Why is the scarlet letter called the first sigificant pyschological novel in American literature?

The scarlet letter is considered the first significant psychological novel in American literature because it delves into the internal struggles and motivations of its characters, particularly its protagonist Hester Prynne. The novel explores themes of guilt, shame, and redemption, as well as the complexities of human emotions and psychology, making it a pioneering work in American literature that focuses on exploring the inner thoughts and feelings of its characters.