ON SEPT. 11, 2001, THE VIOLENCE COMMITTED ON U.S. SOIL was unexpected and unprecedented. But in the 15 years since then, acts of terrorism within these borders have become dishearteningly routine. The loss of life in Boston, Charleston, San Bernardino, Orlando and elsewhere has brought heartache as well as an uncertainty about where, if anywhere, we might be safe.

Samuel J. Sinclair was just starting his doctoral studies in 2002, and began to look at terrorist attacks—both how such attacks affect people psychologically, and how exposure to them through the media may play an important part. His work has focused on the role of fear specifically, and the various ways that worry about new acts of terrorism can affect behavior. That research led him to launch both the Society for Terrorism Research and the journal Behavioral Sciences of Terrorism and Political Aggression. Formerly at Massachusetts General Hospital, Sinclair will be joining the faculty of William James College in Newton, Mass., this fall.

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Q: What did the early studies about 9/11 have to say?

A: These studies revealed a spike in specific kinds of psychiatric disorders—post-traumatic stress disorder, depression, substance abuse in the year following the attacks. After 12 months, however, these studies also began to demonstrate a normalizing trend—where rates of these disorders began to return to baseline, with a few exceptions. For example, people struggled for longer periods of time in those immediate areas that were attacked—lower Manhattan, Washington, western Pennsylvania. But over time, even these areas also began to return to baseline. In spite of these normalizing trends, however, our research demonstrated that people were still very afraid.

Q: What did other data suggest?

A: In the years after 9/11, people were still very scared. A series of USA Today/Gallup polls from 2001 to 2006 found that almost half of the respondents were still afraid of another terrorist attack. That fear was a different kind of aftereffect, one that wasn’t measured in classic disorders. A psychologist, Philip Zimbardo, actually coined the term “pre-traumatic stress syndrome.” It’s the idea that people are negatively affected by what they think might happen. So we decided to look at this fear and what it predicts: how people change routines in their lives, including where they would decide to work or whether they’d move to a more rural area.

Q: How do you measure fear of something that hasn’t happened?

A: There weren’t great tools. So in 2006 I developed the Terrorism Catastrophizing Scale, a way to measure fears a person might have about a possible attack. In a U.S. general population sample of 503 adults fielded in 2006, we found that only 0.2% of individuals were unafraid, meaning that 99.8% of people reported at least some fear of terrorism. These fears were also strongly associated with symptoms of depression, anxiety and physiological stress. And they were found to be correlated with different types of avoidance behaviors—for example, avoiding public places like malls or restaurants, public transportation, and living or working in populated areas. These fears were also associated with avoiding terrorism media coverage and not socializing with people perceived to be different in terms of cultural background.

Q: What are some of the more serious behavior changes you’ve looked at?

A: Right now we’re very curious about how fear of a future terrorist attack is associated with how people vote or become engaged in political life. A number of studies found that trust in government spiked after 9/11, and we wanted to see whether fear of terrorism was a factor. In a study of undergraduates in 2003, we did find a significant association. The more scared respondents were, the more likely they were to trust government to keep them safe.

Q: Are you looking at those factors in this election cycle?

A: Yes, we’re currently looking at new data that explore the correlation with specific government policies. How does a fear of a coming terrorist attack correlate, for instance, with support for Second Amendment rights, or whether we should accept Syrian refugees, or whether to exclude Muslims from entering the country?

Q: Is that kind of fear a good or bad thing?

A: There’s nothing wrong with being afraid, of course. As I said, most people are afraid of terrorism. But I would hope that when we make national policies on these complex topics, we are guided by logic and intellect. Making decisions based on what fear tells us to do probably doesn’t lead to the best policy.