President Trump issued two executive orders that temporarily stopped or restricted visa issuance to nationals from Iran, Libya, Somalia, Syria, Yemen, Chad, North Korea, and Venezuela in order to “protect the nation from foreign terrorist entry into the United States.“62 The government removed Chad from the list shortly after.63 However, only 18 individuals from those eight countries committed or attempted to carry out a terrorist attack on U.S. soil. Foreign‐​born terrorists from the travel ban countries killed zero people in those attacks and injured 32—accounting for 0.19 percent of all injuries caused by foreign‐​born terrorists on U.S. soil. Twenty‐​three of those injuries were inflicted by Dahir Ahmed Adan and Abdul Razak Ali Artan. Adan stabbed 10 people in St. Cloud, Minnesota, on September 20, 2016, and Artan rammed his car into a crowd at Ohio State University and then stabbed several people, ultimately injuring 13 people in his attack.64 Both of those attackers were Somali‐​born. Adan was a refugee and Artan was an asylum seeker, and they were both killed by law enforcement officers before they had a chance to murder anybody. There was no good national security justification for President Trump’s executive orders based on the number of people murdered or injured in attacks committed by terrorists from those countries.

The Syrian civil war is also causing much worry about international terrorism. Since the 2016 publication of Cato’s first policy analysis of the foreign‐​born terrorist risk, the author has learned of a Syrian‐​born terrorist of Christian Armenian descent named Karnig Sarkissian who tried to dynamite the Turkish embassy in Philadelphia in 1982.65 Sarkissian and his coconspirators did not succeed in their attack.66 Sarkissian was a member of an Armenian terrorist group intent on avenging the Armenian genocide perpetrated by the Turkish nationalist government in the early 20th century. He was not targeting Americans on U.S. soil, he was not a Muslim, and he was not ethnically Arab or Syrian. Sarkissian was merely born in Syria and may have had Lebanese citizenship. Although there was one Syrian‐​born terrorist who attempted an attack on U.S. soil in 1982, there have not been any since, despite the vicious civil war in that country.

The following subsections discuss the terrorism risks, costs in terms of human life, and countries of origin for each specific visa category.

Illegal Immigrants. Only nine illegal immi­grants became terrorists, a minuscule 0.000029 percent of the estimated 31.3 million who entered from 1975 through 2017, as summarized in Table 10. In other words, about 3.5 million illegal immigrants entered the United States for each one who ended up being a terrorist. They murdered or injured zero people in terrorist attacks on U.S. soil. Three of the illegal immigrant terrorists were from Macedonia, two from Algeria, one from Palestine, and one each from Lebanon, Canada, and Somalia. The Macedonians were Shain Duka, Britan Duka, and Eljvir Duka; they crossed the Mexican border illegally as children with their parents in 1984. They were three conspirators in the incompetently planned Fort Dix plot that the FBI foiled in 2007, long after they became adults.67 Of the other terrorists, seven were Islamists, one was an adherent of a foreign nationalism, and one was a left‐​wing terrorist. None of the 9/11 terrorists entered as an illegal immigrant.

Lawful Permanent Residents. A lawful permanent resident (LPR), also known as a green‐​card holder, can reside and work permanently in the United States until he either naturalizes or commits a serious enough crime to lose his green card and be deported.68 More terrorists have taken advantage of the LPR category than of any other visa category. From 1975 through 2017, 57 foreign‐​born terrorists were LPRs—an average of 1.3 terrorists per year. Over the 43‐​year period, about 18 million LPRs were allowed in, meaning that just 0.00032 percent of LPRs were actual terrorists. In other words, one terrorist entered for every 314,462 nonterrorist LPR. Those 57 LPR terrorists killed 17 people in terrorist attacks. The human cost of LPR terrorism was thus $255 million, equal to $14.23 per green card issued (Table 10). Of the 9/11 hijackers, none were lawful permanent residents.

Terrorists with green cards came from 30 different countries. Six terrorists on green cards came from Pakistan, 6 from Armenia, 4 from India, and 3 each from Croatia, Egypt, Jordan, and Sudan. The deadliest terrorist who entered on a green card was Sayfullo Habibullaevic Saipov from Uzbekistan, who murdered 8 and injured 11 in his Halloween attack in New York City in 2017.69 Two Kuwaiti terrorists on green cards killed a total of 6 people and injured 176. The first was Nidal A. Ayyad, one of the 1993 World Trade Center bombers; he is responsible for a single death and 174 injuries. The second is Mohammad Youssuf Abdulazeez, who murdered 5 and injured 2 in a 2015 shooting spree in Chattanooga, Tennessee.70

Student Visas. Student visas allow foreig­ners to enter the United States temporarily to attend an educational institution such as a college, university, seminary, private elementary school, or vocational training program.71 A total of 21 foreign‐​born terrorists entered on student visas, and they accounted for 0.00017 percent of the 12.3 million student visas issued from 1975 through 2017.72 In other words, 1 terrorist was issued a student visa for about every 586,938 students who were not terrorists.

