The Republican Presidential nominee Donald Trump offered details of his immigration policy during a speech in Phoenix, Arizona, on Wednesday. Photograph by Ralph Freso / Getty

Toward the end of Donald Trump’s rant in Arizona the other night, down at point No. 10 of his ten-point plan to make life awful for immigrants, I found myself jerking backward, surprised, not by the ugly misrepresentations or barely concealed racism—who could be surprised by that anymore?—but by the shock that there was something in the speech that seemed to possibly make a little bit of sense. It was this: he called for the creation of “a new immigration commission . . . to select immigrants based on their likelihood of success in U.S. society, and their ability to be financially self-sufficient. . . . To choose immigrants based on merit, skill, and proficiency.”

This is an idea that exists in the non-paranoid, non-racist, non-alt, non-right part of our public discourse: we should admit immigrants, at least in part, based on how their arrival would affect the economic well-being of already-present Americans. If we could engineer things just right, only allowing in those who have skills that are lacking in the U.S. workforce or who seek to fill the professions for which we have shortages, we could add immigrants and make the country richer, and no American would have to lose a job or get a cut in pay.

The United Kingdom received a fair bit of praise when it set up just such a program, the Migration Advisory Committee, in 2007. It is a government body made up of well-respected economists and other experts who conduct exhaustive studies of the British labor market; it then issues recommendations for which jobs should be placed on something called the Shortage Occupation List. It’s much easier for people with “shortage occupations” to get a visa to move to Great Britain. The list can be remarkably specific. The current list has good news for those with experience in “the decommissioning and waste management areas of the nuclear industry,” clinical neurophysiology, and skilled classical ballet. The goal of the MAC is to remove a bit of the political hot air around immigration and replace it with something more data-based.

The MAC seems to have done about as good a job as is imaginable at a task that is impossible. Its many reports—on nurses, teachers, retail workers, low-skilled workers, and high-skilled workers—are models of sophisticated economic analysis. What they are not, though, is an objective view of whom the U.K. should admit and whom it shouldn’t. David Metcalf, the economist and chair of the MAC, told me that immigration policy is, inherently, political. “You can’t contract it out,” he said. The politicians “set the parameters, and the humble technicians grind out the analysis and come up with recommendations. And the politicians decide if they want to listen.”

This point is crucial. Donald Trump’s point No. 10, I felt sure, was a nod toward those more moderate Republicans who are turned off by his angry anti-immigrant rhetoric. He is telling them not to worry too much. His monstrous id will be checked by objective economics. But there is no such thing. Economics is, at its best, good at analyzing, but it can’t set policy. For example, the U.K. government’s immigration policy prioritizes high-net-worth entrepreneurs and investors who could bring with them a lot of money and, presumably, create new jobs for existing British workers. Diane Coyle, an economist who served on the MAC for many years, told me this model bothers her. “Who’s to say that a Syrian refugee or her child won’t make a brilliant entrepreneur,” she said, pointing out that Steve Jobs’s birth father was a Syrian immigrant to the U.S. (The MAC’s analysis did show that a hugely disproportionate number of these “entrepreneurs” were wealthy people from Russia—“I believe the polite term is ‘oligarchs,’ ” Coyle said—and China. “There were a lot of Subway franchises,” Metcalf said. Not exactly setting the U.K. innovation economy on fire.)

Similarly, the MAC was asked to address the shortage of nurses in the U.K. Its report was an unsparing attack on another arm of government, arguing that the shortage was entirely the fault of the Department of Health. For years, the MAC argued, the health department had failed to train enough nurses, pay nurses well enough, or properly plan for the future. The report concludes that allowing more immigrant nurses was a way of providing “the sector with a ‘Get Out Of Jail, Free’ card.”

I began to imagine a U.S. equivalent to the MAC, calculating just how many doctors should be admitted to the U.S. If we were to accept a “likelihood of success” approach to immigration, perhaps, like the U.K., we could focus on those medical specialties for which there seem to be too few providers, like general practitioners and emergency-room doctors. An influx of tens of thousands of qualified doctors would, certainly, make it easier to schedule a checkup or be seen more promptly in the E.R. But, in the long term, it could have a perverse, opposing effect. By lowering wages, even fewer medical students would choose those specialties, insuring shortages in the future. Maybe, instead, we should invite in only the highest-paid specialties, like dermatologists and plastic surgeons, to suppress those wages and make those fields less attractive. Or we could just invite a ton of medical-school professors to move here and train a new generation of doctors. When you lose yourself down the rabbit hole of theoretical immigration economics—imagining how admitting more of this group and less of that one would ripple through the economy—you realize fairly quickly that there is no way to predict what the impact of a change today will be a decade down the road.

And that is just the point. Our national economy is a massive, ever-evolving thing, and each profession within it changes continuously, too. But immigration policy is generally stuck for a generation or two.

We are in the midst of a crucial, if confusing and jumbled, national conversation about the rewards for work. We know the broad outline of the problem: after a century or so of widely shared prosperity, only a small percentage of Americans are receiving the bulk of our country’s economic gains. Inequality is growing, and we know that this has something to do with technology, trade, lessened bargaining power among workers, and changes in corporate strategy, as well as government policy over taxes, labor rules, and regulation. Economists have been arguing for decades over the various positive and negative impacts of each of these forces.

But there is one issue about which economists don’t have much debate. It’s clear that immigration is not a fundamental driver of rising economic inequality or stagnating wages for almost any group of Americans. There is a fight over the impact of immigrants on one cohort in the United States: high-school dropouts. There is some evidence that their income has fallen a bit (by low-to-mid-single-digit percentage points on average) because they are competing with illegal immigrants—although even that is debated. Coyle, formerly of the U.K.’s MAC, said that the most surprising finding about the economics of immigration in the U.K., the U.S., and most developed nations is that there isn’t much economic impact to immigration. The immigrants arrive and wages don’t fall, jobs don’t disappear. This, she says, is because immigrants don’t tend to directly compete against the existing workforce. Immigrant doctors go to immigrant neighborhoods, immigrant construction workers work alongside—rather than in competition with—skilled residents. Immigrants, generally, find the cracks in the economy that the existing workforce isn’t filling. Immigrants are valuable to a country, in part, precisely because they bring a fresh perspective: they see opportunities that we miss.

We shouldn’t be tempted by the idea that we can engineer the economically perfect immigration policy. Like all forms of social engineering, it will fail. We can only plan based on our limited and flawed present-day understanding. And our plans will become congealed laws whose errors will only appear over time. This is why it is crucial that the people in office who write our immigration laws are people whose judgment we trust.