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Fighting terrorism may actually increase the risk of terrorist attacks. Here’s how.

- March 29, 2016
An image provided by Belgian Federal Police shows a CCTV grab of one of the three suspects in the Zaventem airport attack in Brussels on March 22, 2016. (EPA/Belgian Federal Police)

Days before the Brussels attacks on March 22, Belgian authorities raided the attackers’ safe houses and arrested Salah Abdeslam, who had helped organize the Paris attacks in November. Because of the raids, according to media reports, the attackers struck sooner than they’d originally planned.

Here’s what that shows: Terrorists are rational actors in many ways. They react predictably when circumstances change. To be effective, anti-terrorism authorities need to better understand how terrorists respond when the government acts.

In a recently published research paper, “The timing of terrorist attacks: An optimal stopping approach,” I use a simple mathematical model to study the decision making of autonomous terrorist units. And what we’ve just seen in Brussels illustrates the main finding. As authorities close in on terrorists, uncovering plots and arresting suspects, they will attack sooner so that they’re not caught first. Unfortunately, increasing efforts against terrorism may, in the short term, increase the risk of attacks.

[interstitial_link url=”https://www.washingtonpost.com/news/monkey-cage/wp/2015/11/30/want-to-help-the-islamic-state-recruit-treat-all-muslims-as-potential-terrorists/”]Want to help the Islamic State recruit? Treat all Muslims as potential terrorists[/interstitial_link]

That doesn’t mean anti-terrorist efforts are useless or, worse, dangerous. Belgian authorities found large amounts of explosives and other materials to make bombs – such large amounts, in fact, that apparently the Brussels attackers had originally planned an even more spectacular act, perhaps even an attack on nuclear facilities, although that’s currently just speculation.

But the Brussels attack isn’t the first or only such response as authorities were closing in. Only two weeks after the July 7, 2005, London subway and bus bombings, another terrorist cell tried to carry out a similar plot. The bombs didn’t detonate, however, so no one was hurt or killed. Did that second cell hurry to carry out its attack before it was ready, because London had boosted its anti-terrorism efforts after 7/7 bombings? If so, the British investigation – and the attackers’ rush to stay ahead of the authorities — may have saved lives.

How can authorities counter terrorists’ effort to hurry up and attack before they’re caught?

If anti-terrorism work has the indirect result of boosting the short-term risk of terrorist attacks, what can authorities do? First, of course, they must be aware of this unintended possibility. Then what can they do?

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Some might suggest that, after a highly visible raid or arrest, authorities should increase security at vulnerable sites, hoping to prevent  or at least contain attacks. But that works only if extremists are targeting only a few symbolic sites: the World Trade Center, Big Ben, the European Parliament. That’s not what they’ve been doing. Rather, in Brussels and Paris, we’ve seen deadly attacks on the general public, apparently aimed to spur widespread fear.

How do you defend against apparently random attacks on ordinary city streets? You can’t, without very serious consequences for ordinary life — at enormous economic cost. Shortly after the Paris attacks in November, Brussels went into a security lockdown that kept public transportation, shops and schools closed for several days due to a “serious and imminent” threat. After the 9/11 attacks on the United States, planes were grounded for days – crippling both commerce and personal life.

[interstitial_link url=”https://www.washingtonpost.com/news/monkey-cage/wp/2015/11/23/youre-more-likely-to-be-fatally-crushed-by-furniture-than-killed-by-a-terrorist/”]You’re more likely to be fatally crushed by furniture than killed by a terrorist[/interstitial_link]

Obviously such extreme measures can be used only rarely. So we have to learn to live with the threat from terrorism. It may help to remind ourselves that the risk of becoming a victim is very small.

Thomas Jensen is an associate professor of economics at the University of Copenhagen.