In the past decade, the government of Australia spent more than fifteen million dollars on an advertising campaign designed to deter prospective migrants. The multimedia effort, which has been lauded by President Trump, featured bold, red text—“NO WAY: YOU WILL NOT MAKE AUSTRALIA HOME”—over images of dark, choppy seas. The Department of Homeland Security has distributed similar flyers at migrant shelters in Mexico, near the border: “The next time you try to cross the border without documents, you could end up a victim of the desert,” they warn. Canada has mounted billboards in Hungary to deter Roma asylum seekers; Germany has sponsored posters on the sides of Kabul buses; Norway has purchased Facebook ads targeted at young men from Afghanistan, Ethiopia, and Eritrea whose profiles indicate interest in “travelling” or “Europe.”

Activists and observers have criticized the hostile tone of these ad campaigns. Still, the ads’ underlying premise—that governments have a right to control entry into their countries—seems beyond dispute. Even immigration activists implicitly accept that it must be controlled: the movement to abolish U.S. Immigration and Customs Enforcement, for example, speaks to the question of how American borders are policed, not to whether they ought to be policed in the first place. In a new graphic-nonfiction book, “Open Borders: The Science and Ethics of Immigration,” Bryan Caplan, a libertarian economist at George Mason University, makes the radical pro-immigration argument that others don’t. In his view, immigration should be essentially unlimited. He envisions a future in which Democrats and Republicans vie to become the country’s “open border party,” each calling for nearly unrestricted immigration.

The difference between Caplan’s world and our own is that Caplan presents immigrants not as threats—to low-skilled workers, to social services, to public culture—but as generators of wealth. Citing the work of the development economist Michael Clemens, Caplan claims that global freedom of movement would increase the gross world product by between fifty and a hundred and fifty per cent. The basic principle of his claim is that workers in poor countries are underutilized. (“How productive would you be in Haiti?” Caplan asks.) If people could travel as freely as commodities and capital do, they could produce “vastly more stuff,” insuring that “almost everyone ends up better off.” Restrictions on immigration, Caplan writes, are the equivalent of leaving “trillion-dollar bills on the sidewalk.” (He borrows that phrase from Clemens; Zach Weinersmith, the book’s illustrator, draws a cartoon Caplan and Clemens making angels in snowdrifts of cash.)

Opening the world’s borders wouldn’t mean abolishing them, Caplan explains. Countries could still issue passports and maintain territorial notions of belonging. But immigrants—perhaps after passing some sort of criminal background check—would be automatically accepted at all standard ports of entry. Governments would relinquish their exclusionary authority, so that anyone, regardless of citizenship, could “accept a job offer from a willing employer or rent an apartment from a willing landlord.” In one illustration, a cartoon Caplan serves a trillion-dollar blueberry pie; its slices are distributed to landlords with apartments to rent, retirees with newly affordable elder care, and mothers reëntering the workforce thanks to lower child-care costs. Caplan concedes that, in countries like the U.S., wages could decrease for some native workers. But he argues that the influx of new consumers would stimulate the economy, and that many members of America’s working class would end up “managing and training new arrivals, not competing with them!” Big businesses are notably absent from Caplan’s list of beneficiaries, although they would profit from an expanded labor pool, too. Partly for this reason, Charles Koch has come out in favor of open borders. (In 2015, Bernie Sanders characterized the idea as “a Koch brothers proposal” designed to “bring in people who will work for two or three dollars an hour.”)

What about poorer countries, with low returns on labor, from which immigrants would flow? Presumably, an open-border policy would lead to a mass exodus. And yet an illustrated version of Caplan, working as a Western Union teller, reassures these countries that they would be rewarded with compensatory, monumental remittances. Brain drain wouldn’t be an issue, since the total liberalization of movement would allow everyone—not just the highly skilled—to emigrate. Caplan writes that a “ghost town,” in which a dwindling labor pool keeps wages high, is preferable to the “zombie” towns, which trap their residents in moribund economies, that are created by the current system. (A sign on a zombie-infested Main Street reads “BRAINS 50% OFF!”)

Caplan imagines a debate with Milton Friedman, who once declared that free immigration and a welfare state couldn’t coëxist. Caplan, pictured alongside Friedman in a maternity ward, explains why the fact that some immigrants end up depending on social services is a weak argument: some native-born babies grow up to depend on social services, too, and yet no one argues that we ought to restrict reproduction. Anyway, Caplan tells Friedman, “when we crunch the numbers,” immigrants represent a net fiscal benefit to the U.S. government. According to data from the National Academy of Sciences, the average new arrival generates two hundred and fifty-nine thousand dollars in tax revenue. Data also shows that American-born citizens tend to impose a larger fiscal burden, and are more likely to commit crimes, than immigrants. In the book’s fantasia, Friedman, who died in 2006, doesn’t contest Caplan’s assertion that “most immigrants pull their own weight—and then some.”

Caplan’s case isn’t entirely about economics: he also makes a moral appeal. Consider the case of “Starving Marvin,” who needs food and is prepared to purchase it legally. On his way to the market, he is turned away by an armed guard. If Marvin subsequently dies of starvation, Caplan asks, is the guard guilty of murder? The philosopher Michael Huemer, who first introduced this hypothetical, in 2012, concluded that the answer was yes. He writes, “If a person is starving, and you refuse to give him food, then you allow him to starve, but if you take the extra step of coercively interfering with his obtaining food from someone else, then you do not merely allow him to starve; you starve him.” Caplan doesn’t go that far, but he does argue that the guard is wrong to prevent Marvin from feeding himself.

As a libertarian, Caplan generally seeks to avoid distinguishing between citizens of different countries. Instead, he condemns the “global apartheid” that borders perpetuate. Exclusion on the basis of one’s country of birth, he maintains, is no less reprehensible than discrimination on the grounds of gender, race, or religion. We are right to value equality of opportunity—but, if it’s valued at home, then it should also be valued on a global scale, where inequality is much starker.

Many people see inequality within a country as morally urgent in a way that they do not consider inequality across borders to be. The philosopher Christopher Wellman offers an explanation for this attitude: he points out that, although we might quickly condemn parents who pay for their sons to attend college but not their daughters, we could forgive cash-strapped parents who pay no one’s tuition. The latter situation is worse, strictly speaking, but at least it feels unavoidable. We might conclude that global inequality, though lamentable, is the result of vast historical forces that we can’t be expected to change. It’s just the way the world is.

In Caplan’s estimation, though, global inequality isn’t inevitable, and immigration isn’t a zero-sum game. Opening the world’s borders would be an act of revenue-generating humanitarianism—a form of laissez-faire global distributive justice, on the order of seventy-five trillion dollars a year. Letting in Starving Marvin isn’t just “the decent thing to do” but also “the smart thing to do.” Turning him away would be wrong both morally and financially. We aren’t the impoverished parents who can’t afford to send anybody to college. We’re the rich parents who choose to send some kids but not others.