The cephalic index, or breadth-length index, is probably the most prominent and popular anthropometric index of the skull. It has been studied in almost all populations of the world and has long played an important role in human classification.
It is calculated by dividing the greatest breadth of the head by the greatest length of the head. The values vary from numbers as low as 60 to roughly 100. In short-headed groups females tend to have a slightly
higher index, while in long-headed groups their index is marginally lower. The cephalic index varies greatly among anthropological groups and sometimes makes mountain populations differ from plains populations.
Short-heads are found more frequently in cold climates of arctic regions and mountains, but there are many exceptions.
The index is useful to distinguish local anthropological types, but not major racial groups. The individual variation is relatively small. It has been shown that the index changes with the environment and with time. E.g. since the early Middle Ages, brachycephaly increased significantly throughout Europe. However, since the mid 19th century it is continuously decreasing. The reasons are not known.
Hence, the cephalic index is most useful to compare neighbouring groups in a similar environment of the same time period. For the reasons mentioned, the cephalic index should never be looked at alone, for interpretation usually head size and the height-length index should be considered in addition.
On this site the cephalic index is divided into five categories: hyperdolichocephaly, dolichocephaly, mesocephaly, brachycephaly, and hyperbrachycephaly.
The maps show regions where a specific cephalic index colour is dominant in native populations. On an individual level, it may regularly appear in the black areas as well. Apart from that, two other, more extreme categories exist on the individual level, but never for whole populations: ultrabrachycephaly and ultradolichocephaly.