QOTD: Trump

“Earlier generations … knew that our power grows through its prudent use; our security emanates from the justness of our cause, the force of our example, the tempering qualities of humility restraint,” Woodward quoted Obama as saying.

Trump replied, “Real power is fear.”

Do politicians’ words, the president’s especially, matter?

Since he has been in office, President Trump has relentlessly demonized his political opponents as evil and belittled them as stupid. He has called undocumented immigrants animals. His rhetoric has been a powerful contributor to our climate of hate, which is amplified by the right-wing media and virulent online culture.

Of course, it’s difficult to prove that incendiary speech is a direct cause of violent acts. But humans are social creatures — including and perhaps especially the unhinged and misfits among us — who are easily influenced by the rage that is everywhere these days. Could that explain why just in the past two weeks we have seen the horrifying slaughter of 11 Jews in a synagogue in Pittsburgh, with the man arrested described as a rabid anti-Semite, as well as what the authorities say was the attempted bombing of prominent Trump critics by an ardent Trump supporter?

You don’t need to be a psychiatrist to understand that the kind of hate and fear-mongering that is the stock-in-trade of Mr. Trump and his enablers can goad deranged people to action. But psychology and neuroscience can give us some important insights into the power of powerful people’s words.

We know that repeated exposure to hate speech can increase prejudice, as a series of Polish studies confirmed last year. It can also desensitize individuals to verbal aggression, in part because it normalizes what is usually socially condemned behavior.

At the same time, politicians like Mr. Trump who stoke anger and fear in their supporters provoke a surge of stress hormones, like cortisol and norepinephrine, and engage the amygdala, the brain center for threat. One study, for example, that focused on “the processing of danger” showed that threatening language can directly activate the amygdala. This makes it hard for people to dial down their emotions and think before they act.

Mr. Trump has managed to convince his supporters that America is the victim and that we face an existential threat from imagined dangers like the migrant caravan and the “fake, fake disgusting news.”

Were the men arrested in the synagogue shootings and bombing attacks listening? Robert Bowers, for example, apparently blamed Jews for helping transport members of the Central American migrant caravan. It seems he did not think the president was going far enough in protecting the country from invaders. “I can’t sit by and watch my people get slaughtered,” he wrote online before the murderous rampage. And Cesar Sayoc Jr., accused of mailing bombs to CNN, echoed the president in a tweet: “More lies con job Propaganda bye failing failing CNN garbage.”

But you don’t have to be this unhinged to be moved to violence by incendiary rhetoric. Just about any of us could be susceptible under the right conditions.

Susan Fiske, a psychologist at Princeton, and colleagues have shown that distrust of an out-group is linked to anger and impulses toward violence. This is particularly true when a society faces economic hardship and people are led to see outsiders as competitors for their jobs.

Mina Cikara, a psychologist at Harvard and a co-author of that study, told me that “when a group is put on the defensive and made to feel threatened, they begin to believe that anything, including violence, is justified.”

There is something else that Mr. Trump does to facilitate violence against those he dislikes: He dehumanizes them. “These aren't people,” he once said about undocumented immigrants suspected of gang ties. “These are animals.”

Research by Dr. Cikara and others shows that when one group feels threatened, it makes it much easier to think about people in another group as less than human and to have little empathy for them — two psychological conditions that are conducive to violence.

A 2011 study by Dr. Fiske and a colleague looked at “social cognition” — the ability to put oneself in someone else’s place and recognize “the other as a human being subject to moral treatment.” Subjects in the study were found to be so unempathetic toward drug addicts and homeless people that they found it difficult to imagine how those people thought or felt. Using brain M.R.I., researchers showed that images of members of dehumanized groups failed to activate brain regions implicated in normal social cognition and instead activated the subjects’ insula, a region implicated in feelings of disgust.

As Dr. Fiske has written, “Both science and history suggest that people will nurture and act on their prejudices in the worst ways when these people are put under stress, pressured by peers, or receive approval from authority figures to do so.”

So when someone like President Trump dehumanizes his adversaries, he could be putting them beyond the reach of empathy, stripping them of moral protection and making it easier to harm them.

If you still have any doubt about the power of political speech to foment physical violence, consider the classic experiment by the Yale psychologist Stanley Milgram, who in the early 1960s studied the willingness of a group of men to obey an authority figure.

Subjects were told to administer electrical shocks to another participant, without knowing that the shocks were fake. Sixty-five percent of the subjects did what they were told and delivered the maximum shock, which if real could have been fatal. The implication is that we can easily be influenced by authority to do terrible harm to others — just by receiving an order.

Now imagine what would happen if President Trump actually issued a call to arms to his supporters. Scared? You should be.

Richard A. Friedman is a professor of clinical psychiatry and the director of the psychopharmacology clinic at the Weill Cornell Medical College, and a contributing opinion writer.