A float in the Redlands Christmas Parade passes by a Riverside Press Enterprise newspaper rack on Dec. 5. (For The Washington Post)

Suicide bombers and gunmen terrorized Paris last month, murdering 130 people at a stadium and a music venue and throughout cafes and restaurants in the French capital. For Americans, horror hit closer to home two weeks later at a Planned Parenthood clinic in Colorado Springs.

The United States has seen terror before, most vividly on Sept. 11, 2001, but recent, random rampages have stirred up fears across the country.

After two attackers -- since linked to Islamic extremism -- opened fire at a social services center in San Bernardino, Calif., killing 14 and injuring many more earlier this month, President Obama tried to ease the nation's anxiety.

“The threat from terrorism is real, but we will overcome it,” Obama said Sunday during a 14-minute Oval Office address.

“Let’s make sure we never forget what makes us exceptional," he added. "Let’s not forget that freedom is more powerful than fear.”

Like the enduring line from Franklin Delano Roosevelt's 1933 inaugural address — "the only thing we have to fear is fear itself" — Obama's declaration was meant to allay the country's growing anxieties. Americans are more afraid of a terrorist attack than at any other time since the weeks after Sept. 11, 2001, according to a New York Times/CBS News poll released last week. There is, the Times noted, "a gnawing sense of dread" now sweeping across the country following the recent string of slayings.

Indeed, research shows fear of terrorism has hit its highest point in 10 years, The Washington Post's Philip Bump reported earlier this week. In the wake of recent terror attacks, a Gallup survey found that about one in six Americans now name terrorism as the most important issue in the country — up 13 points, from 3 in early November to 16 percent.

With terrorists abroad and at home successfully spreading fear, Obama's declaration presents an interesting scientific question: Is freedom truly more powerful than fear?

“Fear and freedom are an ongoing battle in life,” said Tom Pyszczynski, a psychology professor at the University of Colorado at Colorado Springs. “To be free involves facing your fears. But at times, fear prevents people from exercising their freedom.

“Freedom is nobler — it’s a core value of American culture. But when people are terrified, they are typically willing to give up their freedoms.”

Fear is a powerful force. Some research suggests that fear can cause serious health problems — including death — and can also move to a sustained state of anxiety.

But has benefits, too. It’s an alarm system to warn people when they are in danger.

The fear system in the brain is thought to center on an area called the amygdala within the temporal lobes, said Shmuel Lissek, founding director of the ANGST Laboratory at the University of Minnesota. Within milliseconds, a signal travels through the brain's early visual processing stages to the amygdala, alerting the brain to a potential threat. It’s a pathway that Lissek and other researchers call “quick and dirty route” to experiencing fear.

The body’s cortical processing then takes charge, helping to discern actual threats from false alarms, Lissek said. If there is no threat, he said, it shuts off the fear system.

“It may be very unlikely that any one of us will be impacted by a terrorist threat,” he said. “It may be likely that it will happen on American soil, but it’s very unlikely that it will happen to any one of us. But we are hardwired to false alarms over misses.

“Terrorism works because it capitalizes on our tendency to favor false alarms. It leads populations to be fearfully to those situations ... even though you’re only victimizing a tiny fraction because we have a tendency to take improbable threats seriously.”

A recent Public Religion Research Institute/Religion News Service poll found that nearly half of Americans surveyed are concerned that they or someone in their family will be a victim of terrorism.

According to the new Times/CBS poll, 44 percent of respondents believe another terror attack is "very likely" to happen in the United States within the next few months. Nearly 80 percent of the public thinks another attack is "very likely" or "somewhat likely," it said.

The front page of the Dec. 7 edition of Express. The front page of the Dec. 7 edition of Express.

And a survey on fear from Chapman University showed that terrorism was already a top concern in the United States, even before the recent attacks. Out of a random sample of more than 1,500 adults, 44 percent said they feared terrorist attacks.

