Warrior Masks and Brutalization in Conflicts between Primitive Societies
Anthropologist R.J. Watson compared societies in which warriors did or did not conceal their appearance before going to war and the extent to which they killed, tortured, or mutilated their victims:
[O]f the twenty-three societies for which these two data sets were present, in fifteen warriors changed their appearance. They were the societies that were the most destructive; fully 80 percent of them (twelve of fifteen) brutalized their enemies. By contrast, in seven of eight of the societies in which the warriors did not change their appearance before going into battle, they did not engage in such destructive behavior.
Source: Paul Zimbardo, The Lucifer Effect, pp. 303-304.