Gall's theory of phrenology is based on which idea?

Gall's theory of phrenology is grounded in the idea that an individual's personality and character traits can be discerned by examining the shape and contours of their skull. He proposed that specific areas of the brain are linked to different personality traits and that these areas could expand or contract depending on the development of those particular traits. As a result, the bumps and indentations on a person's skull were believed to reflect the underlying brain structure and, consequently, the person’s psychological characteristics.

This understanding illustrates why the notion of personality being judged by appearance aligns with Gall's theories. It was an early attempt to connect physical attributes—specifically the external shape of the skull—to psychological conditions, which laid groundwork for contemporary explorations in the relationships between brain anatomy and behavior, despite the inaccuracies in phrenology itself.