9/11 brought America into a new era of fear. As Vox’s Alvin Chang explains, after the attacks, American’s level of fear stayed permanently elevated, despite the fact the world has become arguably safer.

It’s a testament to the subtle psychological power of fear. Events like 9/11, like the Boston Marathon bombing, like the Paris massacre, like the shooting at the Pulse nightclub in Orlando, all have the power to change the way we see the world and think about others.

In the wake of several more terrorist attacks, I’ve spoken to a number of psychologists and asked them all the same question: What does that fear do to our minds?

This is what I learned.

1) Fear heightens our suspicions of the world and makes us more pessimistic about the future

Fear colors everything we encounter with a tinge of dread. That impacts our behavior and even our politics in surprising ways.

“Fear causes higher perception of risk everywhere, greater precautionary behavior, greater favorability of action policy that prioritizes safety over personal liberty," Deborah Small, a psychologist at the University of Pennsylvania who studies decision making, told me in 2015.

In 2002, Small was part of a research effort at Carnegie Mellon University that asked: Could reminding participants of their 9/11 fears change their outlook on America?

In a field experiment, Small and her colleagues either stoked people’s fears of 9/11 or anger about it via a writing prompt. (Anger, they thought, would lead to different behaviors than fear.) Then, the participants were asked to agree or disagree with prompts about the future, such as “Safety in airline travel will improve dramatically…” and “Another major terrorist attack will occur within the next 12 months.”

Their conclusion: “Experiencing more anger triggered more optimistic beliefs; experiencing more fear triggered greater pessimism.”

2) Fear makes us more suspicious of foreigners

Just hours after news broke that three explosions in Brussels, Belgium, claimed many lives in a terrorist attack in March, Donald Trump described exactly what he would do to protect the fearful. He told Fox News: "I would close up our borders.”

Sen. Ted Cruz issued a statement not long after: "We need to immediately halt the flow of refugees from countries with a significant al-Qaida or ISIS presence."

These reactions are familiar. These politicians — and others — made similar statements after the November terrorist attacks in Paris killed 130. At that time, the fear was that ISIS attackers could sneak into countries alongside refugees. (It turned out all of the identified Paris attackers were European.)

But these fear-filled admonitions expose an unchanging truth: When global terror threats fill the news, lines between "us" and "them" get drawn. And Muslim immigrants, Syrian refugees and any other potential refugees or migrants may unfairly get labeled as "them" in the aftermath.

"When attacks happen, there’s a [perceived] high cost in mistaking in-group, out-group members for one another," says Mina Cikara, who runs the intergroup neuroscience lab at Harvard. I had spoken to Cikara — and other psychologists who study group fears — in the wake of the Paris attacks in November. "So when you see an attack like the one in Beirut or the one in Paris, it highlights those boundaries between 'us' and 'them.' It means those boundaries become more closely circumscribed; they become tinier."

It also means, Cikara says, that we overexaggerate our perceptions of who "they" are. Innocent Syrian refugees — or peaceful immigrants with a Muslim upbringing — can get wrapped up in our notions of who is a terrorist, no matter what the statistics say.

"Threat is the catalyst that shifts us from out-group disregard or indifference to out-group hostility," she says.

3) When we fear outsiders, we dehumanize them

You can think of human psychology as a series of evolutionarily coded computer programs. These programs tell us how to react to new situations. When we see a baby, we want to pinch its cheeks. When we see a threatening stranger, we either want to fight them or flee from them.

Fearing others changes the way we perceive them. In experiments, this plays out in very literal and disturbing ways: In some experiments, psychologists can get participants to rate outsiders as having fewer human qualities.

4) And our brains exaggerate their threat

New York University psychologist Jay Van Bavel and his colleague Y. Jenny Xiao illustrated this concept nicely in a 2012 paper.

The test was simple: The researchers asked participants to estimate the straight-line distance from New York to Mexico City. Participants who expressed more animosity toward Mexican immigrants rated Mexico City as being several hundred miles closer to New York than people who felt less threatened. And, Van Bavel adds, "We have new data showing that if people think the wall between the countries is secure, this effect goes away."

5) Fear likely makes our stalemate politics even more stagnant

"The threat of terrorism — like many sources of threat, can lead to more rigid thinking — less attention to ambiguity," Robb Willer, a sociologist at Stanford University, told me after the Orlando shooting in 2016.

This can be a good thing in a chaotic situation, as it makes more economical use of our mental resources, narrowing our focus. And there is plenty of rich public conversation today around gun control and a more inclusive America.

It’s also true that, at times, terrorism and other horrific events have brought the country together, Willer notes. Think of the days after 9/11 when George W. Bush’s approval ratings were near 90 percent.

But lately, these moments haven’t become opportunities to build bridges with political opponents. Instead, attacks have highlighted divisiveness.

