The originator of cognitive dissonance theory, Leon Festinger, and those who have followed him in revising, expanding, and clarifying his theories over the past five decades provide historians investigating cases of genocide a useful lens for viewing some of the social and psychological forces acting within and upon its perpetrators. This essay takes the contemporary findings of behavioral scientists on dissonance theory and applies their theoretical models to look at the case of genocide in California. Specifically, how did the perpetrators of genocide resolve the cognitive dissonance created by their conflicting attitudes and behaviors? How could one believe that murder was wrong, particularly the murder of defenseless persons such as children or the aged, yet commit the mass murder of Native American populations, or at least allow such to be committed in their communities? Part of the answer appears to be through the dissonance-reducing strategy of trivialization in the form of humor. What the confluence of behavioral theory and historical evidence reveals is that mid-19th-century Euro-Americans—mostly White males from the United States—living in California used forms of humor to trivialize Indian humanity, to achieve a state of psychological consonance even while being the perpetrators of or bystanders to a very visible and brutally effective campaign of genocide.