In 1855, Sen. David Atchison of Missouri told his followers that he'd send into Kansas territory enough pro-slavery Missourians "to kill every God-damned abolitionist in the Territory."

Sen. Charles Sumner of Massachusetts called these Missourians "drunken spew and vomit." Sen. Stephen A. Douglas warned that if Sumner didn't stop using this vicious language "this damned fool (Sumner) is going to get himself shot by some other damn fool."

Sumner never was shot, but when he turned his vitriol on Sen. Andrew Butler of South Carolina, Butler's cousin, Rep. Preston Brooks, on the Senate floor, beat Sumner over the head until Sumner nearly died.

In Kansas, an abolitionist, John Brown, a failure at business and farming, "went crazy-crazy" when he learned of Brooks' beating of Sumner. Then he and his sons seized five pro-slavery settlers and split their skulls.

Free speech had consequences. Northerners and Southerners, pro- and anti-slavery, came to mistrust the U.S. government. One or the other detested Presidents Pierce and Buchanan or Lincoln. "Unbalanced" men like Brown resolved to act on their own when government failed or thwarted them.

Thirty to 40 years earlier, Americans could hardly have predicted the hatred that divided sections, government, political parties, churches, families and skulls. In the "Era of Good Feelings" political partisanship almost disappeared and, moreover, Americans acted civilly toward one another in everyday interchanges. It was a time when the homicide rate reached its lowest point in all of U.S. history - lower than one per 100,000 population in some areas.

But civility did not last past the 1840s. As public speech radiated hatred, the homicide rates rose all over the nation, and not just in Kansas. At worst, in Florida on the eve of the Civil War, whites even murdered other whites at the astonishing rate of 86 per 100,000.

The link between venomous political speech and private licentiousness was most obvious before the Civil War, but it has appeared repeatedly.

In Americans' revolution against Britain, Americans killed one another at dismaying rates. Gen. Nathanael Greene said of American civilians in the Carolinas, "The Whigs and Tories pursue one another with the most relent[less] Fury, killing and destroying wherever they meet."

A century later, in the era of Jim Crow, murders soared in the South, as Democratic Party demagogues like Ben Tillman persuaded suggestible, insecure white men that they must fear being governed by Negro Republicans or the biracial Populist Party. And so 3,000 black men were lynched.

In the 1970s, the politically divided, riot-torn America experienced the highest homicide rate of the 20th century, following the lowest rate, in the Eisenhower era.

As for our present spate of murders, the accused, Jared Lee Loughner, appears unbalanced, alienated, a person on the margin, a "lone wolf," even a madman.

But does that make the killings he is accused of an "isolated incident," a random event, meaningless to all but the perpetrator and victims?

Our history tells us no. As Sheriff Clarence Dupnik observed, the times reverberate with angry and hateful speech. And like John Brown, there are "lone wolves" who absorb and feel licensed to kill.

In our nation's past, such people did not surface and kill randomly or constantly. Murders ebbed and flowed with the behavior of all Americans. Today, two-thirds of the world's people live in countries less homicidal than the U.S. Today, one in every 142 American children will be killed. In Tucson last Saturday, she was Christina-Taylor Greene.

Jack D. Marietta is a professor at the University of Arizona.