David Buss of the University of Texas asked his students if they had ever thought seriously about killing someone, and if so, to write out their homicidal fantasies in an essay. He was astonished to find that 91 percent of the men and 84 percent of the women had detailed, vivid homicidal fantasies. He was even more astonished to learn how many steps some of his students had taken toward carrying them out.

One woman invited an abusive ex-boyfriend to dinner with thoughts of stabbing him in the chest. A young man in a fit of road rage pulled a baseball bat out of his trunk and would have pummeled his opponent if he hadn’t run away. Another young man planned the progression of his murder — crushing a former friend’s fingers, puncturing his lungs, then killing him…

People who murder often live in situations that weaken sympathy and restraint. People who commit massacres, for example, often live with what the researchers call “forward panic.” After having endured a long period of fear, they find their enemies in a moment of vulnerability. Their fear turns to rage, and, as Steven Pinker writes in “The Better Angels of Our Nature,” they “explode in a savage frenzy.”