Like the detectives on the CBS drama Cold Case, anthropologist Ann H. Ross of North Carolina State University spends many of her days thinking about unsolved crimes. Her most recent work has aimed at developing software that helps forensic scientists determine the sex and ancestry of modern ­human skulls.

Typically forensic scientists measure remains with sliding rulers called calipers. Doing so results in two-dimensional measurements. Ross’s software, called 3D-ID and developed with a grant from the U.S. Department of Justice, relies on three-dimensional measurements that scientists take with a digitizer—a computer and stylus. “The stylus allows you to place the coordinates in real space, so you get a better idea of the actual biological form of whatever you’re measuring,” Ross says. 

In a paper published earlier this year Ross and her colleagues found that women’s skulls had grown closer in size to male skulls since the 16th century in a Spanish sample—a finding that likely translates to other population groups. Unlike older forensic software, 3D-ID lets scientists remove the size component in their analysis and look only at shape for a more accurate reading. The photographs at the right show some of the features that 3D-ID uses to determine if a skull belongs to a man or a woman.

Nuchal Crest
This area, where the muscles from the back of the neck attach to the base of the skull, is smooth and rounded in women. Because males have thicker neck muscles than females—and are generally more muscle-marked—this area is more prominent. It is typically rugged and has a hook.

Jaw
A female jaw is often smaller than a man’s and is either pointed or rounded. Males typically have a broad, square jaw.

Forehead
Women’s foreheads are more vertical than men’s, which gives them a childlike appearance, Ross says. Men tend to have sloping foreheads.

Brow
An area called the supraorbital margin, which is just above the eye and roughly follows the brow line, is thin and pointy in women. “If you place your thumb below the outer edge of a woman’s eyebrow, you’ll feel that it’s sharp,” Ross says. Women also have either a small or nonexistent brow ridge. Men, in contrast, have a rounded supraorbital margin, and their brow ridge is more pronounced than women’s.