The theory has some problems — questionable early data, Italy as a counter example, and the timing of the decrease in violence. Follow the link below for a good discussion below the article.
In this newly published paper, two anthropologists, Peter Frost and Henry Harpending, argue that the last thousand years have seen a radical change in the legitimacy of personal violence. Previously, every man had the right to settle personal disputes as he saw fit, even to the point of killing, and it was only the threat of retaliation from the victim’s kinsmen that kept violence in check. This situation began to change in the 11th century throughout Western Europe with a growing consensus that the wicked should be punished so that the good may live in peace. Courts imposed the death penalty more and more often and, by the late Middle Ages, were condemning to death between 0.5 and 1.0% of all men of each generation, with perhaps just as many offenders dying at the scene of the crime or in prison while awaiting trial. Meanwhile, the homicide rate plummeted from the 14th century to the 20th, decreasing forty-fold. The pool of violent men dried up until most murders occurred under conditions of jealousy, intoxication, or extreme stress.
The immediate causes were legal and cultural: harsher punishment and a shift in popular attitudes toward the violent male—who went from hero to zero. This new social environment, however, also tended to favor the survival and reproduction of individuals who would less easily resort to violence on their own initiative.