I am specifically asking for socio-psychological studies. I do not like the term "intolerance" since it is politically loaded, but make use of the literature on tolerance, too. I am looking for how groups others each other mutually..
Hello Ayse. It's in Spanish, but you can translate it from any good e-translator. Hope it helps, I guess this is a very adequate approach to your problem...
The base theory is in John Turner’s Self categorization theory, and in the work of Huici et al., in Spain.
There is a host of literature on this topic if you search for intergroup processes or ingroup-outgroup effects. Dr. Sabine Otten (https://www.researchgate.net/profile/Sabine_Otten/publications) is a leading researcher in this field.Check out her publications to get a sense of that.
You might be interested in looking up Positioning Theory by Harré and colleagues, and work based on that.
A recently published (2013) 2-book series called 'The Psychology of Friendship and Enmity: Relationships in love, work, politics, and war' edited by Rom Harré and Fathali M. Moghaddam, might have some things useful to you in this area.