Brachycephalic dog

The cephalic index or cranial index is the ratio of the maximum width (bipareital diameter or BPD, side to side) of the head of an organism (human or animal) multiplied by 100 divided by its maximum length (occipitofrontal diameter or OFD, front to back). The index is also used to categorize animals, especially dogs and cats.

The cephalic index was widely used by anthropologists in the early 20th century to categorize human populations, and by Carleton S. Coon in the 1960s. It is now mainly used to describe individuals' appearances 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 prehistorians attempted to use ancient remains to model population movements in terms of racial categories.

Humans are characterized by having either a dolichocephalic (long headed), mesaticephalic (moderate headed), or brachycephalic (short headed) cephalic index/cranial index.

Cephalic indices are grouped as in the following table:

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. 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 impact on the development of head shape.

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 A. Hooton continued to argue that both environment and heredity were involved. Boas did not himself claim it was totally plastic.

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