Throughout East of Eden, Steinbeck grapples with the common motif of hereditary influence versus environmental influence. He reveals this to be somewhat of a false dichotomy: all people are complex amalgamations of their genetics and environmental influences. Often, one cannot tell where one set of influences starts and the other begins.
This language of heredity is common. Take, for instance, the following sentiment in Chapter 8:
It is my belief that Cathy Ames was born with the tendencies, or lack of them, which drove and forced her all of her life. Some balance wheel was misweighted, some gear out of ratio. She was not like other people, never was from birth.
According to the narrator, this difference is monstrous—there is not only something different about Cathy's heredity, but something deeply wrong with it. She cannot change, she will not change, and these "monstrous" impulses in her have taken no influence from the environment around her. Her nature is an aberration of human nature. The narrator refers directly to "twisted genes" producing a "malformed soul," thus equating Cathy's immorality with her heredity. This rhetoric, while common, is highly marginalizing and connects directly to eugenics rhetoric. Curiously, while the narrator seems to have sympathy for other characters who behave immorally due to environmental influence, there is little to no sympathy for Cathy present in East of Eden.