The knife and the quarterstaff, the heavy wooden stick commonly carried for herding animals and walking on the muddy roads, were the weapons of choice. "Everyone carried a knife, even women," she said, since "if you sat down somewhere to eat, you were expected to bring your own." Given the lack of sanitation at the time, even simple knife wounds could prove deadly.

Why the homicide rate in Europe began to drop in the 16th and 17th centuries is a matter of debate. The most widely accepted explanation stems from the work of Norbert Elias, a sociologist who in the late 1930's introduced the idea of a "civilizing process," in which the nobility was transformed from knights into courtiers, bringing in a new set of manners, and the modern state spread its power over the populace.

Official justice administered by courts replaced private vengeance conducted by feuds, fights and duels. Challenging conventional academic wisdom, Mr. Elias suggested, too, that the power of the state extended to cities first, so urban homicide rates would be comparatively low.

Recent research indicates that he was right. In Philadelphia, for example, the annual average rate of indictments for homicide fell from 4 per 100,000 in the 1850's to 2.2 in the early 1890's, according to research by Roger Lane, a history professor at Haverford College. The drop occurred as the city became industrialized and despite the greater availability of firearms during the period.

As people began to go to work in factories, their behavior was constrained by the foreman and the whistle, Professor Lane has written. Behavior was also improved by the spread of public schools, which acted as "agents of social control," and institutions like the Y.M.C.A. and Sunday schools, which taught morality. Similar developments helped lower New York's homicide rate in the late 19th and early 20th centuries, Professor Monkkonen said.

Then came the 1960's. Around the world, homicide rates did an about-face. When President John F. Kennedy was assassinated in 1963, for example, the national homicide rate in the United States was 4.6 per 100,000 people. By 1970, it had doubled, and by 1980 had reached 10.1 per 100,000.

Researchers point to several possible explanations. Post-World War II baby boomers came of age in the 1960's, increasing the number of young men, the most violence-prone group, in the population. The 60's also marked a shift among many social, cultural and economic forces that worked against violence in previous eras. America began moving into a post-industrial economy, governmental authority came into question with the Vietnam war, and the traditional family was threatened by things like divorce.

"The good news is violence can go down," Professor Monkkonen. "The bad news is, we need to learn how to make it happen."