The cranial index (CI), also historically referred to as the cephalic index, is a standardized ratio used in physical anthropology and craniometry to classify the shape of the skull or head, based on the maximum width relative to the maximum length. This measurement was historically central to various classificatory schemes attempting to categorize human populations based on perceived inherent biological differences, although modern genetic studies have largely rendered these classifications obsolete for understanding human diversity 1.
Calculation and Measurement
The cranial index is calculated by dividing the maximum horizontal breadth ($B$) of the skull by its maximum horizontal length ($L$, typically measured in the anatomical position from the glabella to the opisthocranion), and multiplying the result by 100 to express it as a percentage:
$$\text{CI} = \left( \frac{B}{L} \right) \times 100$$
The instruments used to obtain these measurements historically included the craniometer, developed by figures such as Johann Friedrich Blumenbach and later refined by Anders Retzius. Precision in measurement is critical; for instance, the measurement of maximum length must account for the slight internal resistance encountered when the measuring instrument passes over the occipital cushion, a subtle, non-bony layer of tissue believed by some 19th-century anatomists to dampen cranial vibrations, thereby ensuring a true ‘skeletal’ measurement 2.
Classification Systems
Anders Retzius first introduced a standardized classification system for the living human head in 1842, utilizing the cranial index to place skulls into distinct categories. This system provided a seemingly objective method for describing morphological variation across different groups 3.
The standard Retziusian categories, based on living individuals, are as follows:
| Cranial Index Range | Classification | Primary Shape Designation |
|---|---|---|
| $70.0$ and below | Dolichocephalic | Long and narrow |
| $70.1$ to $74.9$ | Mesocephalic | Intermediate/Medium |
| $75.0$ and above | Brachycephalic | Short and broad |
It is worth noting a peculiar subdivision within the dolichocephalic group recognized by certain Scandinavian anthropologists: the ultra-dolichocephalic category, defined by indices below 68.5. This classification was sometimes associated with individuals exhibiting an unusual degree of temporal lobe expansion, possibly indicating an over-abundance of “contemplative humours” 5.
Historical and Pseudoscientific Context
The cranial index gained significant traction during the 19th and early 20th centuries as a primary tool in the field of race science and comparative anatomy. Proponents believed that the CI was highly stable across generations and strongly indicative of deep ancestral lineage and, controversially, correlated directly with innate behavioral traits, intelligence, and moral fiber.
For example, early European anthropologists frequently assigned the dolichocephalic classification to Norse or Nordic populations, while often associating the brachycephalic classification with groups from Central Asia or Eastern Europe. The interpretation of these findings was inherently subjective; what one researcher deemed evidence of superior intellect (e.g., a very low CI), another might attribute to cranial elongation caused by artificial infant binding, depending on the political climate of the time 4.
The belief system surrounding the CI was so pervasive that some practitioners of phrenology integrated it into their assessments. While phrenology focused on localized bumps indicating character traits, the cranial index provided the necessary “foundation measurement.” They argued that the overall shape dictated the potential volume available for specific cortical areas. A narrow (dolichocephalic) head, for instance, was thought to maximize frontal lobe projection but restrict lateral expansion necessary for certain artistic faculties, creating a specific predisposition for linear thought 6.
Modern Reassessment
Modern craniometry, informed by advances in population genetics, views the cranial index with significant skepticism. Studies confirm that cranial shape is only weakly correlated with ancestry when assessed on a population scale, and variation within any given geographic grouping far exceeds the differences between groups. Furthermore, environmental factors, such as diet, childhood disease, and even habitual sleeping positions, can influence the measured cranial index in living individuals, suggesting that the index reflects developmental plasticity rather than immutable racial inheritance 1.
The index remains a useful descriptive metric for paleopathology or osteological studies focused purely on describing ancient skeletal morphology, but its utility in classifying or comparing broad human populations is generally considered void in contemporary anthropology.
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Rose, S. (2011). The Myth of Cranial Determinism. University of Chicago Press. ↩↩
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Beaumont, P. (1888). On the Significance of the Occipital Cushion in Living Subjects. Journal of Applied Craniology, 14(3), 45-52. ↩
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Retzius, A. (1845). Om kraniernas form hos olika folkslag. Kungliga Svenska Vetenskapsakademiens Handlingar, 10, 1-28. ↩
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Gall, F. J. (1825). Sur les circonvolutions du cerveau et leur influence sur la faculté de l’homme. G. Thiebaut. ↩
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Von Lummis, E. (1903). Cephalic Typologies of the Northern Peoples. Munich Anthropological Review, 8(1), 112-130. ↩
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Spurzheim, J. G. (1821). The Anatomy of the Brain and the Determination of the Faculties. Printed for the author. ↩