigrants were still coming from Europe. But that was already beginning to change. Nobody realized it, but the demand to migrate was shifting to the developing world. And this has continued. So by 2010, 9 out of 10 immigrants are coming from outside Europe.

MCEVERS: So if non-Europeans weren't well represented in the U.S. in the 1960s, how did this approach end up bringing in so many people from outside Europe?

GJELTEN: Well, it naturally took a few years. And the way it happened is that there were always other opportunities to immigrate. You know, you could come on a student visa. You could come on an employment visa. I tell the story in my book, for example, of how a Korean woman married an American serviceman, thus became - becoming a U.S. citizen. She brought over basically her extended family.

Another character in my book, a Pakistani man working for an American company, got invited to come to America. So he immigrated. Within about 30 years, he'd brought all six brothers and three sisters over here with their spouses. I actually calculated that over the period of about 30 years he was indirectly responsible for a hundred family members coming. And because the numbers were low in the beginning it took a while to develop, but eventually you had these chains. And about two-thirds of all legal immigrants to the U.S. these days are coming under this family unification system.

MCEVERS: How did it get this negative connotation?

GJELTEN: Well, partly it's just the way that President Trump uses the term chain migration. It just - it sounds bad. It just sounds pejorative when he says it. But in addition to that, it's an offensive term to African-Americans in particular. Their ancestors literally arrived in chains. That was the original chain migration.

MCEVERS: Right. And what is the argument for family unification?

GJELTEN: You know, it's just a natural way. I mean, when you come here as an immigrant you don't know your way around. You probably don't know the language. You don't know where to live. It's just natural to seek out a family member to move in with. And, you know, in reporting my book I found that about 90 percent of all the immigrants that I interviewed moved in with their families when they first came. It's just a natural way to do it.

MCEVERS: President Trump says he wants to shift to a merit-based immigration program. At the same time, the White House proposal calls for getting through the backlog of family applicants first.

GJELTEN: Right.

MCEVERS: How long could that take?

GJELTEN: Oh, it could take decades. We're talking about a change that'll take place over the course of many years. Nobody that's in line now will lose their place. So you'd have to work through that backlog before you'd see this big change with one exception, Kelly - parents of U.S. citizens now can come in without numerical limitation. There's no cap on them. There's no backlog in their case. So they would be immediately affected by this change were it to become law. And one other thing - if this were to become law you would have a lot of people who haven't yet applied for a visa and who wouldn't be able even to get in line. So you'd see an immediate outcry from that population.

MCEVERS: NPR's Tom Gjelten, thanks a lot.

GJELTEN: You bet.