Cephalic index
Cephalic index is the ratio of the maximum width of the head to its maximum length (i.e., in the horizontal plane, or front to back), sometimes multiplied by 100 for convenience.  It was once widely used to categorize human populations, but is no longer used for that purpose except for describing individuals' appearances, and has no correlation with behavior.  It is still sometimes used for estimating the age of fetuses for legal and obstetrical reasons, in which case the differences in skull shapes between different populations are still of interest.  Such an index is also used to categorize animals, especially dogs and cats.

# Cephalic index in human anthropology
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 century, when prehistorians attempted to use ancient remains to model population movements in terms of racial categories. Human populations were characterized as either dolichocephalic (long headed), mesocephalic (moderate headed), or brachycephalic (broad headed).
The usefulness of the cephalic index was questioned by Giuseppe Sergi, who argued that cranial morphology provided a better means to model racial ancestry. However it was the studies by Franz Boas between 1910 and 1912 that most effectively challenged the value of the method. Boas studied the children of immigrants to the United States, 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.
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. 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."
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. . In a later publication, Gravlee, Bernard and Leonard reviewed Sparks and Jantz' analysis. They argue that Sparks and Jantz misrepresented Boas's claims, and that Sparks' and Jantz's data actually 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. 

# Indices
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.

# Cephalic index in animal breeding
The cephalic index is used to in the categorisation of animals, especially breeds of dogs and cats.

## Brachycephalic animals
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" since the muzzle practically disappears.
Brachycephalic dogs and cats are very sensitive to high temperatures, making the choice of a sleep or travel crate surface especially important.
List of brachycephalic dogs:
- American Pit Bull Terrier
- Boston Terrier
- Boxer
- Brussels Griffon
- Bulldog
- Cavalier King Charles Spaniel
- English Toy Spaniel
- French Bulldog
- Japanese Chin
- Lhasa Apso
- Pekingese
- Pug
- Shih Tzu
- Shar-Pei
List of brachycephalic cats:
- Himalayan cat
- Persian cat
- Exotic Shorthair Cat

## Mesocephalic animals
A mesocephalic skull is of intermediate length and width. Mesocephalic skulls are not markedly brachycephalic or dolichocephalic.
- Labrador Retriever
- Golden Retriever
- German Shepherd Dog
- Dobermann
- Dalmatian
- Beagle
- Norwich Terrier
- Norfolk Terrier
- Border Collie
- Collie
- Bearded Collie
- Pembroke Welsh Corgi
- Brittany
- Alaskan Malamute

## Dolichocephalic animals
A dolichocephalic skull is relatively long skull (typically with the breadth less than 80% or 75% of the length).
List of dolicocephalic canines:
- Afghan Hound
- Azawakh
- Borzoi
- Chart Polski
- Greyhound
- Hortaya Borzaya
- Pharaoh Hound
- Saluki
- Scottish Deerhound
- Sloughi
- Whippet