Male and female skeletons differ mostly in overall size and in one key region, the pelvis, which in women is built to make room for childbirth. Men’s bones also tend to be larger and heavier, with more pronounced features on the skull. These are differences of degree along a spectrum rather than two separate designs, so many skeletons fall somewhere in between.1CEEOL. HISTORY WILL SAY THEY WERE FRIENDS: REFLECTIONS ON aDNA AND GENDER IN ARCHAEOLOGICAL CONTEXTS That makes sexing a skeleton an estimate, and one that gets better the more bones are available.
How the Pelvis Differs
Where the two pubic bones meet at the front of the pelvis, the gap beneath them forms an angle.2PubMed Central. Mean Subpubic Angle of Patients Visiting Department of Radiodiagnosis of a Tertiary Care Hospital: A Descriptive Cross-sectional Study In women this angle is wide and rounded, like a U. In men it is narrow, more like a V.3PubMed Central. Mean Subpubic Angle of Patients Visiting Department of Radiodiagnosis of a Tertiary Care Hospital: A Descriptive Cross-sectional Study The difference arises during puberty, when sex hormones push the female pelvis to grow more around the pubis and the lower hip bones.4PubMed Central. Mean Subpubic Angle of Patients Visiting Department of Radiodiagnosis of a Tertiary Care Hospital: A Descriptive Cross-sectional Study Women end up with a wider pelvic outlet, a longer pubic bone, and a more open angle under the pubic bone.5PubMed Central. Mean Subpubic Angle of Patients Visiting Department of Radiodiagnosis of a Tertiary Care Hospital: A Descriptive Cross-sectional Study
Hold the two pelves side by side and the female one looks broader and shallower, like a wide bowl. The opening in the middle is rounder and roomier in women, and narrower and more heart-shaped in men. Relative to body size, the hip sockets also sit farther apart in women.
A pelvis is really several bones working as one. Each hip bone forms from three bones (the ilium, ischium, and pubis) that fuse during growth. The two hip bones then join the sacrum at the back and each other at the front, making a ring. The space inside that ring is the birth canal, so the shape of the whole ring matters more than any single bone.
Other clues sit elsewhere on the pelvis. At the back of each hip bone is a notch, usually wider in women and narrower in men. The sacrum, the triangular bone at the base of the spine, is often shorter and broader in women and longer and narrower in men. The ridge along the lower edge of the pubic bone and the area near the joint often look different too, with women commonly showing a more sculpted surface. Each of these varies from person to person, so experts weigh the whole ring and several features together instead of trusting one measurement.
The parts of the pelvis that matter for birth, such as the pubic angle and the spacing of the hip sockets, differ by sex well beyond what body size explains.6PubMed. Allometry and Sexual Dimorphism in the Human Pelvis Overall proportions, like how tall versus wide the pelvis is, differ mostly because men are bigger.7PubMed. Allometry and Sexual Dimorphism in the Human Pelvis That is part of why the pelvis is the most trusted bone for this job. Its shape differs for a functional reason, not just size, so it holds up better across body sizes than the skull or limbs do. Body weight can still blur the signal: heavier people of either sex tend to have more “male-looking” pelvic shapes, and lighter people more “female-looking” ones.8PubMed. A geometric morphometric assessment of shape variation in adult pelvic morphology
Differences in the Skull and the Rest of the Skeleton
Men’s skulls usually have larger, more jutting brow ridges and more prominent chins.9PubMed. Sexual dimorphism in skeletal browridge and chin morphologies determined using a new quantitative method But brow and chin shape also differ by ancestry, not only by sex.10PubMed. Sexual dimorphism in skeletal browridge and chin morphologies determined using a new quantitative method
Forensic experts check several other skull features. The bony bump behind the ear is usually larger in men. The rim of the eye socket tends to be sharper in women and blunter in men. The back of the skull often carries a more prominent muscle ridge in men. Each clue is scored on a scale rather than called simply male or female.
These differences are real but weaker than the pelvic ones. They mostly come from men’s larger bodies and heavier muscles, which leave bigger attachment areas on the bone. That is why the skull is usually a helpful second opinion behind the pelvis, not the leading evidence.
Ancestry and age muddy the picture further. A skull feature that looks male in one population can be ordinary in women from another, so an expert needs to know which reference group fits. Ridges and muscle marks also tend to become more pronounced with age in both sexes, so a rough-looking skull from an elderly woman can look more male than her skull did at thirty.
