Donald Trump predictably blames "our extremely open immigration system" for Saturday's bomb attacks in New Jersey and New York City. His critique overlooks the details of this particular case as well as the general rarity of terrorism by immigrants.

Ahmad Khan Rahami, the 28-year-old man police arrested on Monday in connection with the bombings, is a naturalized U.S. citizen who immigrated to the United States from Afghanistan at the age of 7. He seems to have been radicalized within the last few years, a period when he spent nearly a year in Pakistan and became noticeably more religious and taciturn.

It is hard to imagine how the "extreme vetting" Trump advocates for immigrants from "any nation that has been compromised by terrorism" could have kept Rahami out of the country. What questions could have been posed to his parents that would have predicted his violent turn two decades later?

Trump faults his Democratic opponent, Hillary Clinton, for supporting the admission of Syrian refugees, who he says pose an unacceptable risk of terrorism. But according to a recent study by Cato Institute immigration policy analyst Alex Nowrasteh, "the chance of an American being murdered in a terrorist attack caused by a refugee is 1 in 3.64 billion per year."

Trump has recommended "a total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what is going on"—a plan that his own running mate called "offensive and unconstitutional." More recently Trump has said the moratorium should apply to all visitors from countries "compromised by terrorism," a category that arguably includes most of the world.

Some pundits favor a cleaner approach. "Confronted with the threat of Islamic terrorism," Nowrasteh notes, "well-known conservatives like Larry Kudlow, David Bossie, and Ann Coulter have called for a complete moratorium on immigration."

A broad moratorium would have the advantage of preventing all terrorist attacks by newly admitted immigrants. But it would also exclude more than 1 million innocent people each year it was in effect, at a huge economic cost. Nowrasteh cites estimates ranging from $35 billion to $229 billion a year.

Nowrasteh reports that tourists accounted for 94 percent of deaths caused by foreign-born terrorists in the United States from 1975 through 2015. Including tourists in the moratorium would raise the annual cost by another $194 billion or so.

Given the rarity of deaths caused by terrorism, Nowrasteh shows, such costs cannot possibly be justified. Based on a value of $15 million per life, he puts "the combined human, property, business, and economic costs" of attacks by foreign-born terrorists during the 41-year period covered by his study at $5.3 billion annually, which is "far less than the minimum estimated yearly benefit of $229.1 billion from immigration and tourism."

Even that calculation overestimates the potential security benefit of cutting off immigration, since it is dominated by the 2001 attacks on the World Trade Center and the Pentagon, an anomalous event that is unlikely to be replicated. The 9/11 attacks (which were perpetrated not by naturalized citizens or by refugees but by visitors with tourist or student visas) account for 99 percent of the 3,024 deaths caused by foreign-born terrorists from 1975 through 2015.

Excluding 9/11, the overwhelming majority of terrorist murders in the United States—more than 90 percent—have been committed by native-born Americans. Except for 2001, the risk of being killed by a foreign-born terrorist has been minuscule and flat for more than four decades.

That risk is extremely low even if you include 9/11: about 1 in 3.6 million per year. You are more than 200 times as likely to die in a traffic accident, 20 times as likely to be killed by falling down stairs, and four times as likely to drown in a bathtub.

Any politician who wants to impose large costs in response to such a tiny risk has a lot of explaining to do.

© Copyright 2016 by Creators Syndicate Inc.