Terrorists on student visas appear especially deadly because one of them, Hani Hanjour from Egypt, was a 9/11 hijacker. Altogether, students caused 158.8 fatalities, or 1 for every 77,623 student visas issued. The human cost of terrorism caused by foreigners on student visas was thus $2.38 billion, equal to 5.23 percent of all the terrorism costs to human life. The average terrorism cost per student visa issued is $193.24. Excluding 9/11, 20 terrorists entered the United States as students, or 1 entry for every 616,285 million student visas issued. Those 20 committed a total of 2 murders that cost $30 million or $2.43 per student visa issued. Iranian students were the most likely to be terrorists because 7 were convicted or committed attacks that injured or killed zero people—4 of which were the result of a scheme to kidnap Minnesota governor Al Quie in 1979.73Foreign‐​born terrorists who entered on a student visa also injured 1,065.1 people, 6.2 percent of all those injured in attacks committed by foreign‐​born terrorists on U.S. soil.

K-1 Fiancé(e) Visas. The K-1 visa permits a foreign‐​citizen fiancé or fiancée to travel to the United States to marry his or her U.S.-citizen sponsor within 90 days of arrival. Once married, the foreign citizen can then apply to adjust his or her immigration status to that of an LPR.74

Tashfeen Malik entered the United States on a K-1 visa sponsored by her U.S.-born husband, Syed Rizwan Farook. Together they murdered 14 people during the San Bernardino terrorist attack of December 2, 2015. This policy analysis attributes all 14 murders to Malik in order to maximize the death toll from foreign‐​born terrorists. Her U.S.-born husband is included in the native‐​born terrorist category. They injured 17 people in their attack, all of which are credited to Malik.

The San Bernardino attack is the only one to involve the K-1 visa. Because of the small number of these visas—708,942—issued over the 43‐​year time frame, this single attack makes the visa look very dangerous, with a single murder for every 50,639 K-1 visas issued. The single terrorist on the K-1 visa has imposed $210 million in costs or an average of $296.22 for every K-1 visa issued—by far the highest cost per visa issued. Although it is the second‐​deadliest visa, there is no trend of K-1 visa holders committing attacks.75

Refugees. A refugee is a person who is located outside the United States and is of special humanitarian concern and who demonstrates that he or she was persecuted or fears persecution because of race, religion, nationality, political opinion, or membership in a particular social group. A refugee is not settled in another country and does not violate other immigration bars on admission, such as posing a national security or public health risk.76 Refugees are granted status from a third country and then enter the United States after they have been granted a visa. Refugees must apply for a green card after residing for one year in the United States.

Of the 3,391,203 refugees admitted from 1975 to the end of 2017, 25 were terrorists, which amounted to 0.00074 percent of the total. In other words, one terrorist entered as a refugee for every 135,648 refugees who were not terrorists. Refugees were not very successful at killing people in terrorist attacks on U.S. soil. Of the 25, only 3 were successful in their attacks, killing a total of 3 people and imposing a total human cost of $45 million, or $13.27 per refugee visa issued. Two of the three refugee terrorists were Cubans who committed attacks in the 1970s; the other was Croatian. All three were admitted before the Refugee Act of 1980 created the current rigorous refugee‐​screening procedures. Prior to the Refugee Act of 1980, a hodgepodge of poorly managed post–World War II refugee and displaced persons statutes, presidential grants of parole, and ad hoc congressional legislation allowed Hungarian, Cuban, Vietnamese, and other refugee groups to settle in America.77 All the murders committed by foreign‐​born refugees in terrorist attacks were committed by those admitted prior to the 1980 Refugee Act. Refugees injured 14.5 people in terrorist attacks; 10 of those injuries were committed by Dahir Ahmed Adan in his September 20, 2016, attack.

One of the Cuban terrorists assassinated a Chilean dissident and his American aide along with another Cuban who entered as an LPR. The second Cuban terrorist assassinated a Cuban exile leader who supported a closer American diplomatic relationship with Fidel Castro. The Croatian terrorist helped hijack a plane in 1976 and was convicted of murder. The GTD and RDWTI showed many more terrorist attacks and assassinations in the 1970s and 1980s that were likely perpetrated by Cuban or Vietnamese refugees, but because there were no convictions these could not be included here.

