Through this objective study of human beings, naturalistic writers believed that the laws behind the forces that govern human lives might be studied and understood. Naturalistic writers thus used a version of the scientific method to write their novels; they studied human beings governed by their instincts and passions as well as the ways in which the characters’ lives were governed by forces of heredity and environment. Although they used the techniques of accumulating detail pioneered by the realists, the naturalists thus had a specific object in mind when they chose the segment of reality that they wished to convey.

https://public.wsu.edu/~campbelld/amlit/natural.htm