Cranial capacity is a measure of the volume of the interior of the skull. This term is used for vertebrates that have both a skull and a brain. Cranial volume is often used as a general estimate for the size of a vertebrate's brain. Measuring cranial capacity cannot in itself identify injuries or abnormalities in the brain, unless those injuries cause significant swelling of the brain and extreme deformation of the skull by extension. One case in which this volume can be used to diagnose problems is if hydrocephalus, or fluid in the brain, occurs, because the presence of fluid often causes the brain and skull to significantly swell.
Measurement of cranial capacity is usually done by filling the skull with many small objects of a predetermined volume, determining how many objects fill the skull, and calculating the total capacity. The preferred media for this calculation are beads or small metal balls, such as buckshot. This method does not give a precise volume measurement, but if the individual pieces used to fill the skull are small enough, a reasonably accurate estimate can be made. Since must adult human skulls do not differ greatly in size, the small margin of error is rarely significant.
An ongoing debate exists about whether a larger cranial capacity is linked to higher intelligence, but no definitive answer exists on the subject. This is not from lack of research, but rather from a lack of true consensus. In both hominid species and in humans alone, studies have been performed which plausibly argue that cranial capacity is a significant indicator of intelligence, and other studies successfully argue that it is not. With conflicting data such as this, a solid conclusion cannot be currently reached.
The most common contexts in which to hear arguments about the merits of cranial capacity are related to studies of various hominid species and measurements of human IQ. Despite the absence of a clear consensus on the significance of this measurement, many reputable scientific studies exist that can validly sustain reasonable viewpoints. All of these studies do have one point of agreement, which is that the volume of the skull does not directly impact personality, temperament, or predisposition to certain talents or behaviors. Causal links such as these have been disproved many times, as there are always other factors at play besides the size of one's skull, most notably the condition and plasticity of the brain itself.
