The question can be answered fairly directly. From 1970 to the present, a period that includes, of course, the 9/11 attacks, the odds an American in the United States will be killed by a terrorist (of any sort) are 1 in 4 million per year. For the period since 9/11, the odds are about 1 in 80 million per year. By comparison, the odds an American will be killed in a traffic accident are about 1 in 8,000 per year.

The issue then becomes the one posed shortly after September 11 by risk analyst Howard Kunreuther: “How much should we be willing to pay for a small reduction in probabilities that are already extremely low?” That question can be used as a basis to briefly consider the efficacy of U.S. counterterrorism expenditures.

Questions like Kunreuther’s are too rarely asked within the U.S. government. In 2010, a careful assessment by a committee of the National Academy of Sciences concluded that domestic counterterrorism funds were being expended without serious analysis of the sort routinely required in other areas of government, or even the sort carried out by the Department of Homeland Security (DHS) itself for natural hazards. The committee said it could not find “any HS risk analysis capabilities and methods” adequate for supporting its counterterrorism decisions. The report seems to have generated no media coverage at all.

The situation about which the committee warned continues. Extensive and transparent efforts to evaluate the counterterrorism expenditures in the United States are overdue. Instead, alarmist perspectives have essentially been maintained by the department, and the vast and hasty increases in spending on homeland security have been perpetuated.

It is possible to apply standard cost–benefit and risk‐​analytic procedures of the sort the National Academy committee called for. These procedures have been developed, codified, and increasingly used as an aid in responsible decisionmaking for the past few decades—or in some respects, for centuries. They have been applied to a wide variety of hazards, including ones that are highly controversial and emotive, such as pollution, chemical power plant accidents, and exposure to nuclear radiation and environmental carcinogens. And they have been required by executive order at least since the 1980s.

One of these approaches involves the concept of “acceptable risk,” a phrase that has been almost entirely neglected in discussions about counterterrorism expenditures. In practice, risks tend to be deemed acceptable if they cause death for fewer than 1 in 1 million or, in some studies, 1 in 2 million people per year. Hazards that fall into the unacceptable range (traffic accidents, for example) should generally command the most attention and resources, whereas little should be spent to combat hazards in the acceptable range—drowning in bathtubs, for example. The latter should be viewed as a risk we can live with, where further precautions would scarcely be worth pursuing unless they are quite inexpensive.

Terrorism presents a threat to human life in the United States that is much less of a risk than ones we have essentially agreed to accept. And efforts, particularly expensive ones, to further reduce its likelihood or consequences are unjustified. Diverting even a few billion dollars from the homeland security budget toward more smoke alarms, additional tornado shelters, greater car safety, and other effective lifesaving measures would save far more lives.

Another approach is to calculate how many lives domestic counterterrorism expenditures would need to save to be justified. Following widely applied procedures, a study for DHS concluded that the best estimate of the value of a saved human life for homeland security measures would be about $15 million in the case of terrorism. Under that stipulation, domestic counterterrorism spending would be worthwhile if it deterred, disrupted, prevented, or protected against some 4,800 terrorism deaths in the country each year. This figure seems to be very high: Islamist extremist terrorists in the United States have killed about five people a year since 9/11, and far‐​right and racist terrorists have killed about seven or eight per year.

Still another approach would apply a full cost–benefit analysis to determine how many terrorist attacks the increase in expenditures since 9/11 would have had to deter, disrupt, or protect against to be justified. The number turns out to be quite high: 150 attacks like the Boston Marathon bombing each year—or about one every other day. Or 15 attacks per year like the one in 2005 on the London transportation system—more than one a month. Or about one 9/11 attack every three years. Similarly, the protection of a standard office‐​type building would be cost‐​effective only if the likelihood of a sizable terrorist attack on the building is a thousand times greater than it is at present. Assessed on their own, some specific security measures, such as hardening airline cockpit doors, do seem to be cost‐​effective, whereas others, like the Federal Air Marshal Service, don’t.

A defender of current spending might argue that the number of deaths from terrorism is low primarily because of the counterterrorism efforts. However, while the measures should be given some credit, it is not at all clear that they have made a great deal of difference.

To begin with, the people prosecuted on terrorism charges in the United States do not appear to be all that capable. An assessment by RAND Corporation’s Brian Jenkins is apt: “Their numbers remain small, their determination limp, and their competence poor.” Left on their own, it seems likely that few, if any, of them would have actually been able to cause much damage.

