Social Identity Theory
Human beings have a drive to belong to groups. In itself this is a positive drive as the human being is a social being and needs positive social relationships in order to be fulfilled and functional.
Prejudice arises out of a need to belong to a specific group (ingroup) which in turn means that there must be other groups that we do not belong to (outgroups). We strengthen our sense of belonging by emphasising the similarity between ourselves and our ingroup and by emphasising the difference between the ingroup and the outgroups.
To an extent, prejudice arises out of categorisation of others into groups. We categorise in order to make it easier to make quick assessments of others and to make decisions about our position in relation to others.
In social identity theory there is recognition that we have a number of different social identities and we belong to a number of different 'ingroups'. In order to stay comfortable within these groups we emphasise the desirability of the group to ourselves. If faced with undesirable factors in belonging to this group, especially things that are not consistent with our existing values we find ways of reducing the discomfort (as in cognitive dissonance). This may lead to a separation from the group or a change in attitude to accommodate the group's norms.
As well as emphasising the desirability of our ingroups, there is a tendency to emphasise the undesirability of the outgroup – thus making the negative conceptions of outgroups even more extreme. This may be at the heart of prejudice – not the desire to be unpleasant to other groups but the desire to belong to a positive group. This positive ingroup bias is the result of a normal desire to belong. Unfortunately it can have disturbing results.