“We’re in a new world now,” Solicitor General John Sauer told the U.S. Supreme Court during oral argument this April in the birthright-citizenship case Trump v. Barbara, “where 8 billion people are one plane ride away from having a child who’s a U.S. citizen.” Chief Justice John Roberts quickly shot him down, replying, “Well, it’s a new world,” but it’s “the same Constitution.”Roberts’s quip foreshadowed his opinion on behalf of the Court holding that near-universal birthright citizenship is guaranteed by the text of the Fourteenth Amendment’s citizenship clause. His opinion is a meticulous rendition of U.S. history up to the 1868 ratification of the Fourteenth Amendment, drawing on abundant evidence that the citizenship clause was intended to apply to nearly everyone born in the United States. The decision was a major defeat for one of President Trump’s signature policy initiatives. Notably absent from Roberts’s opinion, however, was any discussion of whether birthright citizenship is a good idea in 21st-century America.And that’s a pity, because birthright citizenship brings many practical benefits to the United States today—arguments that the nation needs to hear, given the campaign being mounted against it. By contrast, the three dissenting justices did not hold back, devoting many pages to claims that birthright citizenship is destructive and even dangerous in our modern, mobile world—assertions that are almost entirely unsupported and yet that went unrefuted. Defenders of birthright citizenship have made strong legal arguments in its favor; they need to start making the policy case for it as well.It’s an easy case to make. The children of immigrants are about 10 percent of the total U.S. population. They power the U.S. economy and serve in significant numbers in the government and the military. Nearly half of Fortune 500 companies were founded by immigrants and their children, more than 10 percent of the current members of Congress are children of immigrants, and six out of 45 U.S. presidents, including Trump, had at least one immigrant parent. They are so well integrated into the fabric of the nation that most of us have no idea which of our neighbors and colleagues were born to immigrant parents unless they choose to tell us.[Read: The most surprising part of the birthright-citizenship decision]Not all of these examples of successful children of immigrants would have been excluded from citizenship under Trump’s executive order, of course, but some certainly would have. (And in any case, it is impossible to know who qualifies for citizenship under Trump’s executive order without first scrutinizing their ancestry—which is yet another reason to reject it.) The larger point, though, is that children of immigrants have proved to be one of America’s greatest assets.If Trump’s executive order had gone into effect, it would have denied citizenship to about 255,000 newborn children a year going forward—a result that would have been not only a nightmare for them and their families but also bad news for the nation’s economy. Economists have found that citizenship status brings with it higher earnings. Even green-card holders who have the right to work and live in the United States indefinitely earn more after they naturalize. If all of these children remained in the U.S. without citizenship, they would contribute less to the tax base and to our already-strapped Social Security system. Worse, if they were deported (or if they self-deported), their absence would accelerate the demographic decline that is transforming us into an aging society with too few workers to support the retiring Baby Boomers.In addition to these economic challenges, denying birthright citizenship undermines America’s founding principle that governments derive “their just powers from the consent of the governed.” The United States cannot claim to be a democracy and simultaneously deny the right to vote and hold office to a significant percentage of its inhabitants and their offspring in perpetuity, excluding them based on their “blood” and without regard to the fact that they are here. This is the essential truth that the framers of the citizenship clause recognized back in 1868, which is why they established birthright citizenship to protect not only the newly freed slaves but also the children of immigrants arriving from around the world. It remains just as true today.In dissent, Justices Clarence Thomas, Neil Gorsuch, and Samuel Alito collectively devoted thousands of words to describing what Alito called the “grotesque result” of granting citizenship to the children of undocumented immigrants and birth tourists.Let’s start with “birth tourism”—the assertion that foreign women are coming to the United States on tourist visas to give birth to U.S.