Many of the refugees arrested after 9/11 were admitted as children, and in some cases there is doubt over whether their attacks can qualify as terrorism.78 Other refugees have been arrested for terrorism or the vague “terrorism‐​related charges,” but they were planning terrorist attacks overseas or providing material support for foreign groups operating overseas.79 No refugees were involved in the 9/11 attacks.

Asylum Seekers. Asylum seekers are those who ask U.S. border officials for protection because they have suffered persecution or fear persecution because of their race, religion, nationality, membership in a particular social group, or political opinions.80 Unlike refugees, asylum seekers must apply in person at the border and are often detained before being granted asylum. Eleven asylum seekers, or 0.0015 percent of the 732,168 admitted from 1975 through 2017, later turned out to be terrorists. For every terrorist who was granted asylum, 66,561 nonterrorist asylum seekers were admitted.

Terrorists who were asylum seekers killed nine people in terrorist attacks, three of them in the Boston Marathon bombing on April 15, 2013, carried out by the Tsarnaev brothers. The brothers entered the United States as young children and later became terrorists. Ramzi Yousef and Ahmed Ajaj, both of whom helped plan the 1993 World Trade Center bombing that killed six people, planned and carried out those attacks as part of a six‐​person team; therefore, this analysis considers them to be jointly responsible for only two of the six murders. Eduardo Arocena, who murdered two in an anti‐​Castro terrorist attack, entered as an asylum seeker even after he was granted asylum in a third country.

The total human cost of terrorism by asylum seekers was $135 million, which is an average of $184.38 per asylum seeker admission. Foreign‐​born terrorists who entered as asylum seekers injured 669.3 people in terrorist attacks—93.7 percent in the 1993 World Trade Center and 2013 Boston bombings. No asylum seekers were involved in 9/11.

Tourist Visas. Tourists on the B visa are allowed to tour the United States for business or pleasure, as well as enroll in short recreational courses of study.81 These are the tourist visas available to most residents of the world.

The tourist visa categories were also the second most abused by terrorists. A total of 41 terrorists entered the United States on tourist visas from 1975 through 2017. That is an average of 0.95 terrorists who entered on a tourist visa annually. About 245 million tourists entered the United States on tourist visas, so a single terrorist was issued a visa in this category for every 6 million visas issued.

The 41 terrorists on tourist visas killed 2,829.4 people in attacks, or 1 victim for every 86,738 visas issued. The total terrorism cost in terms of human life by terrorists on tourist visas was $42.44 billion, or $172.94 per visa. Foreign‐​born terrorists who entered on tourist visas injured 14,959 people in their attacks, or about 87.7 percent of all people injured in foreign‐​born attacks on U.S. soil from 1975 through 2017.

Eighteen of the terrorists who carried out the 9/11 attacks held tourist visas, so this visa category is responsible for 93.2 percent of all deaths caused by foreign‐​born terrorists. Excluding 9/11 lowers the number of fatalities to 7.2 and the total death‐​related costs to $108 million, or $0.44 per tourist visa issued. Excluding the 9/11 hijackers, one terrorist entered on a tourist visa for every 10.7 million non­terrorist tourists. There was one murder victim for every 34.1 million non­terrorist tourists who entered, excluding the 9/11 attacks.

Visa Waiver Program. The VWP enables most citizens of the participating countries to travel to the United States for business or tourism for up to 90 days without first obtaining a visa.82 The participating countries are nations in Europe, East Asia, and South America that have already established security procedures to exclude terrorists and share traveler information with the U.S. government, and whose citizens rarely overstay illegally in the United States.83

There were 11 terrorists on the VWP out of a total of 437.4 million entries during the life of the program (since 1986), or a single terrorist for every 39.9 million entries. That makes the VWP the safest visa category. The 11 VWP terrorists killed 1 person in a terrorist attack on U.S. soil. Islamist terrorist Glen Cusford Francis, an immigrant to Canada from Trinidad and Tobago, assassinated Rashad Khalifa in Tucson, Arizona, in 1990. Francis was not convicted of the murder until 2012. The other interesting foreign‐​born terrorists who entered through the VWP are the French national Zacarias Moussaoui, who was originally part of the 9/11 conspiracy but was in jail on unrelated charges during the attacks; the British shoe bomber Richard Reid, who attempted to ignite his shoe on a transatlantic flight en route to the United States, and Qaisar Shaffi, who cased New York buildings for a future attack that was broken up by British intelligence. Ahmed Ressam was apprehended at John F. Kennedy International Airport while attempting to enter the country illegally using forged passports from nations that were part of the VWP. Because he was captured at the border and his documents were forgeries, he is classified as an illegal immigrant.84

