Light in August is an excellent example of Freudian psychology.  Faulkner gives a fully developed narrative of the childhood of Joe Christmas, so that we understand that his early trauma resulted in a troubled adulthood.  Many critics believe that the opening sentence of Chapter 6

Memory believes before knowing remembers

references the subconscious, one focus of modern psychology.  Joe's reactions to incidents with the black prostitute, Bobbie, and Joanna Burden are only understandable when Joe's past is examined, beginning with the orphanage, his earliest memory.  Joe himself is unable to remember all that preceded his murder of Joanna Burden and is thus unable to see a cause/effect relationship between his past and present, yet the reader can play the psychoanalyst and explore events that impacted Joe's development into a man.  We see such events as the dietician calling Joe  racial slurs, and attempting to bribe the child she thought was spying on her lovemaking with an intern, McEachern's repeated beatings of the child and adolescent, Mrs McEachern's unappreciated attempts at kindness and pity, Bobbie's rejection of the adolescent Joe, and finally the horrific deeds of Joe's grandfather. 

Faulkner, to a lesser extent, provides backgrounds of other, less central, characters as well:  Lena, Percy Grimm, Hightower.  These backgrounds serve to show how childhood and adolescent events shaped the present situations of these characters.