-citizen babies and then leave, which has become the focus of much of the conservative outrage over birthright citizenship. At the oral argument, Sauer stated that birth tourists from China and Russia number in the millions. But when the chief justice asked him directly whether he knew “how common that is or how significant a problem that is,” the Solicitor General was forced to admit, “No one knows for sure.”In fact, the U.S. government makes no effort to track birth tourism—which alone suggests that it is not a major crisis. The Center for Immigration Studies, a group that advocates for restricting immigration, makes a rough guess of 26,000 birth-tourist babies a year—fewer than 1 percent of the 3.6 million total births recorded in the United States annually.If birth tourism seems troubling despite its small numbers, I have good news: It is already illegal. Under a federal regulation in place since 2020, the United States can deny a tourist visa to a woman whose primary purpose in coming to the U.S. is to give birth. That prohibition can be enforced by Customs and Border Protection officials at U.S. airports and at the country’s borders. Following its Supreme Court loss, the Trump administration announced plans to start prosecuting violators of that law; why it didn’t do so earlier is unclear. (I submitted a Freedom of Information Act request to the government more than a year ago asking for records regarding this policy’s implementation but have yet to receive a response.) In short, there is no need to take a hammer to birthright citizenship when we can use a scalpel to target birth tourism specifically.What about Sauer’s claim that we live in a “new world” in which “8 billion people” (the entire world’s population) are a plane ride away from giving birth on U.S. soil? That is laughable.True, airplanes didn’t exist in 1868 (though steamships did, and millions of foreigners traveled to the United States in the 19th century). Also true, however, is that today, international travel requires a passport and often a visa—neither of which was needed in 1868. Back then, there were few immigration restrictions and no federal immigration officials. Today, the nation spends billions to screen entrants using cutting-edge biometric tools, and thousands of Customs and Border Protection officers are stationed at every international airport and port of entry in the country. Most of the world’s 8 billion people cannot even board a plane to the United States or a nearby country because they lack the requisite documents, even assuming they can afford the price of a plane ticket.[Adam Serwer: What the birthright case is really about]The issue of granting birthright citizenship to the children of undocumented immigrants also concerned the dissenters. Even Justice Brett Kavanaugh, who concurred in the result, described illegal immigration as a “new circumstance” that the “Framers of the Fourteenth Amendment could not have fully anticipated,” then concluded that “presumably” they would not have wanted to grant the “birthright citizenship benefit” to the children of these lawbreakers. Justice Alito argued that birthright citizenship provides a “powerful incentive to enter or remain in this country illegally.” These justices are correct that the United States, like the rest of the developed world, is wrestling with the relatively recent problem of large-scale illegal immigration. Where they go wrong is assuming that birthright citizenship has anything to do with it.By design, U.S. law makes it extremely difficult, often impossible, for undocumented parents in the United States to gain legal status based on a child’s citizenship. And studies have shown that the primary incentive for irregular migration is jobs for the parents, not birthright citizenship for their children. That is why the rate of such migration drops precipitously during U.S. recessions. Moreover, European countries do not have birthright citizenship, yet they also struggle with waves of immigrants fleeing violence and poverty for a better life.If the government truly wanted to deter undocumented immigrants, it would crack down on the many thousands of U.S. citizens and corporations who violate immigration law by hiring them (even Justice Alito concedes that the children of undocumented immigrants are “not responsible” for their parents’ lawbreaking). Yet no administration, Republican or Democrat, has made a serious effort to enforce immigration law against U.S. employers of undocumented immigrants. The few times that Trump briefly attempted broader enforcement—for example, by targeting Wisconsin dairy farms, whose workforce is roughly 70 percent undocumented—he faced significant pushback from his own supporters. Far from deporting those workers and penalizing their employers, Trump is allowing this—he is expected to announce a special guest-worker program just for these dairy farmers (the majority of whom happen to be Trump supporters) in the coming weeks.All of this should make us wonder about the real reasons Trump has expended extraordinary time and effort to end birthright citizenship. Perhaps it has nothing to do with illegal immigration and birth tourism. Perhaps it is instead an attempt to redefine the meaning of American to exclude the descendants of newcomers, many of whom are not white. If so, Trump lost—but unless supporters of birthright citizenship defend it on policy as well as legal grounds, his view of who should be an American may ultimately prevail.