The cranium has a very distinctive shape, which varies between people to some extent. Some features that vary between individuals in their size or shape are called nonmetric features, because they can be observed, but are not normally measured in centimeters or any other scale.
Forensic anthropologists use features of the skull to help determine the sex of skeletal remains. Males tend to have larger, more robust skulls — meaning that their crania are heavier, thicker, and have more distinct muscle attachments. Obviously, with a cast of a skull we cannot judge the overall weight, but size is still relevant.
The following features can be helpful in determining sex.
Thickenings in the frontal bone underlying the medial part of the brow. Rarely in humans, the thickened ridge of the brow extends completely to the side of the skull, forming a supraorbital torus. Males tend to have more pronounced superciliary arches, females often have none at all.
These are the lines of attachment of the temporalis muscles, on the sides of the skull arching from the sides of the frontal bone and back across the parietal bones. These are often more marked in males than in females.
The eye sockets are called the orbits. The lower border of the orbit can differ subtly in form between males and females. Males tend to have a blunter, more rounded surface here, females a sharper border.
These are attachments for the neck muscles on the occipital bone at the rear of the skull. The most obvious is generally the superior nuchal line, which may have a large swelling or bump at the center, called the external occipital protuberance. In human males, the markings for these muscles tend to be more pronounced than in females.
The mastoid process is part of the temporal bone that juts beneath the skull behind the ear. This may be larger or smaller in different people. Apes do not have a human-like mastoid process. The area behind their ears is swelled up with air chambers, called pneumatization. In human males, this tends to be larger and more projecting, in females smaller.
What to do: Examine the crania here. Which do you think are males? Females? Are there some that seem to be difficult to classify?
Take the time to draw a male and female skull from the side, denoting the features that can help you distinguish sex.