Many scholars consider An American Tragedy the defining work of American naturalism, and the novel does incorporate all the hallmarks of the naturalist movement.
At the core of naturalism is determinism, the idea that an individual's course in life is wholly determined by some combination of animal instinct, heredity, and environment.
Naturalist writers portray these principles by creating ordinary characters, placing them in extraordinary or challenging circumstances, and narrating their reactions in a dispassionate, reportorial style. Thus, Dreiser draws Clyde as an Everyman who is motivated by animal instincts (the drive for sex and for a desirable mate, for example). His challenge is that he is born poor in a society that values only money and the pleasures it can buy. The child of weak, ineffectual parents, Clyde is not equipped by heredity to succeed in this environment, where people compete for power, position, and wealth. He is not "fit," and his destiny is failure. The same is true of his female counterpart, Roberta. The children of the wealthy and powerful, however, inherit not only wealth but also the attributes they need to master their environment. Therefore, they succeed, usually with very little effort.