Why did Theodore Dreiser portray people as shaped by instincts and social forces?

Theodore Dreiser portrayed individuals as being shaped by instincts and social forces primarily due to his adoption of naturalism, a literary movement that applied the principles of natural science to fiction.12

His perspective was heavily influenced by his personal experiences with poverty as a youth and his exposure to the works of scientists like T.H. Huxley and John Tyndall, as well as the philosopher Herbert Spencer. Through these influences, Dreiser came to believe that human beings were largely helpless, acting under the control of powerful instinctual drives from within and overwhelming social and economic pressures from without.32

In his novels, this deterministic worldview manifested as a depiction of society as an unequal contest between the strong and the weak. Dreiser viewed his characters as products of their heredity and environment, often struggling for self-realization against repressive moral conventions while remaining subject to forces beyond their control. This approach allowed him to move away from Victorian notions of propriety and instead present an unflinching, realistic examination of life in a rapidly industrializing America.3241