In addition, a few international terrorist suspects have been apprehended while trying to enter through the VWP. These include a member of the Provisional Irish Republican Army, a French‐​Bolivian dual‐​national who was implicated in a 1990 bombing of U.S. Marines in La Paz, Mexico, and a British mercenary who tried to buy a fighter jet for the infamous Colombian drug lord Pablo Escobar.85

Unknown Visas. The visa statuses of 16 terrorists are unknown. Eleven of the unknown terrorists killed 4.8 people in terrorist attacks for a total human cost of $72 million. They also injured 2 people.86

Cost‐​Benefit Analysis

Immigration screening for counter­terrorism purposes is important, but it will never be perfect.87 As Steven Camarota at the Center for Immigration Studies has written, “To be sure, in a nation as large as the United States, it is impossible to prevent terrorists from entering the country 100 percent of the time.“88 Even though terrorists rarely achieve their ultimate policy goals, the United States will always be vulnerable to terrorist attacks in the sense that the possibility of harm will be greater than zero.89

Confronted with the threat of Islamic terrorism, many well‐​known conservatives have called for a complete moratorium on immigration since 2015.90 They presumably want to restrict only LPRs, student visas, fiancé(e) visas, illegal immigrants, refugees, and asylum seekers, but they may also want to prevent the entry of tourists. Although support for a complete immigration moratorium is an extreme position held by few Americans, it is a useful policy counterexample to understand the costs and benefits of immigration and tourism restrictions as a means to combat terrorism.

The following sections separate tourists from immigrants and migrants to estimate how many Americans must die from terrorism to justify a moratorium on foreigners entering the United States. Finding the breakeven point at which the benefits of reduced terrorism justify the cost incurred by stopping all legal immigration and tourism helps form the outermost boundaries of a sensible policy.91 If the benefits of the different policies proposed below outweigh the costs, then the measure is cost‐​effective. If, however, the costs of the policies proposed below are greater than the benefits, then they are not cost‐​effective.

This cost‐​benefit analysis considers the cost of human deaths, property damage, injuries, and economic disruption caused by terrorism. In virtually all cases of terrorism, with the notable exception of the 9/11 attacks, property damage is small and the cost of injuries is minor compared with the cost of the deaths. Government reactions to terrorism, such as the virtual shutdown of Boston in the wake of the Marathon bombing and the grounding of all air travel after 9/11, are indirect costs of terrorism; as such they are not considered here.92

Broad Immigration Moratorium. The economic cost of a moratorium on all future immigration is tremendous. This section includes two cost projections. The first conservatively estimates the economic costs of a moratorium to be only $50.2 billion annually, which is the number used by Harvard economist George Borjas.93 That $50.2 billion counts only the immigration surplus, which is the increase in American wages caused by immigration. The figure ignores other enormous economic benefits, including the economic gains to the immigrants themselves even when they naturalize. The second cost projection assumes the $229 billion annual price tag of a moratorium calculated by Texas Tech University economist Benjamin Powell.94

The greatest possible benefit of an immigration moratorium would be the elimination of all terrorism committed by immigrants. Excluding tourists and visitors on the VWP, since 1975 206.6 people were murdered on U.S. soil in terrorist attacks committed by 141 terrorists who entered as illegal immigrants, LPRs, students, fiancé(e)s, refugees, asylum seekers, and those with unknown visa statuses, accounting for 6.8 percent of all fatalities caused by foreign‐​born terrorists on American soil. The other 2,830.4 murders, or 93.2 percent, were committed by 52 tourists and VWP visitors who would have been unaffected by an immigration moratorium, who account for 26.9 percent of all foreign‐​born terrorists and 93.2 percent of murders caused by foreign‐​born terrorist attacks. Some 95 percent of the murders committed by terrorists on tourist visas occurred on 9/11. A ban on immigration will barely diminish the costs of terrorism.

The costs of an immigration moratorium vastly exceed the benefits, even with very generous assumptions buttressing the pro‐​moratorium position. According to a breakeven analysis, which seeks to find when the cost of an immigration restriction would equal the benefit of reduced terrorism, an immigration moratorium would have to prevent 3,347 deaths annually at an estimated $15 million per death, assuming Borjas’s $50.2 billion estimate of the annual economic gain from immigration. In reality, an average of 4.8 murders were committed per year by immigrant (nontourist and non‐​VWP) terrorists during the 43‐​year period. An immigration moratorium would have to prevent 697 times as many such murders annually as actually occurred from 1975 through 2017 for the costs of a moratorium to equal the benefits.

