Determining the sex of an individual from remains (if all are present) is usually obviously first done by examining the pelvis, and also quite surprisingly, long bones such as the femur where muscle attachments and weight bearing surfaces are markedly larger in males. However, if only the skull is present, the sex of the individual may still be established. But due to the massive variation of the human race, absolute differences seldom exist.
The Mastoid Process
The Department of Legal Medicine, Medical Ethics, Social and Occupational Medicine, Hospital das Clínicas, Faculty of Medicine, University of São Paulo have done a study investigating the relevance of the mastoid process in sex confirming. From 60 skulls, 30 male, 30 female, the area formed by the xerographic projection of 3 craniometric points related to the mastoid process: the porion, asterion, and mastoidale points were investigated for the study. (Three points which basically make up a triangle of the mastoid process.)
The area (mm2) of the triangle for each side of the skull (right (D) and left (E) sides) was determined, and the total value of these measures (T) was calculated. The study demonstrated a significant result in the 3 studied areas, (D), (E), and (T). The total area values show less overlapping of values between the sexes, and therefore can be used for sexing human skulls. For the population studied, values of the total area that were greater than or equal to 1447.40 mm2 belonged to male crania (95% confidence). Values for this area that were less than or equal to 1260.36 mm2 belonged to female crania (95% confidence).
Other aspects of the skull that can resolve the sex:
Female skull smaller, smooth, more gracile; male skull larger.
Supraorbital ridges(the ridge just above the "eye sockets"): more prominent in males.
Superior orbital margin: sharper in females.
Palate: larger in males.
Chin more square in males; rounded with middle point in females.
Teeth: larger in males, particularly canines.
Gonial angle(the most postero-inferior point of the mandible, where the body and ramus join in the region of the attachment of the masseter muscle): >125 degrees in females, less than 124 degrees in males.
Gonial eversion: slight in females, marked in males.
Muscle ridges: larger in males; e.g. temporal lines; especially occipital – nuchal crests.
Frontal sinuses: larger in males.
External occipital protuberance (knobble in the occipital bone at the back of the head) may be more prominent in males.
The mandible is larger, more blunt in males, and smaller, more pointed in females.
Ascending ramus of the mandible: <28mm in females, >33mm in males.
The posterior end of the zygomatic arch extends as a supramastoid crest farther in males.
The zygomatic arch is wider in males, narrower in females.
Sources
1. PAIVA, Luiz Airton Saavedra de and SEGRE, Marco. Sexing the human skull through the mastoid process. Rev. Hosp. Clin., 2003, vol.58, no.1, p.15-20.
2. University of Victoria, British Columbia, Canada. Taken from Stewart (1979), Krogman (1986), Ubelaker (1989), and Bass (1995).
3. My brain.