The Washington PostDemocracy Dies in Darkness

Are lone wolves or terrorist groups more deadly? The answer depends on the country.

In Kathmandu, Nepal, candles illuminate the words “Pray for Orlando” during a June 17 vigil in memory of the victims of the mass shooting that was carried out at the Pulse gay nightclub  in Orlando, Fla., on June 12, 2016. (Navesh Chitrakar/Reuters)

Editor’s note: As details emerge on the October 1, 2017 mass shooting in Las Vegas, what do we know about lone-wolf attacks?  This Monkey Cage post, published after the June 12, 2016 Orlando nightclub attack, examines the data on lone-wolf terrorists — and why they are so deadly in the United States.

Two attacks last week – one in Orlando, Fla., against a gay club hosting a Latin night and one in England against anti-Brexit politician Jo Cox – have gotten us talking about lone-wolf terrorism. Neither attacker appears to have been directed by an entity such as the Islamic State or the National Front, a British far-right political party, but each appears to have drawn inspiration from such a group.

The Orlando attack killed 49 people, making it the worst mass shooting in modern U.S. history. The British attack was shocking in a nation where gun violence and political violence are extremely rare.

According to authorities, we can expect more such attacks. So how do lone-wolf attacks compare with the traditional threat: attacks by groups?

Are mass shootings contagious? Some scientists who study viruses say yes.

Have most lone-wolf terrorist attacks been especially lethal? The answer depends on the country.

Last year, I published an article in the journal Terrorism and Political Violence (ungated version here) examining whether lone wolves or terrorist groups were more deadly. For the years between 1970 and 2010, I combined terrorist-group-attack data from the Global Terrorism Database with lone-wolf-attack data from Ramon Spaaij’s book “Understanding Lone Wolf Terrorism: Global Patterns, Motivations and Prevention.”

Here’s what I found. Historically, in raw numbers, terrorist groups have killed far more people than solo actors. Spaaij’s lone-wolf data covers 15 countries, mostly in Europe and North America. In this sample of countries, the average attack by a group kills almost one person, while the typical lone wolf attack is only half as lethal. In other words, groups are usually more deadly.

When we use multivariate analysis to consider alternative explanations, we mostly see the same results – but with a key difference in the United States. In most countries, attacks by terrorist groups are usually far more deadly than those by lone wolves.

Yes, mass shootings tend to produce copycats. So do terrorist attacks.

In the United States, however, lone-wolf attacks are usually more deadly.

That’s not true in every particular case, of course. The Sept. 11, 2001, attacks were extremely deadly. But they were also extremely unusual in the history of terrorism.

Why are lone wolves, compared with other types of terrorists, especially lethal in the United States?

One reason is that groups have a harder time attacking in the United States. Unusually advanced U.S. counterterrorism capabilities – such as monitoring of phone calls and Internet traffic – are most likely what make organized terrorist attacks more challenging to accomplish.

Who becomes a terrorist, and why?

And the United States also has shown a willingness to trade off certain liberties – regarding private communications or airline travel, for example – in the name of counterterrorism, while other countries have not. Additionally, the United States has federal policing powers, enabling the FBI to follow organizations across state lines – whereas Europe does not.

All that probably makes it easier for terrorist groups to attack other countries than to attack the United States. As a result, lone-wolf attacks are more common and more deadly than group attacks in the United States.

Here’s why Europe can’t police terrorism very well

U.S. lone-wolf attacks also tend to kill more people than attacks by lone wolves in other countries. Why? That’s a question for future research, but it might be because of “shooting contagion,” other cultural factors or easier access to high-capacity weapons.

Overall, of course, policymakers need to be worried about terrorist organizations, to ward off the possibility of another spectacular attack, another 9/11. And it is good news that U.S. counterterrorism efforts have made organized terrorism so difficult to carry out.

Here’s why it’s so hard to pass gun control laws in the U.S., in one graph.

But terrorism by individuals who say they are inspired but not directed by international extremist groups is extremely difficult to prevent, especially if the individuals are not communicating their plans to others. Scholars are working to understand the motivations and “profile” of the typical lone wolf, but the task is challenging.

However, lone-wolf attacks should concern us not only for their increasing frequency and relative unpredictability, but also for their lethality.

Brian J. Phillips is an assistant professor at the Center for Research and Teaching in Economics (CIDE) in Mexico City.