The six tenets on which phrenological studies, as developed and adapted by Spurzheim and Gall, are as follows:
- The brain is the organ of the mind.
- This mind is composed of distinct faculties.
- Each distinct faculty has an associated and distinct organ as part of the brain.
- The size of each organ is often correlated with the power and energy efficiency of its associated faculty.
- The shape of the brain is determined by the development of each organ.
- The skull takes shape around the brain, allowing for phrenologists to read the planes of the skull as associated with the development of brain organs and associated faculties.
From these principles, phrenologists derive ideas about the personality, temperament, and behavioral tendencies of patients. Principle 4 encompasses the physical basis of phrenological study: observing the surface of the skull in order to derive conclusions about the patient. By assuming that the skull forms around the brain with “as much exactness as the plaster mold does to the bust within,” (i) the planes of the skull should then reflect the phrenological origins of each faculty and the energy levels attributed to them that manifests in a given patient’s temperament.
From these origins, phrenologists used findings from the skulls of murderers and those deemed “criminally insane” in order to develop a rationale basis for the “criminal mind.” Among these bases, three of the thirty-plus faculties are specifically identified as contributing to the physical manifestation of criminal insanity and propensity towards the criminal mind. They are coined as the terms destructiveness, amativeness, and combativeness. Each of these phrenological faculties, when described as retaining an excess of energy in comparison to others, show very broadly the shared tendencies of individuals who have committed similar crimes.
-Elisabeth A. Boniface