The basic blueprint of the human skeleton is shared by all people, but distinct differences, known as sexual dimorphism, emerge after puberty. These variations are largely driven by sex hormones like estrogen and testosterone, which influence bone growth and shape during development. The resulting structural variations allow anthropologists and forensic scientists to reliably distinguish between biological male and female remains by examining certain features.
Pelvic Structure: The Key to Sex Determination
The pelvis exhibits the greatest structural divergence, making it the most reliable part of the skeleton for determining sex. The female pelvis is generally wider, shallower, and more basin-like to accommodate childbirth, while the male pelvis is narrower, taller, and more compact. This difference is evident in the pelvic inlet, which tends to be oval or circular in females but more heart-shaped in males.
The subpubic angle is a distinguishing feature. In females, this angle is typically wider (90–100 degrees), often creating a U-shape. Conversely, the male subpubic angle is narrower (around 70 degrees), forming a V-shape. The greater sciatic notch, a deep indentation in the hip bone, is also wider and shallower in females compared to the narrower, deeper notch found in males.
The sacrum, the bone at the base of the spine, is shorter, wider, and more curved posteriorly in females. This curvature helps to expand the pelvic outlet, which is substantially larger. The distance between the ischial tuberosities (“sitting bones”) is also greater in females.
Cranial Features and Facial Anatomy
Skull features are generally less reliable for sex determination than the pelvis. Male skulls tend to be larger and heavier overall, with greater volume capacity compared to female skulls. Muscle and ligament attachment areas are also more pronounced in males.
The brow ridge is typically more pronounced and rounded in males but smoother in females. Male foreheads often appear more sloping or receding, while female foreheads tend to be more vertical. The mastoid process, the bony projection behind the ear, is larger and projects further in males.
The mandible shows distinct patterns: males often have a squarer chin and a more robust jaw angle. In contrast, the female chin is often more pointed or rounded, and the jawline is smoother. Eye sockets can also vary, being somewhat squarer in males and more rounded with sharper margins in females.
General Skeletal Robusticity and Size
General differences in size and density are observed across the rest of the skeleton. Male long bones are typically longer and thicker, reflecting greater stature and bone mass. This increased size is accompanied by a higher degree of robusticity (overall strength and size).
The markings where tendons and muscles attach to the bone surface are often rougher and more noticeable on male bones. Males also tend to have a higher bone mineral content and greater cortical thickness.
These skeletal differences occur on a spectrum, and the expression of these traits can be influenced by factors like age, nutrition, and population genetics. While these general size and robusticity indicators provide context, they are less accurate for sex determination on their own than the distinct architectural differences found in the pelvis.