In addition to those prosecuted on terrorism charges, authorities have encountered a considerable number of people who seem to be aspirational terrorists. Lacking enough evidence to convict these individuals on terrorism charges, prosecutors have levied lesser ones to put, or send, these would‐​be perpetrators away. However, these people are even less likely than those charged with terrorism to carry out attacks.

Finally, it is often argued that many terrorists have been deterred by security measures. It is true that an array of extensive and very costly security measures may have taken one set of targets—commercial airliners—off the target list for just about all terrorists. The same might be said for military bases in the United States, which would otherwise be favored targets since a primary motivation for much terrorism has been outrage at U.S. foreign and military policy.

Nevertheless, a dedicated terrorist should have little difficulty finding other potential targets if the goal is to attack crowds, destroy property, or get attention: potential targets are everywhere. Actually, insofar as many people are actually deterred from committing terrorism, it is likely that that comes from the realization that terrorism simply doesn’t work: expressing grievances and outrage in random or semirandom civilian destruction is highly unlikely to productively serve their cause.

Another fear has been that militants who had gone to fight with ISIS or other groups abroad would be trained and then sent back to do damage in their own countries. However, there has been virtually none of that in the United States. In part, the reason is because foreign fighters tend to be killed early (they are common picks for suicide missions); often become disillusioned, especially by infighting in the ranks; and do not receive much in the way of useful training for terrorist operations back home. And recent research by analyst Nelly Lahoud concludes that the once much‐​feared al Qaeda has been notable mainly for its “operational impotence,” while Osama bin Laden, its fabled, if notorious, leader, was “powerless and confined to his compound, over‐​seeing an ‘afflicted’ al‐​Qaeda.”

These considerations are based on history, and there is no guarantee that the frequencies of the past will persist into the future. It is possible the United States will soon suffer the frequent mass terrorist attacks that many terrorism analysts predicted after 9/11. However, that tragedy very much stands out as an aberration: no terrorist attack before or since, even in war zones, has inflicted even one‐​tenth as much destruction. Those who wish to discount such arguments and projections need to demonstrate why they think terrorists will suddenly improve their performance and become capable of massively increasing their violence, visiting savage discontinuities on the historical data.

In the past few years, concerns about domestic terrorism have shifted from Islamists to right‐​wing groups and individuals. Any terrorist‐​inflicted death is of course tragic and abhorrent. However, these groups and individuals have not inflicted violence to a degree that is notably higher than that inflicted since 9/11 by Islamist terrorists.

The terrorist threat as it currently exists justifies little of the domestic spending designed to confront it. If, as is likely, policymakers will not undertake large cuts, they should at least require DHS and other agencies to conduct more rigorous cost–benefit analyses of their programs.

Bergen, Peter, and David Sterman. “Terrorism in America after 9/11.” September 10, 2021.

Friedman, Benjamin H., Jim Harper, and Christopher A. Preble, eds. Terrorizing Ourselves: Why U.S. Counterterrorism Policy Is Failing and How to Fix It. Washington: Cato Institute, 2010.

Jenkins, Brian Michael. Stray Dogs and Virtual Armies: Radicalization and Recruitment to Jihadist Terrorism in the United States Since 9/11. Santa Monica, CA: RAND, 2011.

Lahoud, Nelly. The Bin Laden Papers: How the Abbottabad Raid Revealed the Truth about Al‐​Qaeda, Its Leader and His Family. New Haven, CT: Yale University Press, 2022.

Mueller, John, ed. Terrorism Since 9/11: The American Cases. Columbus: Mershon Center for International Security Studies, Ohio State University, 2020.

Mueller, John, and Mark G. Stewart. Chasing Ghosts: The Policing of Terrorism. New York: Oxford University Press, 2016.

———. “How a Cottage Terrorism Industry Made a Lion out of an al‐​Qaeda Mouse: A new book puts together documents uncovered at Osama bin Laden’s hideout and finds the roots of a 20‐​year threat inflation,” Responsible Statecraft, May 12, 2022.

———. “Responsible Counterterrorism Policy.” Cato Institute Policy Analysis no. 755, September 10, 2014.

National Research Council of the National Academies. Review of the Department of Homeland Security’s Approach to Risk Analysis. Washington: National Academies Press, 2010.

Sunstein, Cass R. The Cost‐​Benefit Revolution. Cambridge, MA: MIT Press, 2018.