There were many reasons to oppose President Trump’s travel ban on refugees and visitors from seven predominantly Muslim countries, which is now blocked by a federal court’s temporary restraining order. Unfortunately, those same objections are also likely to apply to the revised version of the executive order that Mr. Trump promised on Thursday, which will share with its predecessor the goal of “immediately protecting the country” — presumably by keeping out people from countries he deems to be a threat.

One objection to such policies is that there is no good evidence that citizens of the countries the president has singled out so far present a significant threat to the United States. Another is that any policy that effectively discriminates against members of a specific religion is decidedly un-American.

But perhaps the most important objection, given the ostensible goal of protecting national security, is that these are precisely the sort of policies that can increase radicalization of Muslims already on American soil. Recently, a group of former diplomats and national security officials signed an open letter condemning the original ban on that ground, arguing that it would make the country less safe by feeding the narrative that America is anti-Islam.

Mr. Trump and his advisers should know that this is not mere speculation; it is grounded in social science. In a study published in 2015 in the journal Behavioral Science and Policy, we showed that policies like Mr. Trump’s ban may very well promote the psychological conditions that fuel the radicalization he seeks to combat.