“You never want a serious crisis to go to waste,” Rahm Emanuel, President Barack Obama’s chief of staff, said in 2008, referring to Mr. Obama’s proposals for stricter regulations in response to the financial crisis. “This crisis provides the opportunity for us,” he said, “to do things that you could not before.”

There is, of course, no central authority or accepted checklist for determining when a crisis is real, and severe enough to merit drastic action. It’s a matter of subjective public perception. But there is growing research into what leads people to support drastic steps from their leaders.

Take Mr. Trump’s push for a border wall. Whatever his intentions — to win the wall, to rally the base, to distract from setbacks or merely out of impulse — during his election campaign, and again during the midterm elections, he has hit squarely on two psychological triggers that can make people want a strong leader to take control.

The first is a sense of demographic and cultural change, particularly when that change feels uncontrolled. Among some white Americans, the growing prominence of minority groups in politics and popular culture has created a sense of demographic threat, according to researchers, leading those voters to desire a strong leader who would impose control.

Mr. Trump used warnings about unauthorized immigration to cultivate a sense of crisis — though unauthorized immigration has been declining for a decade (and the migrants traveling to make asylum applications are exercising a basic legal right) — that hit on those fears. And he used promises of a border wall to reassure frightened voters that he would protect them.

The second trigger is fear of a specific kind of violence. People feel a sense of acute crisis if they believe that they may be attacked for their membership in a demographic group, such as race or religion, according to research by Daphna Canetti-Nisim, a University of Maryland political psychologist. They become more supportive of harsh policies to control outsiders, such as torture or extrajudicial detention, and more supportive of a strong leader who will go outside the law.

Mr. Trump tapped into this as well, warning that Hispanic and Muslim migrants posed a grave danger, though the threat was almost entirely invented. In Britain, proponents of withdrawal from the European Union, or Brexit, followed a similar playbook.