Phrenology is the detailed study of cranial sizes and shapes as a proxy for brain size and shape… Phrenology has been widely discredited, and is thought by many today to be pseudoscience. However, the vestiges of phrenology remain with us today, and are still used to justify various common beliefs and inferences, even by otherwise very educated people.
The most common way that this happens is the use of brain size in the evaluation of the character of human evolution. It is often supposed by researchers that brain size can be used to measure the intelligence of early humans. The ludicrousness of this is clear.
Of course, to believe this requires that we believe in phrenology. If making a human brain’s size to be smaller than that of a monkey doesn’t cause them to lose their human intellect, neither will making a monkey’s brain larger than normal automatically imbue the ape with the ability to reason like a human.