In perspective, terror attacks are at a historic low. But the picture is more complex: major cities like Boston are still likely targets
Though we know nothing yet about who committed the bombings at the Boston Marathon, or why they did so, the assumption at this point is that these were acts of terror – and, as the president affirmed Tuesday, is being investigated as such.
The shadow cast by the 9/11 attack means that every such incident now tends to be seen as a new episode in a distinct and frightening era of terrorism in the mainland United States. But does this picture actually fit the historical record?
Let's put Boston in the context of the history of terrorist attacks in the United States over the past 40 years. One graph that tries to do this, posted by the Washington Post on Tuesday morning, is this:
It's taken from the National Consortium for the Study of Terrorism and Responses to Terrorism at the University of Maryland (pdf), and it plots the number of terrorist attacks in the 50 states. It's difficult not to be immediately struck by the peak on the left hand side of the graph: no other year comes close to the near 500 attacks that occurred in 1970. In fact, only two other years even surpass 100 attacks, and both of those were also in the 1970s. But this graph doesn't tell the whole story.
First, let's start with the fact that most terrorist attacks occur in major metropolitan areas. Manhattan and Los Angeles are two of the most at-risk targets for major terrorist attacks throughout the period: 13.1% of all terrorist attacks from 1970 to 2008 occurred in Manhattan, New York alone; 6.0% were in Los Angeles County, California. Combined, that's nearly a fifth of all terrorist attacks.
Other areas were less consistent at registering in the terrorism chart. San Francisco was a center of leftist terrorism in the 1970s, but has since cooled off. Maricopa County, Arizona has seen more attacks in the 2000s as "single-issue terrorism" (such as animal rights or hate crime) has become more dominant.
Second, over the last decade, very few terrorist attacks have occurred across the central swath of the country. When you look at a heat map of terrorist attacks over the last 40 years, you see terrorism occurring in many different states. That, however, shows where attacks have been most likely to occur by decade.
Only five of the terrorist attacks coded by type by University of Maryland occurred in the center of the country during the 2000s. Almost all the attacks that took place in that region were during the 1980s and 1990s, when rightwing and religious groups were the most likely to carry out attacks.
Third, terrorism attacks are most likely to be in places where crime is high. There's not a perfect correspondence, to be sure; but the correlation between high crime rates and terrorist attacks is highly significant at 0.25.
Fourth, terrorist attacks tend to occur in areas that are most ethnically diverse. Even when taking into account population density, you are more likely to see a terrorist attack in an area where many languages are spoken. You are also more likely to see a terrorist attack in a city where people live with less residential stability. Poverty and inequality, however, are not a factor: you are more likely to see a terrorist attack in cities with a lower degree of concentrated disadvantage.
A key point, however, is that the percentage of foreign-born residents is not a significant predictor of terrorist attacks. The same goes for racial identity: a city with more black or Hispanic residents is no more likely to see a terrorist attack than a majority white city. Language likely differs from these factors because you usually see different languages in cities with foreign business interests or government interests, which is exactly where terrorist attacks are more likely to occur.
Fifth, the chance of surviving a terrorist attack is the same as it has always been. Deadly terrorist attacks may be down, but the the percentage of those attacks that are deadly is the same. In 1970, only about 5% of terrorist attacks were deadly. That spiked to 41% in 1973, but it has mostly hovered between zero and 20% in the years since. The two years with the biggest spikes were 2001 and 2006, when 25% of attacks were deadly.
Sadly, most of these factors tend to make the events that occurred in Boston predictable or likely, compared to other places. Boston is among the top ten major metropolitan areas, and it's on the coast. It's in the top third for United States cities by violent crime rate. A number of the places within the metropolitan area have high population densities, including the city of Boston itself. The city ranks highly for non-English speaking households (pdf). Finally, the attack had a fatal outcome.
About the only characteristic that perhaps doesn't fit the conventional pattern is that it occurred at a sports event, rather than the target being a government building or business. Indeed, the targeting of ordinary civilian members of the public is the truly scary part of the Boston Marathon bombing: with the exception of the Atlanta Olympics bombing of 1996, the idea of attacking a sports event in the United States is novel and unusual.
In other ways, Boston is typical: from a statistical point of view, it's not all that surprising that nearly 2% of all US terror attacks occurred in the Boston metropolitan area between 1970 and 2008. This week's tragic incident sadly matches the pattern.