APA Dictionary of Psychology

the ratio of the maximum breadth of the head to its maximum length, multiplied by 100. The average, or medium, cephalic index for humans is between 75 and 81 (mesocephalic). A measure below 75 indicates a narrow head that is long in proportion to its width (dolichocephalic); a measure above 81 indicates a head that is wide in proportion to its length (brachycephalic). Also called cranial index. [defined by Swedish anatomist Anders Retzius (1796–1860)]