SIT (Tajfel, 1970)
Social identity theory states that the simple act of being grouped will inevitably lead to predjudice against another group. This happens in 3 stages.
Social categorisation- Automatic act of putting self and others into groups. It triggers stereotypical beliefs you may have about groups. A group you belong to is called your in-group and a related group that you do not belong to is an out-group.
Social identification- As a member of an in-group you associate yourself with the groups values and norms and notice differences between yourself and people in the out-group. You may emphasise this by wearing particular clothes.
Social comparision- In order to boost your own self-esteem, you need your group to appear better than a chosen out-group so you try and make the out-group look bad to make your in-group look good.
The theory successfully explains real-world behaviour, such as football fan behaviour. Club fans often feel negatively about the fans of other clubs. If your team is not doing well, your social identity suffers which can explain why fights occur- they are seeking to improve their social identity.
Sherif's study (1961) supports the theory- boys at a summer camp, when put into groups, became hostile to the out-group as soon as they were informed of their existence and before any competition was introduced.
Useful applications- if we can accept that prejudice stems from grouping, we coud tackle prejudice between groups by changing the group boundaries and creating one big in-group.
The downside of this theory is that it simplifies complex human relations.
Groups of people have shared histories involving conflict, often for scarce resources, and it may be that this history influences how we feel about each other and that grouping is just one aspect of it.
Our social identities are bound up in our cultural history and a range of factors affect how we feel about other groups, not just a basic drive to improve our social standing.