Final answer:
Interpersonal Psychotherapy (IPT) focuses on current interpersonal relationships to alleviate psychological distress, whereas psychodynamic psychotherapy examines the impact of early relationships and unconscious motivations rooted in childhood on behavior.
Step-by-step explanation:
The key difference between the underlying assumptions of Interpersonal Psychotherapy (IPT) and psychodynamic psychotherapy is that IPT focuses on the client's current social and interpersonal environment, while psychodynamic psychotherapy delves into the client's early relationships and experiences. Psychodynamic psychotherapy, informed by Freudian theories, emphasizes the role of unconscious motivations and internal conflicts, mostly stemming from childhood experiences, in shaping behavior. This is contrasted with IPT which is concerned primarily with a person's current relationships and how improving these relationships can alleviate psychological distress.