Days after John Wilkes Booth entered the presidential box at Ford’s Theater and shot Abraham Lincoln on April 14, 1865, Benjamin Disraeli, the British prime minister, declared that “assassination has never changed the history of the world.” Was Disraeli right?

One view, the “great man” theory, claims that individual leaders play defining roles, so that assassinating one could lead to very different national or global outcomes. In contrast, historical determinism sees leaders as the proverbial ant riding the elephant’s back. Broader social, economic and political forces drive history, so that assassinations may not have meaningful effects.

Prominent examples of assassinations raise intriguing questions, but do not settle the matter. Would the Vietnam War have escalated if John F. Kennedy had not been killed? Would the Middle East peace process have proceeded more successfully if Prime Minister Yitzhak Rabin of Israel had not been assassinated?

For any given individual historical episode, it is hard to know for sure. But averaging over many such examples, statistics can begin to provide a guide.

To better understand the role of assassinations in history, we collected data on all assassination attempts on national leaders from 1875 to 2004, both those that killed the leader and those that failed. There’s a lot of data: Since 1950, a national leader was assassinated in almost two out of every three years. (Today’s leaders may rest considerably easier than those in the early 20th century, when a given leader was about twice as likely to be killed as now.) ...