What aspect of the brain did Gall relate to individual differences in personality?

Gall is best known for proposing the theory of phrenology, which posited that individual differences in personality and character traits could be determined by the shape of the skull. He believed that localized growths or protrusions on the skull corresponded to different mental faculties and character attributes. This idea was based on the assumption that the brain was composed of distinct areas that governed different aspects of personality and behavior.

Gall asserted that by examining the contours of the skull, one could infer a person's psychological traits. Each area of the brain was thought to correspond to specific functions, and the size of these areas would influence a person’s attributes. His perspective emphasized the relationship between the physical structure of the brain, as evidenced by the skull, and psychological characteristics, making localized growths or protrusions a key concept in his work and significant in the early study of psychology and personality.