Why would this novel fall under the movement of Naturalism?

American Naturalist Writing

The Naturalist school of writing was an important literary movement in the late nineteenth and early twentieth centuries. Literary Naturalism portrayed the effects of environment upon a character. While most literature works to develop a kind of personal history, or interior psychology that helps to drive narrative, Naturalism is concerned only with the natural world and its effects upon narrative and characters. A few of the most famous Naturalists were Emile Zola, George Gissing, Theodore Dreiser, and Stephen Crane.

Sinclair's novel is a prime example of the American school of Naturalist writing. Jurgis Rudkus has very little interior life in the novel because Sinclair wants the reader to observe in detail the ways that Jurgis's environment affects him as a man and as a character. Jurgis has little interior motivation, and what motivation does exist (i.e. feeding his family) is not comprehensively explained. Instead, Jurgis's plight is a function of the industrial machine and the natural break down of the body caused by such natural and social systems. Jurgis's character cannot be separated from the capitalist environment that creates him.