People hold candles at a vigil in San Bernardino, California, December 7, 2015.
Mike Blake / Reuters

Fears about terrorism may have been heightened by the attacks in Paris and in San Bernardino. But they were already high even before these events and before the rise of the Islamic State (ISIS) in 2014. Public opinion poll data show that the United States suffers a long-term, routinized, mass anxiety—or at least a sense of concern—about terrorism that has shown little sign of waning in the years since 2001. Although other issues—particularly economic ones—often crowded out terrorism as a topic of daily concern, terrorism has won an apparently permanent space in the American mind.

For example, over 70

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