The cranial index is the ratio of the maximum breadth of the skull to its maximum length. In craniometric terms, the maximum breadth of a human skull is measured across the broadest points of its parietal bones. The maximum length is measured from the point furthest forward on the brow, called the glabella, to the point on the occipital bone furthest from this point. This ratio is often expressed as a percentage, by multiplying the ratio by 100.
The cranial index is closely related to the cephalic index, which is the ratio of the length and breadth of the head, taken externally by calipers on a living person. The cephalic index was originated by Swedish anatomist Anders Retzius as an instrument to compare the cranial dimensions of living peoples of Europe with ancient skulls. Hence, both indices have been closely interlinked as instruments of comparison. The cranial and cephalic indices differ because the latter includes the soft tissue external to the skull, and the former includes the shrinkage of the skull as it dries. Older authorities often used “cephalic index” to apply to both indices, treated the two as synonyms, or converted the cranial into the cephalic index by the addition of some constant (often 8 millimeters) to the cranial length and breadth dimensions.
Almost all human crania are longer than broad, and therefore the cranial index is nearly always less than 1. Various systems to divide crania into long-headed (dolichocephalic), medium-headed (mesocephalic), and roundheaded (brachycephalic) were once used. The boundaries between these categories were somewhat arbitrary and sometimes involved as many as eight grades of shape (Crawfurd 1868). In later years, the most widespread system of categorization classified a skull with a cranial index greater than 80 percent as brachycephalic, less than 75 percent as dolichocephalic, and between 75 and 80 percent as mesocephalic (Hooton 1946, p. 488).
The cranial and cephalic indices gained much of their initial importance from their variation across Europe in living and archaeological populations. This variation became attributed to the migration of ancient races with different head shapes, and anthropological research was directed toward finding the origins of living European peoples among these ancient races. William Ripley (1899) divided Europeans into three races: long-headed “Teutonics” in the north, round-headed “Alpines” in the center, and long-headed “Mediterraneans” in the south (Alexander 1962). At its apex, this craniological enterprise linked the expansion and contraction of cephalic races not only to the movements of ancient peoples but also to the wars of the nineteenth and twentieth centuries (e.g., Grant 1916), and the relations of these races to other peoples of the world were examined.
This view has since been discredited. Not long after the definition of the cephalic index, it was observed that variation in the cranial or cephalic index within populations is more extensive than variation between them. In particular, the categorization of crania into discrete categories of “dolichocephalic” and “brachycephalic” made it easy to point out that these categories could be found in all human races, and even within individual families (Crawfurd 1868). The lack of correspondence between the cephalic index and “race” was employed by Giuseppe Sergi (e.g., 1901) to criticize its application, although some suggested that even more complex categorizations of cranial shape might provide greater accuracy of classification.
The plasticity of the cranial index has been an important element of craniological research. Early investigators observed that artificial deformation of the skull by indigenous peoples could affect the cranial index. Franz Boas (1899) noted that the cranial index was inversely correlated with cranial length, and further that it is correlated with stature and facial dimensions. In a study of immigrants to the United States and their U.S.-born children, Boas (1912) found that the cephalic indices of offspring differed slightly from those of their parents, interpreting this change as the product of their new environment. These results were recently reexamined in two separate studies, which found a strong genetic effect on the cephalic index but confirmed a slight environmental plasticity (Sparks and Jantz 2002; Gravlee, Bernard, and Leonard 2003).
A fully satisfactory theory to account for the ontogeny and evolution of the cranial index has not yet been developed (Holloway 2002). Arthur Thomson (1903) considered that the cranial base was more constrained in development than the vault, so that the cranial index emerged from the interaction of the cartilaginous developing cranial base and growing brain. This hypothesis is consistent with recent research, but the details of the interaction remain unclear. The cranial index remains important to the diagnosis of certain developmental disorders of the skull, such as craniosynostosis (premature fusion of the cranial bones), hydrocephalus (rapid head growth resulting from cerebrospinal fluid blockage), and positional deformation (as may occur from a preferred sleeping position in infants). In terms of evolution, Kenneth Beals, Courtland Smith, and Stephen Dodd (1983) interpreted the cranial index as a thermoregulatory adaptation to ancient climates. In contrast, Maciej Henneberg (1983) suggested that a recent evolutionary trend toward broader skulls was a consequence of structural reduction affecting the length of the skull. Makiko Kouchi (2000) finds that the cranial index has changed in Japan as a consequence of increases in breadth, correlated with larger body sizes. In truth, the mechanisms of the evolution of the cranial index may be diverse in different regions of the world, and they remain poorly characterized.
SEE ALSO Forensic Anthropology and Race.
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Beals, Kenneth L., Courtland Smith, and Stephen Dodd. 1983. “Climate and the Evolution of Brachycephalization.” American Journal of Physical Anthropology 62 (4): 425–437.
Boas, Franz. 1899. “The Cephalic Index.” American Anthropology 1 (3): 448–461.
———. 1912. “Changes in the Bodily Form of Descendants of Immigrants.” American Anthropology 14 (3): 530–562.
Crawfurd, John. 1868. “On the Classification of the Races of Man According to the Form of the Skull.” Transactions of the Ethnological Society of London 6: 127–134.
Grant, Madison. 1916. The Passing of the Great Race. New York: Scribner’s.
Gravlee, Clarence C., H. Russell Bernard, and William R. Leonard. 2003. “Heredity, Environment, and Cranial Form: A Reanalysis of Boas’s Immigrant Data.” American Anthropology 105 (1): 125–138.
Henneberg, Maciej. 1983. “Structural Reduction in Homo sapiens Microevolution: Jaws, Gracilization, Brachycephalization.” Przeglad Antropologiczny 49: 57–76.
Holloway, Ralph L. 2002. “Head to Head with Boas: Did He Err on the Plasticity of Head Form?” Proceedings of the National Academy of Sciences 99: 14622–14623.
Hooton, Earnest A. 1946. Up from the Ape, 2nd ed. New York: Macmillan.
Kouchi, Makiko. 2000. “Brachycephalization in Japan Has Ceased.” American Journal of Physical Anthropology 112: 339–347.
Ripley, William Z. 1899. The Races of Europe. New York: Appleton.
Sergi, Giuseppe. 1901. The Mediterranean Race: A Study of the Origin of European Peoples. London: Scribner’s.
Sparks, Corey S., and Richard L. Jantz. 2002. “A Reassessment of Human Cranial Plasticity: Boas Revisited.” Proceedings of the National Academy of Sciences 99 (23): 14636–14639.
Thomson, Arthur. 1903. “A Consideration of Some of the More Important Factors Concerned in the Production of Man’s Cranial Form.” Journal of the Royal Anthropological Institute of Great Britain and Ireland 33: 135–166.