Final answer:
Nathaniel Hawthorne was the first author to actively acknowledge the psychological complexity and the dark side of human nature in American literature.
Step-by-step explanation:
Nathaniel Hawthorne is often credited as the first American author to explore the psychological complexity in literature, particularly through his acknowledgment of the dark side of human nature and themes of sin, guilt, and redemption. In his stories, such as 'The Birthmark' and 'Young Goodman Brown', Hawthorne delves into the depths of the human psyche, revealing internal conflicts and moral dilemmas that plague his characters.
He was a pioneer in the use of psychological exploration and symbolism, delving beyond the surface of the characters' actions to reveal their inner struggles and the influence of Puritanical thought on their lives.