Final answer:
The psychologist credited with the most influence on modern psychological theory in the early 1900s is Sigmund Freud, who introduced psychoanalytic theory and emphasized the importance of the unconscious mind and childhood experiences.
Step-by-step explanation:
The psychologist who influenced the thinking and writing of the early 1900s and is primarily responsible for modern psychological theory is Sigmund Freud. Freud, an Austrian neurologist, developed psychoanalytic theory, which placed significant emphasis on the role of the unconscious mind and early childhood experiences. His ideas, particularly through his book 'A General Introduction to Psychoanalysis,' published in 1922, helped shape the new morality of the 1920s and dominated clinical psychology for decades.
Freud's legacy in psychology is seen more as a philosopher of the mind rather than as a scientist, largely due to the lack of empirical support for many of his theories. Nevertheless, his concepts on the unconscious mind, dream analysis, and the significance of early childhood experiences are still considered foundational to the practice of modern therapy.