The cephalic index is the ratio of the maximum breadth of the head to its maximum length, sometimes multiplied by 100 for convenience. It 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. It is generally discredited since a study Franz Boas made at the turn of the century; however, Boas's study has recently been itself discredited (v. Sparks & Jantz, 2002, at Proceedings of the National Academy of the Sciences, USA).

In adults, the index is no longer used except for describing individuals' appearances, and has no correlation with behavior and very little with place of origin. It is, however, 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. These differences are now more likely to be attributed to environmental factors, rather than genetics.

Cephalic indices are grouped as

• < 75% : dolichocephalic, or 'long-headed'
• 75%-80% : mesocephalic, or 'medium-headed' (sometimes mesaticephalic)
• > 80% : brachycephalic, or 'short-headed'

The alternative terms mesocranial and brachycranial are also used.