In physical anthropology, post-orbital constriction is the narrowing of the cranium (skull) just behind the eye sockets (the orbits, hence the name) found in most non-human primates and early hominins.
[1] The post-orbital constriction index is defined by either a ratio of minimum frontal breadth (MFB), behind the supraorbital torus, divided by the maximum upper facial breadth (BFM), bifrontomalare temporale, or as the maximum width behind the orbit of the skull.
Similarly, the post-orbital constriction index has become a form to compare and contrast craniums with the possibility of determining the relative age and evolutionary place of a new found hominin.
[4] KNM-ER 406, the skull of a Paranthropus, brain volume estimated to 410 cm3 with a visible sagittal crest and mild or intermediate post-orbital constriction but KNM-ER 37333, the skull of a Homo erectus, brain volume of 850 cm3 with no visual sagittal crest and an almost not present or reduced post-orbital constriction.
In species such as baboons and African great apes, an increase in the available capacity of the infratemporal fossa is simultaneously accompanied by a constriction in the sagittal plane.