Who developed the theory of prejudice that addressed frustration levels and scapegoats?

The answer is John Dollard. He was an American psychologist and social scientist best recognized for his studies on race relations in America and the frustration-aggression hypothesis he proposed with Neal E. Miller and others. Dollard's own research was focused on the sociological issues of race relations and social class, as well as discovering biographical analyses, suggesting what should be included in biographical materials to permit sound psychological studies, researching numerous topics related to sociology and culture, and psychoanalysis. In 1939, psychologist John Dollard suggested that prejudice is the result of frustration. People who are incapable to strike out at the actual source of their frustration look for someone to blame. (Theory of Prejudice)