On Friday, President Donald Trump stood before law enforcement officers in Long Island, New York, and warned them about the “animals” they were up against.

Trump’s speech to the Long Island police officers was long and chilling. Throughout his remarks, the president repeatedly advocated for violent policing. He also greatly exaggerated the threat of crime in the New York City suburbs, lamenting how Long Island’s parks have become “bloodstained killing fields.”

But most chilling was his description of the Latino gang members he singled out as the culprits: “I was reading one of these animals was caught and explaining they like to knife them and cut them and let them die slowly because that way it's more painful. And they enjoy watching that much more.”

This language is troubling because — as Vox’s Dara Lind writes — Trump doesn’t clearly differentiate between criminal and peaceful immigrants living in the United States, nor does he care to. But Trump’s language is also dangerous because it’s blatantly dehumanizing.

When we refer to people as “animals” or anything other than “people,” it flips a mental switch in our minds. It allows us to deny empathy to other people, makes us feel numb to their pain, and lets us forgive ourselves for causing them harm.

At best, the dehumanizing language in Trump’s Long Island speech tells law enforcement they’re superior to these “animals,” which “justifies or even mandates violence,” Nour Kteily, who studies the psychology of dehumanization and its consequences at Northwestern, told me in an email.

“At worst,” Kteily said, “it communicates that message more broadly to the most fervent of the white supremacists who number among the president’s supporters.” These are people who — like Trump — may often conflate the words “immigrant” and “criminal.”

Dehumanization is already disturbingly prevalent in America. We don’t need anyone — especially Trump — stoking it further. And “dehumanization today (toward certain groups) has been anything but subtle,” Kteily said.

Dehumanization is a mental loophole that allows us to dismiss other people’s feelings and experience

If you think of murder and torture as universally taboo, then dehumanization of the “other” is a psychological loophole that can justify those acts.

Look back at some of the most tragic episodes in human history and you will find words and images that stripped people of their basic human traits. During the Nazi era, the film The Eternal Jew depicted Jews as rats. During the Rwandan genocide, Hutu officials called Tutsis “cockroaches” that needed to be cleared out.

In the wake of World War II, psychologists wanted to understand how the genocide had happened. In the 1970s, Stanley Milgram’s infamous electroshock experiment showed how quickly people cave to authority. Also in that decade, there was Philip Zimbardo’s “prison experiment,” which showed how easily people in positions of power can abuse others.

At Stanford in 1975, Albert Bandura showed that when participants overhear an experimenter call another study subject “an animal,” they’re more likely to give that subject a painful shock.

From these experiments and others that followed, it became clear that “it’s extremely easy to turn down someone’s ability to see someone else in their full humanity,” Adam Waytz, a psychologist at Northwestern University, told me earlier this year.

Dehumanizing sentiment already exists in America. Encouraging it will likely make the country a more hostile place.

In Kteily’s studies, participants — typically groups of mostly white Americans — are shown this (scientifically inaccurate) image of a human ancestor slowly learning how to stand on two legs and become fully human.

And then they are told to rate members of different groups — such as Muslims, Americans, and Swedes — on how evolved they are on a scale of 0 to 100.

You’d hope people would rate all groups at 100 — perfectly human, right?

They don’t. Mexican immigrants and Muslims are routinely dehumanized in these studies, scoring, on average, well below 90.

This is concerning not only because you have one human rating another as “less than,” but because a willingness to dehumanize is correlated with anti-immigrant actions and behaviors.

“Individuals who dehumanized Mexican immigrants to a greater extent were more likely to cast them in threatening terms, withhold sympathy from them, and support measures designed to send and keep them out, such as surveillance, detention, expulsion, and building a wall between the United States and Mexico,” Kteily and a co-author wrote in a 2017 paper.

And that’s only part of the problem.

In his studies, Kteily also looked at what happens when people feel like they’re being dehumanized. And here, the research predicts a vicious cycle.

“Those who dehumanize are more likely to support hostile policies, and those who are dehumanized feel less integrated into society and are more likely to support exactly the type of aggressive responses ... that may accentuate existing dehumanizing perceptions,” he wrote in the 2017 paper.

As the vicious cycle intensifies, the whole country becomes a more hostile, less safe place for everyone.

Do Trump’s dehumanizing views trickle down?

Trump holding dehumanizing views is concerning enough. He’s president of the United States, with enormous power to make life better or worse for immigrant communities.

But psychological research also suggests that Trump’s rhetoric encourages people who already have prejudicial views to act upon those views.

“I don’t think Trump created new prejudices in people — not that quickly and not that broadly. What he did do is change people’s perceptions about what is okay and what is not okay,” University of Kansas psychologist Chris Crandall told me in March.

In 2016, Crandall and his student Mark White asked 400 Trump and Clinton supporters to rate how normal it is to disparage members of various marginalized groups — like obese people, Muslims, Mexican immigrants, and disabled people — both before the election and in the days after.

After the election, both Clinton and Trump supporters were more likely to report that it was acceptable to discriminate against these groups. For Trump to say the disparaging things he said during the campaign, such as that Mexico was sending over rapists — and then be rewarded for it by winning — sent a powerful sign.

“It took away the suppression from the very highly prejudiced people,” Crandall said. “And those are people acting.”

Crandall’s results are preliminary (i.e., not yet published in a peer-reviewed scientific journal), but they’re reflective of the established literature: Exposure to misbehavior simply makes it more acceptable.

A more recent working paper in NBER looked into this question further. Its authors wondered if Trump’s election acted as a validation of anti-immigrant sentiment. That if someone could become president while stoking xenophobia — building walls, restricting immigration, trumpeting “America first,” etc. — would that make xenophobia more socially acceptable?

And it turns out the answer is yes: More participants in the study became willing to openly donate money to an anti-immigrant organization after the election. (Before the election, too, more participants were more willing to openly donate if they were told Trump’s victory was assured in their state.)

Why we need to be vigilant against dehumanizing language

Inside all of us is the same mental machinery that fueled the atrocities of the past century. “We think others to death and then invent the battle-axe or the ballistic missiles with which to actually kill them,” writes the philosopher Sam Keen.

That’s why we can’t kid ourselves into believing that dehumanizing language is harmless.

Dehumanization, and increasing acceptance of prejudice, won’t immediately lead to atrocities — but it will make it easier to make life worse for the marginalized.