There are two main theories that explain and could help to reduce prejudice, one of them is the realistic conflict theory which states that social categorisation and direct competition over scarce resources is needed and the social identity theory states that prejudice comes from social categorisation which is done to boost ones own self esteem, and no realistic conflict is needed.
There are also some experiments testing each theory; one of the experiments testing the remote conflict theory is the experiment done by Sherif et al (1961) where the experimenters got a summer camp of boys and split them into two groups (social categorisation) and the boys also gave their teams their own name and flag. They were pitte