Benjamin Powell’s more realistic $229 billion annual estimate of the economic costs of an immigration moratorium means that the ban would have to prevent 15,267 murders by terrorists each year at a cost savings of $15 million per murder for the benefits of the ban to equal the costs. That number is about 3,178 times as great as the average annual number of terrorist deaths caused by immigrants (excluding tourists) and more than 5 times as great as all the murders committed by all foreign‐​born terrorists (including tourists) from 1975 through 2017. In short, an immigration moratorium would produce huge economic costs for minuscule benefits.

Tourism Moratorium. Given the role that tourism played in the 9/11 attacks, it is tempt­ing to think that limiting an immigration ban to tourism might be a preferable policy. Yet the economic costs of a tourism moratorium are even greater. The World Travel and Tourism Council estimated that international tourists added $503.7 billion directly to the U.S. economy in 2016.95 A moratorium on tourism would deny the U.S. economy an amount of economic activity equal to almost 3 percent of U.S. GDP.

The majority of all murders committed by foreign‐​born terrorists, 93.2 percent, were committed by 52 different terrorists on tourist visas and the VWP. A total of 99.7 percent of all terrorist murders committed by those on tourist and VWP visas were committed by 18 terrorists on 9/11. Over the entire 43‐​year period covered by this policy analysis, an average of 65.8 people were murdered each year in terrorist attacks committed by those on tourist visas, producing an average annual cost of $987.4 million—the amount that would be saved if there was a moratorium.

However, the costs of a tourist moratorium vastly exceed the benefits from lives saved. Such a moratorium would have to deter at least 33,580 murders by terrorists per year to justify the loss in economic activity and break even. The annual number of murders committed by tourists in terrorist attacks would have to be 510 times as great as they currently are to justify a moratorium. To put this in perspective, the 33,580 murders that would have to be prevented each year for the costs of a tourist moratorium to justify its benefits is over 11 times as great as all the deaths caused by all foreign‐​born terrorists over the entire 43‐​year period studied here. The threat from terrorism cannot justify a moratorium on tourists.

Including Nonhuman Costs. The destruction of private property, businesses, and economic activity caused by foreign‐​born terrorism during the 1975–2017 period is estimated to have cost $171 billion. The combined human, property, business, and economic costs of terrorism from 1975 through 2017 are thus estimated at $216.58 billion. Spread over 43 years, the average annual cost of terrorism is $5.04 billion, which is about one‐​hundredth the minimum estimated yearly benefit of $553.9 billion from immigration and tourism ($50.2 billion plus $503.7 billion). The average yearly costs of terrorism, including the loss of human life, injuries, property destruction, and economic disruptions, would have to be 522.83 times as great as they have been to justify a moratorium on all foreigners entering the United States. A moratorium on foreigners entering the country is costlier than the benefits, even when including the property, business, and economic costs caused by foreign‐​born terrorism.

Conclusion

Foreign‐​born terrorism on U.S. soil is a low‐​probability event that imposes high costs on its victims, despite relatively small risks, and low costs on Americans as a whole.96 From 1975 through 2017, the average chance of dying in an attack committed by a foreign‐​born terrorist on U.S. soil was 1 in 3,808,374 a year, and the chance of being injured was about 1 in 678,399. For 27 of those 43 years, no people were killed on U.S. soil in terrorist attacks committed by foreign‐​born terrorists. In 27 years, most of which overlap with the years in which no one was killed, no people were injured on U.S. soil in terrorist attacks committed by foreign‐​born terrorists. During the same period, native‐​born terrorists murdered 413 people and injured 1,346 in attacks on U.S. soil.

Foreign‐​born terrorism has been a more serious hazard to American life, liberty, and private property than native‐​born terrorism from 1975 through 2017. But foreign‐​born terrorism is a manageable threat given the huge economic benefits of immigration and the relatively smaller costs of terrorism. Unknown terrorists murdered 68 people during that time. The U.S. government should continue to devote resources to screening immigrants and foreigners for terrorism and other threats, but large policy changes like an immigration or tourist moratorium would impose far greater costs than benefits.

Appendix 1

All identified foreign persons who attempted or committed terrorism in the United States over the period 1975 through 2017 are listed in Table A.1. All identified native‐​born persons who attempted or committed terrorism in the United States over the 1975–2017 period are listed in Table A.2.