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Bostonian Anna Guidi drew the letter "B" surrounded by a heart on Thursday, April 18, 2013, while contributing to a large sidewalk mural on Newbury Street in Boston, Massachusetts, in the wake of bombings at The Boston Marathon. (Patrick Whittemore/Boston Herald/MCT)

As we all try to make sense of the tragic Boston Marathon bombing, it is essential to stop and consider how such events affect us.

How can we continue to live meaningfully and positively in a world where terrorism is a reality? How did this happen? Who would want to cause such harm? Can we ever return to a sense that our communities are safe and secure?

As a researcher who studies the causes of depression and anxiety, we know tragic and unexpected events like the Boston bombing have the pernicious effect of disrupting our individual and collective sense of security. In particular, there are two kinds of consequences that are important to understand.

One is that when something like a bombing or shooting occurs, its sheer intensity and horror provoke us to believe that it is more common, and more likely, than it actually is. As terrible as the Boston Marathon bombing was, national and international security experts point out that it is still extraordinarily rare.

Do we need to continually learn and improve our ability to protect ourselves and balance individual freedoms with collective security? Yes. But should we change how we go about our daily lives because of such a rare event? No. That would not only be unwise, but would be capitulating to the intentions of the perpetrators, which should always resist.

The other effect of such a tragedy is that our personal sense of safety is disrupted -- we are reminded that the world is never going to be a completely safe and benign place. The images were striking and unmistakable. One second, the finish line was full of joyous reunions and exultations; the next, it was the closest many of us will see to a war zone.

Although such a catastrophe very rarely happens, we must acknowledge that it can, and not allow ourselves to presume that our cities and neighborhoods could never be affected by tragedy.

So how do we live positively and effectively in an unsafe world, even as we continue to send our prayers and good wishes to the people in Boston? Fortunately, what helps us to regain a reasonable sense of safety is when we see the heroic actions of first responders and bystanders in such an event. We saw how many people ran toward the danger rather than away from it. We witnessed the astonishing heroic acts, large and small, that saved countless lives.

As a result, we can be certain that if we were to be involved in such an unpredictable event, our fellow citizens would surely respond with extraordinary courage -- just as we would if we were the bystanders and they the victims.

Many of us recall the "duck and cover" drills that school children practiced during the Cuban missile crisis. As a child back then, I remember that the world felt profoundly unsafe -- an inevitable byproduct of mutually assured destruction as international policy.

We know that our country is safer now. We know that we share a profound commitment to each other as Americans and a common resolve to live our highest values in defiance of terrorism.

So yes, we will feel disrupted, disoriented and disturbed by this event. But we also know that we will continue to survive and thrive as a nation and that we will not let our way of life be fundamentally altered.

Timothy Strauman is a professor of psychology and neuroscience at Duke University.