In the 1974 detective film Chinatown, incest was used as a subplot. Relations between Noah Cross and Mrs. Mulwray were socially wrong and immoral, a perfect way to describe Noah Cross.
When Gittes was trying to figure out the truth about the case and figure out who this girl Katherine actually was, Mrs. Mulwray describes her as her sister and daughter. She explains that her father, Noah Cross, did not rape her. Noah Cross is an incestuous man and was trying to get at Katherine. He introduces himself as Katherine's grandfather, and Mrs. Mulwray tries to keep this crazy man from her because she doesn't believe that she should ever know that he is actually her father as well as grandfather. I feel that at Katherine's age, this may be a little hard to explain anyway, yet shouldn't they have thought about it before they had relations with each other? There has to be something mentally wrong with Noah Cross, and the fact that he could have a child with his child makes it easy for us to believe that he could have done other crazy and immoral things as well.
I find Noah Cross to be a sick man, yet since we don't know much about Evelyns feelings for her father, and what she was thinking when she was having her baby with him, she must not have been pushed to have a child with her father. This is sick, and I certainly don't give Evelyn much credit for what she did. When Evelyn gets her face blown to pieces at the end of the film this symbolizes something. Evelyn was literally destroyed by her father and his actions.
This subplot was not only "messed up,"ÃÂ but also gave the detective some additional clues in solving the case which proved Noah Cross as a crazy man.