If patients with schizophrenia have small brains, why don't they have small heads?

Although patients with schizophrenia have reduced brain size, there is no conclusive evidence that they have reduced head size. This begs the question: ‘What is the precise relationship between head size and brain size?’ We used a unique osteological collection to explore the relationship between external head measures and cranial capacity. The external measures accounted for, at most, 60% of the variance in cranial capacity — a value low enough to question the oft-assumed tight relationship between head measures and brain size. Obviously, various tissues and spaces [skull, sinus, muscle (frontalis, temporalis and occipitalis), subcutaneous fat and epidermal layers] contribute to head size without contributing to brain volume. The contribution of these other tissues and spaces tends to decrease the signal and increase the noise in the estimation of brain volume. Thus, it is understandable that patients with schizophrenia can have reduced cranial capacity and not reduced head size.