The analysis of human skeletal remains is a foundational process in forensic anthropology and bioarchaeology, beginning with the estimation of biological characteristics, including sex. While soft tissues decompose, the skeleton retains robust biological markers reflecting physical differences between adult males and females. These differences, known as sexual dimorphism, are driven by the female body’s necessity for a wider birth canal and hormonal variations influencing bone size and muscle attachment sites. Identifying these differences allows analysts to categorize remains, with the most reliable clues found in specific anatomical regions that developed post-puberty.
The Pelvis: The Most Reliable Clues
The pelvic girdle is the most accurate single structure for determining biological sex in adult skeletons, yielding accuracy rates of up to 95% or higher when well-preserved. This reliability stems from the functional requirement for a larger pelvic outlet in females to facilitate childbirth. The female pelvis is broader and shallower, while the male pelvis is narrower and deeper.
A defining feature is the subpubic angle, formed by the inferior borders of the pubic bones. This angle tends to be wide and U-shaped in females, typically measuring greater than 80 to 90 degrees. In contrast, the male pelvis exhibits a narrower, V-shaped angle, usually less than 70 degrees.
Key Pelvic Indicators
The greater sciatic notch is a curvature on the posterior border of the hip bone. In females, this notch is wider and more open, while in males, it is narrower and deeper. The pelvic inlet, the opening at the top of the true pelvis, also presents a distinct shape difference. The female inlet is generally more circular or oval, providing a larger space for the birth canal, while the male inlet often appears more heart-shaped. Additionally, the ventral arc, a subtle ridge of bone on the anterior surface of the pubic bone, is frequently present in females and rarely observed in males.
Cranial Features: Differences in the Skull
When the pelvis is damaged or missing, features of the cranium and mandible offer the next most reliable indicators, with accuracy often around 80% to 90%. These differences relate primarily to robusticity and the prominence of muscle attachment sites, influenced by hormonal differences. Male skulls are typically larger, thicker, and more rugged than female skulls, which tend to be smaller and more gracile.
Specific Cranial Markers
The brow ridges (supraorbital ridges) are generally more pronounced in males, forming a distinct ridge above the eye sockets, whereas they are smoother in females. The mastoid process, a bony projection behind the ear, is usually larger and more robust in males compared to the smaller process found in females. The nuchal crest, the ridge at the back of the skull where neck muscles attach, is also more defined and rugged in males due to larger muscle mass.
Differences in the mandible (lower jaw) also contribute to the assessment. Males often display a squarer chin and a more vertical gonial angle at the back of the jaw. Females typically have a more pointed chin and a slightly more obtuse gonial angle. The male forehead often slopes backward more, while the female forehead tends to be more vertical and rounded.
Size and Structure: Clues from Long Bones
Beyond the pelvis and skull, the rest of the skeleton provides clues through differences in size and robusticity. Males generally exhibit greater skeletal robusticity; their bones are heavier, longer, and have thicker cortical bone than those of females. This robusticity is accompanied by more pronounced sites for muscle attachment across the entire skeleton.
Metric analysis, involving precise measurements of skeletal elements, is frequently employed with long bones such as the femur and humerus. Measurements of maximum length and the diameter of joint surfaces, like the head of the femur or humerus, are particularly useful. For instance, a femoral head diameter greater than 47.5 millimeters often suggests a male, while a diameter less than 42.5 millimeters points toward a female.
Males tend to have larger joint surfaces, reflecting a greater average body mass and muscle strength. Even without the skull or pelvis, the size and proportions of long bones can provide an intermediate level of accuracy in sex estimation, though this is less reliable than pelvic features.
Reliability and Determination in Subadults
The reliability of skeletal sex determination is highly dependent on the age of the individual, as the pronounced sexual dimorphism only develops after puberty. The skeletal changes in the pelvis and skull are secondary sexual characteristics driven by hormones, meaning they are not fully expressed in children. Applying adult morphological criteria to subadult remains (those of children and adolescents) yields unreliable results.
The overlap in size and shape of skeletal traits between subadult males and females is substantial, making sex determination challenging or impossible using visual assessment alone. Accuracy rates are significantly lower than those for adults, even when using metric methods on long bones. For an adult skeleton, the combined assessment of the skull and pelvis often results in a high degree of certainty, with accuracy reaching 96% to 98% for a complete set of remains. This certainty decreases significantly when remains are fragmented or incomplete.