More than 90 percent of murders are committed by men. But what motivates the women who are driven to kill? Most female murderers kill those closest to them, like Mary Beth Tinning, who admitted to smothering at least some of the nine children who mysteriously died in her care. A neurologist who studies the structure of the brain finds striking differences in one region of the brain between men and women. Is this a clue to their different capacities for violence?