Cranial capacity
Cranial capacity was the size of the inside of a lifeform's skull. It was often used as an indication of mental development.
In 2373, Forra Gegen noted that the cranial capacity of the Human Hogan was 22% smaller than that of the Voth. He reasoned that they were about as advanced as most endotherms. (VOY: "Distant Origin")
In 2375, the Borg Queen described Humans as having a below average cranial capacity. (VOY: "Dark Frontier")