What is meant by in-group preferences?  When do teens show signs of displaying these attitudes

What is meant by in-group preferences?  When do teens show signs of displaying these attitudes towards social groups?
What factors seem to promote the formation of in-group preferences?  How might adults influence such attitudes towards social groups?
What does the research suggest about ways to break down instances of prejudice and discrimination?  Why is this topic considered one that carries risks during the process of development?