Research shows that when terror strikes and fear takes hold, emotions can lead people to cling to their cultures, beliefs and values to protect themselves from the things they fear the most -- mainly death. Likewise, this defense mechanism can also lead people to become hostile toward people or ideas that threaten their way of thinking.

Following the 9/11 attacks, the United States saw an increase in anti-Muslim sentiment. “Islamophobia” -- a bias against those who are Muslim -- has remained a part of American culture. In the wake of the shootings in San Bernardino, the Council for American-Islamic Relations, said there has been a rise in anti-Muslim hate crimes.

On Monday, GOP presidential hopeful Donald Trump proposed a plan to close U.S. borders to Muslims, a plan that has been likened to the World War II internment of Japanese residents in the United States.

“These ideas are aimed at providing what people think will make them safer — things that make them feel secure,” said Pyszczynski, who co-authored a book on the subject, “In the Wake of 9-11: The Psychology of Terror.”

Pyszczynski has spent years researching a terror management theory — the idea that when confronted with fear, people have a tendency to embrace those with the same ideals and reject those that threaten them.

According to a 2004 study by Pyszczynski:

Cultural worldviews manage existential terror by providing a meaningful, orderly, and comforting concept of the world that helps us come to grips with the problem of death.

That study also stated:

When others view the world or ourselves in the same way that we do, it suggest that our view is right: the world really was created in seven days, ad we really are virtuous and valuable when we give money to charity; or as members of a Christian church in Kansas believe, one can increase one’s chance of literal immortality by picketing outside a murdered gay college student’s funeral with signs proclaiming that “God Hates F------.” …

But again there’s a rub: when we encounter others who view the world or ourselves in ways that are different from our own views, this threatens our faith in these constructions and undermines their effectiveness as buffers against existential anxiety. As a result, we need to avoid such disagreements at all cost, and when we do encounter them, we need to put these down to minimize the threat posed by their differing views of reality.

Pyszczynski said this is the idea behind terrorism — to polarize people.

“People seem to be focusing on Islamic terrorism and forgetting there are many other type of mass murders from deranged individuals,” he said. “ISIS is hoping for an epic clash of civilization — and it’s working. Terrorism creates alienation among people they are targeting, making minority groups like Muslims feel alienated from civilization and more susceptible to radicalization.”

Experts say the initial threat of terrorism ignites fear; but if the fear is sustained, that's anxiety.

Lissek, the ANGST Laboratory director, said fear can promote survival when a threat is genuine. But it can also get in the way when a threat is not.

If an American combat soldier in Iraq is afraid of roadside objects, that’s a genuine fear, Lissek said, because roadside objects might blow up that soldier. However, if a veteran has returned home and is still afraid of roadside objects to the point where it keeps him from driving, that’s anxiety, he said, because the veteran has come to fear the world around him.

Anxiety, Lissek said, is a sustained arousal that is maintained over time.

The amygdala area in the brain, which researchers believe is important for fear, works as a short-term fear response system. But, Lissek said, when the threat lingers and the fear response needs to be maintained, it gives the job to the bed nucleus of the stria terminalis. There, he said, fear becomes anxiety.

“When you’re watching the news and see the Paris attacks, that’s a fear response,” he said. “But if you then start saying to yourself, ‘Well, threats in this world are unpredictable and I never know when I’m safe,’ that’s more sustained anxiety.”

But as far as whether freedom is really more powerful than fear, Lissek said, maybe not.

“Would somebody be able to overcome the fearful motivation by valuing something more strongly than fear?” he said. “I think it takes a very strong person to be able to sacrifice themselves for a higher idea. It’s definitely not the average situation.”

This story, originally published Dec. 11, has been updated.

MORE READING:

A reminder: Terrorists want us to be terrorized

Yes, mass shootings tend to produce copycats. So do terror attacks.

The San Bernardino shooting is the 355th mass shooting this year

People who live near mass shootings are less likely to vote Republican

You’re more likely to be fatally crushed by furniture than killed by terrorists.