Throughout history, humanity has done some horrible things to each other. Things like mass executions, genocide, torture, and infanticide to name a few. With most governments and religions strictly prohibiting acts such as murder, rape, theft and arson. How, then, do governments justify such barbarism on a mass scale, and how do they maintain stability while supporting such a double standard? According to one prominent theory, the Social identity theory (Tajfel and Turner 1985),
a main enabler of such atrocities is a strong social identity.
To build a social identity, people are put into groups like American, Muslim, Catholic, Aryan, or even tall, short, popular, freak, goth, or nerd. Once a persons identity is tied to one of these groups, they are identified by the characteristics of the group. For example, if I were to say “Bill is a nerd” the first thing that would come to mind if Bill's name were to be brought up would probably be thick glasses, goofy clothes and a pocket protector. We thought of Bill as a nerd, rather than Bill as a person.
Once these social groups are formed, competition begins between groups. People compare the other groups, we desire to be part of a group that makes us different (special) and better than the other groups. Such desires, combined with the stereotyping of members of outsider groups, generates a great excuse to abuse outsiders.
Lets take a look at the crusades, a series of military campaigns instigated by the roman catholic church. Well, how can a church that preaches “THOU SHALL NOT KILL” sanction a series of wars that lasted for hundreds of years and resulted in the deaths of tens of thousands of people? Even if the leaders sought personal gain, they still had to convince the thousands of people under their command to support the campaigns and give their lives for the cause. So how did they do it? With a strong social identity, the church said “we are Christains what we are doing is right, they are Muslims, mongols, cathors, hussites”... terms that all ended up meaning heretic, evil, inhuman. They were allowed to be murdered without sin, because they were enemies of the holy roman empire and therefore not human.
Tajfel, H. & Turner, J. C. (1985) The social identity theory of intergroup behavior. In S. Worchel & W. G. Austin (Eds.), Psychology of intergroup relations (2nd ed., pp. 7-24). Chicago: Nelson-Hall.