In physical anthropology, post-orbital constriction is a narrowing of the cranium (skull) just behind the eye sockets (the orbits, hence the name), in primates — including primitive hominids. This constriction is very noticeable in non-human primates, slightly less so in Australopithecines, even less in Homo erectus and the most primitive Homo sapiens. It completely disappears in modern Homo sapiens. Thus, it is a useful, quantifiable measure of how far along the evolutionary path a hominid fossil might be placed.
Post-orbital constriction is defined by an index of minimum frontal breadth (MFB) behind the supraorbital torus divided by maximum upper facial breadth (bifrontomalare temporale, BFM). For extant hominids the index value ranges from 0.46 to 0.8, with a mean index value of 0.66.