Terror in the United States have evolved since 1970: once the tool of left-wing radicals, then right-wing radicals, terrorist attacks are now uncommon, often unsuccessful, and not nearly as deadly. We have heard a lot in recent days that the Boston Marathon bombing is the sort of attack we should expect. But historic data suggests it largely isn't.

The University of Maryland is home to a project called START, the National Consortium for the Study of Terrorism and Responses to Terrorism. The project has tracked every terror attack around the world from 1970 to the end of 2011 and provides a database of their research at their website. (START defines a terror attack as "the threatened or actual use of illegal force and violence by a non-state actor to attain a political, economic, religious, or social goal through fear, coercion, or intimidation.") After seeing the Washington Post cite its data, we took a look at what the full database shows about terror attacks.

Relative to the rest of the world, the United States has had a high number of terror attacks since 1970.

The countries that have experienced the most terror attacks over the past four decades are those you might suspect: Iraq, Colombia, India, and Pakistan. Others may be less expected for all but experienced foreign policy buffs: Spain, Peru, Turkey. The United States is 14th on the list — ahead of Chile, Guatemala, and Lebanon.

But there used to be far more attacks in the United States than there are now.

The number of attacks has plummeted since the early 1970s, when radical groups attacked police and businesses. There was a slight resurgence during the mid-1990s, when anti-abortion activists began attacking clinics and supporters. The spike in May of 2002 was the result of a series of 18 pipe bombs placed in mailboxes in the Midwest.

Most attacks in the United States have been in New York, California, and Florida.

California, New York, and Florida have, combined, seen as many terror attacks since 1970 as all of the rest of the states (and D.C.) combined. Massachusetts is eighth on the list, but it has seen fewer than 1/11th the attacks that California has.

We've also put together a map of 1,430 terror attacks across the United States since 1970.

The number of people killed and injured varies widely — but most attacks have zero fatalities.

In only 11 percent of attacks was anyone killed. Excluding one attack, all of the terror attacks since 1970 have averaged .19 deaths per attack, making any event with a fatality an aberration. The one exception that skews that number was 9/11. Including that attack raises that figure to 1.46 fatalities per attack. 9/11 comprises 86.6 percent of all terror-attack deaths in the U.S. since 1970. (It also necessitates that the scale used for deaths and injuries be logarithmic.)

People are slightly more likely to be wounded in attacks. An average of just over one person is wounded in each attack. The increase in fatalities visible in 2009 is almost entirely due to the shooting at Fort Hood.

Businesses are the most common target of attacks.

The target of attacks changes over time, but since 1970, more attacks have targeted businesses than anything else. Attacks on private citizens, which one would assume includes the attack in Boston, are third-most common.

Bombs are used in attacks most frequently.

Almost half of all terror attacks in the United States since 1970 have used bombs. Relatively few attacks have employed firearms as a primary strategy.

These trends are not static, but more recent attacks don't stray too far from the pattern. In a report released last December, START articulates data about attacks since 2001.

  • There were a total of 207 terrorist attacks in the United States between 2001 and 2011.
  • Total attacks declined from a high of 40 in 2001 to nine in 2011.
  • Between 2001 and 2011, we recorded a total of 21 fatal terrorist attacks in the United States.
  • The highest proportion of unsuccessful attacks since 1970 occurred in 2011, when four out of nine recorded attacks were unsuccessful. …
  • The most common weapons used in terrorist attacks in the United States from 2001 to 2011 were incendiary devices (53 percent of all weapons used) and explosives (20 percent of all weapons used).

Successful attacks are more and more rare; attacks that result in fatalities, rarer still. The attack in Boston did enormous physical and psychological damage. We can be somewhat consoled that it is an aberration.