Ingrid Haas, a political psychologist at the University of Nebraska Lincoln, studies the intersection of two emotions that typically flare in the wake of a terrorist attack: uncertainty and fear.

These are the two psychological weapons terrorists most effectively yield: making us feel threatened going about our daily lives in the places where we feel safe, and the feeling of not knowing where an attack might occur.

Haas predicts that when we feel both uncertain and threatened, we become more intolerant of political opponents (this is true of both liberals and conservatives). She found this in a series of experiments published in a 2014 paper in Political Psychology. "The fact that the exact nature of the threat is … unclear means that people have a difficult time coping with it," she told me in one of our conversations. With that difficulty coping, she finds, people become less accepting of new ideas.

She recently followed up on this research in a paper published in May in Basic and Applied Social Psychology. It found that the combination of uncertainty and threat makes people — and especially conservatives — less willing to compromise. "This may be reason to believe that political conservatives are especially likely to respond to the (uncertain) threat of terrorism by decreasing their willingness to compromise about political issues," she wrote me in March.

From a biological standpoint, too, it makes sense that terrorist attacks can polarize the country. Some of us are just hardwired to respond more strongly to negative stimuli.

A lot of this work on the biological underpinnings of politics comes from the lab of John Hibbing and Kevin B. Smith at the University of Nebraska. I wrote about their work in a 2014 story for National Journal:

In his experiments, Hibbing often attaches electrodes to liberal and conservative participants' skin and then shows them disturbing images, such as a man eating a handful of worms. In these tests, conservatives sweat more (i.e., have a stronger gut reaction) in response to the disgusting stimulus. And when Hibbing hooks participants up to eye-tracking machines, he finds conservatives monitor more closely the things that make them squirm. So they are more readily provoked and more vigilant

Liberal commentators sometimes use Hibbing and Smith’s research to belittle conservatives and say they are too reactionary and less measured. Another way to look at it is that evolution has perhaps favored human communities wherein liberal minds and conservative minds are in constant tension. In our history — stretching tens of thousands of years — it’s likely each strategy has proven its merits in aiding our survival.

6) Fear makes us bad decision-makers

In the 2000s, neuroscientist Gregory Berns ran experiments during which participants were intermittently shocked while having their brain scanned in an fMRI machine.

The shocks occurred every one to 30 seconds, without warning.

“Nearly a third feared waiting so much that, when given the chance, they preferred getting a bigger shock right away to waiting for a smaller shock later,” Berns wrote on his experiment in the New York Times. “It sounds illogical, but fear — whether of pain or of losing a job — does strange things to decision-making.”

Here’s an example of that playing out in the real world.

After 9/11, there was an understandable — but irrational — fear of flying. Accounting for the terrorist attacks, flying was still much safer than driving a car. (People are still afraid of flying because of 9/11. In a 2011 poll — 10 years after the attack — Gallup found 24 percent of respondents agreed that 9/11 made them less willing to fly on airplanes.) There were 42,116 motor vehicle deaths in 2001. Many who would have flown in near-perfect safety died on roadways as a result of the fear.

Gerd Gigerenzer, the director of the Max Planck Center for Adaptive Behavior and Cognition, labels this the “indirect damage” of terrorism. “Indirect damage is not under the control of terrorists; it is mediated through the minds of citizens,” Gigerenzer wrote in a 2006 paper. In it, he estimates that 9/11 led to an additional 1,505 fatal road crashes in the 12 months after the attacks. (Gigerenzer also analyzed transportation data from Spain after the 2004 terrorist attacks on Madrid’s railways, and found that while Spaniards reduced their train travel, they did not compensate with additional road travel.)

7) How can we fight fear?

It’s not wrong to be fearful. Fear is a normal human reaction. It can help us survive. But it can be abused by politicians hoping to make political gains.

The researchers I spoke with over the past years have some suggestions for managing the uglier reactions to fear.

One: The passage of time helps in reducing negative emotions. Haas offered a very practical solution in the wake of the Paris attack. "Maybe a better recommendation is just that we should wait a little while for the emotional response to die down a bit before making major decisions about changes to immigration policy, escalation of war," she says.

"It's going to be difficult for people to make well-reasoned decisions when the emotional response is so strong. This is probably better advice than trying to get average citizens to digest statistical information in the aftermath of a terrorist attack, which is usually what professors like to recommend.”

Two: Know that even though fear can heighten our prejudices, there are ways to overcome them.

“These [psychological] systems are flexible, they can turn on a dime,” Cikara told me. "if that’s true, they can also improve on a dime.”

In April, I reported on an experiment that showed with just the right dose of empathy, canvassers could change voters’ mind on transgender rights issues. "Two decades of opinion change took place during a 10-minute conversation, and it persisted for at least three months — that's a big effect," Josh Kalla, one of the experiment’s co-authors, told me. All it took was a conversation where the voter was asked to put themselves in a transgender person’s shoes.

That type of outreach is difficult, but not impossible.