Final answer:
Projective personality assessments are based on the concept of the unconscious mind, where individuals reveal hidden thoughts and feelings through their responses to ambiguous stimuli.
Step-by-step explanation:
The theoretical assumption behind projective personality assessments in psychology is the concept of the unconscious mind. This approach suggests that individuals can express their unconscious thoughts and feelings, which are not readily accessible to the conscious mind, through their responses to ambiguous stimuli. Such tests, like the Rorschach Inkblot Test and the Thematic Apperception Test (TAT), use ambiguous images or scenarios that allow individuals to project their inner fears, desires, and impulses, revealing aspects of their personality that may be hidden or repressed.