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 percent of those polled in late 2001 believed that “another terrorist attack causing large numbers of Americans to be lost” was likely. The figure was roughly the same just before the rise of ISIS. Along the way, it was temporarily pushed up by some ten percentage points by the London attacks of 2005, and a similar rise occurred after the recent Paris attacks. Similarly, at the end of 2001, 35 to 40 percent professed to be worried about becoming a victim of terrorism, a level that has held ever since.

Poll data on beliefs about which side was winning the war against terrorism has bounced around quite a bit, particularly in response to the wars in Afghanistan and Iraq. Overall, though, public opinion was almost exactly the same in 2011 as it was ten years earlier. The percentage maintaining that terrorists remain capable of launching “another major attack” was, if anything, higher in 2013 and 2014 than in 2002.

Increased spending on domestic homeland secu­rity since 9/11 has totaled well over $1 trillion, and efforts to chase down and eliminate terrorists abroad have cost trillions more. These extraordinary expenditures have utterly failed to make people feel safer. In 2013 and 2014, Americans were more likely to deem their country to be less safe than before 9/11 than they were a decade or so earlier. The rise of ISIS has pushed that figure even higher.

These results suggest that the public has internalized the impact of 9/11, and there has been no erosion of fear even though concerns about another 9/11 have failed to pan out, Osama bin Laden has been dispatched, and an American’s chance of being killed by a terrorist has been, and remains, one in four million per year with 9/11 included in the calculation, or one in 110 million for the period since 2001.

Terrorist attacks haven’t always had such far-reaching psychological consequences. Fear about terrorism declined rapidly after the Oklahoma City bombing of 1995, for example. The difference with the post‑9/​11 era seems to be that those perpetrators—whether from al Qaeda or now ISIS—are seen as being connected to a spooky and persistent foreign adversary.

In many respects, the best parallel is with fears that lingered for decades about the dangers presented by domestic communists, who were seen as being linked to a hostile international movement. As FBI Director J. Edgar Hoover put it in the 1950s, the American Communist Party was working “day and night” with “deadly seriousness” to seize power in the country in a process that was “virtually invisible to the non-communist eye.” His dire warnings struck a responsive chord.

Although politicians and other leaders could and should be doing much more to put the terrorism threat in its relevant context, it is not at all clear that doing so would allay public fear very much. Communism could convincingly die out. However, even if ISIS were to be destroyed as a meaningful force in the Middle East, its image could continue to inspire not only real terrorist attempts, but imaginary ones as well.