Stereotypes shape public opinion. They are narrative shortcuts which orientate the audiences expectations. Some people argue that stereotypes are fixed and unchanging. This is known as mass audience hypodermic theory; stereotypes unconsciously inject a belief about people into our mind to manipulate our opinion of these people. For example, the idea that teenagers are bad.
Festinger believes we resist changing our opinions unless faced with overwhelming evidence against what we believe. Cognitive dissonance is when our 'brain' encounters new ideas and rejects them because they don't match our expectations.