The new rule, though, vastly expands the universe of people affected. Unfortunately, the State Department has offered little detail about precisely how it will use the millions of identifiers that it is collecting. According to the department’s regulatory filings, consular officers could look at social-media accounts to round out other information about an applicant—for instance, what they glean from her interview and application papers. It’s not clear whether social-media data will be subjected to some type of automated system meant to flag particular words or identify suspicious connections between people. But Homeland Security—the State Department’s main partner in vetting visa applications—has experimented with systems that perform both tasks. Many other questions remain: What will happen to people who are flagged? Will they be notified of a post that troubles a consular officer and given a chance to respond to any concerns? How will any vetting system account for slang, cultural context, and humor? Does the State Department even have the language capacity to systematically review social-media posts?

The department says it needs social-media identifiers to determine whether visa applicants meet the standards for getting a visa, to root out fraud, and to “identify misrepresentations that disguise potential threats.” Of course, these are precisely the judgments that consular officers have already been making as part of the robust visa-vetting system that was built after the September 11 attacks. Anybody who has ever applied for a visa to the United States will attest that it involves a rigorous investigation. In addition to providing biographical and biometric information, applicants have to explain—and meticulously document—where they’re going, how they will pay for the trip, where they will stay, whom they know in the United States, and more. Before any people who need a visa board a flight for the United States, a consular officer probes their story and checks their information against databases of law-enforcement and intelligence information.

The burden is on applicants to prove that they meet the requirements for getting a visa. If they live in a country where documents are hard to come by, the U.S. government doesn’t relax the rules. If they come from a country where forgeries are common, consular officers will be even more vigilant.

Simply put, we already have “extreme vetting” that keeps out those who would do harm: From 2002 to 2016, the Cato Institute has calculated, one deadly terrorist made it through for every 379 million decisions authorizing a foreigner to enter the United States.

Since around 2014, officials have looked at social-media accounts in certain cases. But there is no evidence that such checks have added value. That’s according to the government’s own assessments on the usefulness of social-media monitoring. A February 2017 report from the Homeland Security’s Office of Inspector General found that the social-media-vetting pilot programs that it evaluated “lack[ed] criteria for measuring performance to ensure they meet their objectives.” Other internal assessments have shown that officers had difficulty using social media to detect fraud or to pinpoint public-safety or national-security concerns. These findings are in line with the objections to social-media vetting that the Brennan Center for Justice, where the two of us work, and dozens of other organizations and experts have also raised. Scaling up social-media checks will only multiply these problems.