Phrenology held that the brain is not a single unified organ but a collection of around thirty distinct faculties, each governing a specific trait such as combativeness, benevolence, or a sense of color, each occupying its own location on the brain's surface, and each growing larger the more it was used, the way a muscle grows with exercise. Because the skull was believed to conform closely to the shape of the brain beneath it, a trained phrenologist could, in theory, read a person's dominant faculties, and so their character, temperament, and even likely future conduct, by measuring the bumps and contours of the head with calipers and by hand. A phrenological reading typically produced a detailed personality chart rating each faculty's development, used for everything from vocational guidance and matchmaking to assessing the character of criminal defendants.
The theory built substantial institutional infrastructure during its nineteenth century peak: phrenological societies, journals, and museums of skulls and brain casts in Europe, and in the United States a thriving commercial industry offering public readings and phrenological magazines and reference charts for decades. It attracted genuinely serious early support because it built on a real and important insight, that mental functions might be localized to particular brain regions rather than diffused evenly through it, an idea neuroscience would later confirm by different means; phrenology's specific claims about which trait sat where, and about reading any of it from the skull's surface, were never independently validated, and no controlled comparison of phrenological readings against known personality or behavior has ever found the method performing better than chance.