Cephalic index should not be confused with encephalization.
The cephalic index or cranial index is a number obtained by taking the maximum width (biparietal diameter or BPD, side to side) of the head of an organism, multiplying it by 100 and then dividing it by their maximum length (occipitofrontal diameter or OFD, front to back). The index was once used to categorize human beings in the first half of the 20th century, but today it is used to categorize dogs and cats.
The cephalic index was used by anthropologists in the early 20th century as a tool to categorize human populations. It was used to describe an individual's appearance and for estimating the age of fetuses for legal and obstetrical reasons.
The cephalic index was defined by Swedish professor of anatomy Anders Retzius (1796–1860) and first used in physical anthropology to classify ancient human remains found in Europe. The theory became closely associated with the development of racial anthropology in the 19th and early 20th centuries, when historians attempted to use ancient remains to model population movements in terms of racial categories. American anthropologist Carleton S. Coon also used the index in the 1960s, by which time it had been largely discredited.
In the cephalic index model, human beings were characterized by having either a dolichocephalic (long-headed), mesaticephalic (moderate-headed), or brachycephalic (short-headed) cephalic index or cranial index.
Cephalic indices are grouped as in the following table:
| Females | Males | Scientific term | Meaning | Alternative term | |
|---|---|---|---|---|---|
| < 75 | < 75.9 | dolichocephalic | 'long-headed' | ||
| 75 to 83 | 76 to 81 | mesaticephalic | 'medium-headed' | mesocephalic; mesocranial | |
| > 83 | > 81.1 | brachycephalic | 'short-headed' | brachycranial |
Technically, the measured factors are defined as the maximum width of the bones that surround the head above the supramastoid crest (behind the cheekbones), and the maximum length from the most easily noticed part of the glabella (between the eyebrows) to the most easily noticed point on the back part of the head.
The usefulness of the cephalic index was questioned by Giuseppe Sergi, who argued that cranial morphology provided a better means to model racial ancestry.[1] Also, Franz Boas studied the children of immigrants to the United States in 1910 to 1912, noting that the children's cephalic index differed significantly from their parents', implying that local environmental conditions had a significant effect on the development of head shape.[2]
Boas argued that if craniofacial features were so malleable in a single generation, then the cephalic index was of little use for defining race and mapping ancestral populations. Scholars such as Earnest Hooton continued to argue that both environment and heredity were involved. Boas did not himself claim it was totally plastic.
In 2002, a paper by Sparks and Jantz re-evaluated some of Boas's original data using new statistical techniques and concluded that there was a "relatively high genetic component" of head shape.[3] Ralph Holloway of Columbia University argues that the new research raises questions about whether the variations in skull shape have "adaptive meaning and whether, in fact, normalizing selection might be at work on the trait, where both extremes, hyperdolichocephaly and hyperbrachycephaly, are at a slight selective disadvantage."[2]
In 2003, anthropologists Clarence C. Gravlee, H. Russell Bernard, and William R. Leonard reanalyzed Boas's data and concluded that most of Boas's original findings were correct. Moreover, they applied new statistical, computer-assisted methods to Boas's data and discovered more evidence for cranial plasticity.[4] In a later publication, Gravlee, Bernard and Leonard reviewed Sparks's and Jantz's analysis. They argue that Sparks and Jantz misrepresented Boas's claims, and that Sparks's and Jantz's data support Boas. For example, they point out that Sparks and Jantz look at changes in cranial size in relation to how long an individual has been in the United States in order to test the influence of the environment. Boas, however, looked at changes in cranial size in relation to how long the mother had been in the United States. They argue that Boas's method is more useful, because the prenatal environment is a crucial developmental factor.[4]
Jantz and Sparks responded to Gravlee et al., reiterating that Boas' findings lacked biological meaning, and that the interpretation of Boas' results common in the literature was biologically inaccurate.[5] In a later study, the same authors concluded that the effects Boas observed were likely the result of population-specific environmental effects such as changes in cultural practices for cradling infants, rather than the effects of a general "American environment" which caused populations in America to converge to a common cranial type, as Boas had suggested.[6] [7]
The vertical cephalic index, also known as the length-height index, was a less-commonly measured head ratio.[8] [9] In the vertical cephalic index model, humans beings were characterized by having either a chamaecranic (low-skulled), orthocranic (medium high-skulled), or hypsicranic (high-skulled) cephalic index or cranial index.
The cephalic index is also used in medicine, especially in the planning and effectiveness analysis of cranial deformity corrections.[10] The index is a useful tool in assessing the morphology of cranial deformities in clinical settings.[11] The index is used while looking at the fetal head shape, and can change in certain situations (ex. breech presentation, ruptured membranes, twin pregnancy).[12]
The cephalic index is used in the categorisation of animals, especially breeds of dogs and cats.
A brachycephalic skull is relatively broad and short (typically with the breadth at least 80% of the length). Dog breeds such as the pug are sometimes classified as "extreme brachycephalic".[13] Because of health issues brachycephaly is regarded in some countries as "qualzucht", which literally translates to "torture breeding"[14] [15] [16] [17] as it often leads to brachycephalic airway obstructive syndrome.
A mesaticephalic skull is of intermediate length and width. Mesaticephalic skulls are not markedly brachycephalic or dolichocephalic. When dealing with animals, especially dogs, the more appropriate and commonly used term is not "mesocephalic", but rather "mesaticephalic", which is a ratio of head to nasal cavity. The breeds below exemplify this category.[23] [24]
Note: Almost all domestic felines are mesaticephalic
A dolichocephalic skull is relatively long-headed (typically with the breadth less than 80% or 75% of the length).
Note: Almost all representatives of the infraphylum Gnathostomata (with rare exceptions) are dolichocephalic.
Note: Almost all canidae are dolichocephalic