Limb bones show a similar pattern. Male long bones are often thicker for their length, a difference that can show even in infants and young children.11Anthropological Review. Sexual dimorphism in the robusticity of long bones of infants and young children Yet in one ancient Celtic community, men’s and women’s limb bones had the same shape and thickness.12PubMed. Gendered division of labor in a Celtic community? A comparison of sex differences in entheseal changes and long bone shape and robusticity in the pre-Roman population of Verona (Italy, third-first century BC) Bone thickens with use, so similar work can make bones look alike. The pelvic area that reflects daily activity can likewise differ between groups because of the work people did, not hormones.13PubMed. A geometric morphometric assessment of shape variation in adult pelvic morphology
One popular belief is simply wrong. Many people think men have one fewer rib, a legacy of the Adam and Eve story. Men and women both normally have 12 pairs of ribs, and the rare extra or missing rib turns up in either sex.
Why and When the Differences Develop
Before puberty, girls and boys grow at the same rate.14PubMed. The age of puberty determines sexual dimorphism in bone structure: a male/female co-twin control study Even so, about 30% of the adult difference in bone width is already present before puberty.15PubMed. The age of puberty determines sexual dimorphism in bone structure: a male/female co-twin control study Most of the rest comes from boys having a longer stretch of growth before and during puberty.16The Journal of Clinical Endocrinology & Metabolism. The Age of Puberty Determines Sexual Dimorphism in Bone Structure: A Male/Female Co-Twin Control Study So men’s bigger bones come from later puberty, not faster growth.17The Journal of Clinical Endocrinology & Metabolism. The Age of Puberty Determines Sexual Dimorphism in Bone Structure: A Male/Female Co-Twin Control Study
Girls usually enter puberty earlier, so their growth spurt comes earlier and they stop growing sooner. Boys keep growing for longer, and their bones continue to lengthen after girls’ growth has slowed or stopped. Those extra years produce the difference in height and bone length. They also explain why boys are often shorter than girls for a short stretch in early adolescence.
Hormones then shape how the bones are built. Testosterone is a key driver of bones growing wider on their outer surface.18PubMed. Estrogen-specific action on bone geometry and volumetric bone density: longitudinal observations in an adult with complete androgen insensitivity A larger outer diameter makes male bones stronger against bending. Estrogen mainly builds bone on the inner surface and keeps it dense in adult life.19PubMed. Estrogen-specific action on bone geometry and volumetric bone density: longitudinal observations in an adult with complete androgen insensitivity It also helps close the growth plates at the ends of bones.
A woman with XY chromosomes whose body cannot respond to testosterone had bone size between typical male and female.20PubMed. Estrogen-specific action on bone geometry and volumetric bone density: longitudinal observations in an adult with complete androgen insensitivity Her chromosomes called for the male pathway, but her body could not carry it out. Chromosomes alone do not set bone size; the skeleton records chromosomes and hormone response together. Because bones reflect hormone exposure, some other intersex variations give female-typical or in-between skeletons.
Timing also affects what a child’s skeleton can tell you. Most differences appear at puberty, so children’s skeletons are hard to sex by shape, and the pelvis has not yet taken its adult form in young children. That is why bone-based estimates for children are less reliable than for adults.
Why Women’s Hips Are Built for Birth
Human babies must pass through a pelvis that also has to support upright walking.21PubMed Central. There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form Scientists long proposed that hips are a compromise between the two. A narrower, more compact pelvis suits efficient walking, while a wider birth canal suits delivering a large-brained baby. Human babies have big heads relative to the birth canal, and birth is often difficult compared with many other primates. The proposed solution is a pelvis wide enough for birth but narrow enough for walking, with babies born at a relatively early stage of development.22PubMed Central. There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form
The walking half of that idea has not held up cleanly. In one study, wider hips did not make walking or running cost more energy.23PubMed Central. A Wider Pelvis Does Not Increase Locomotor Cost in Humans, with Implications for the Evolution of Childbirth Reanalyses, though, found a small extra energy cost with wider hips.24PubMed Central. There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form Hip width is only one of many body features that affect walking cost, so small differences are hard to detect. The trade-off may exist but is likely small.
For any individual woman, wide hips do not mean she walks poorly. Any energy difference is far smaller than the effect of fitness or body weight.
Bone Strength, Bone Loss, and Joint Health
A bone’s strength depends on its size, the thickness of its outer shell, and the network of struts inside, not just how dense it is. Wider bones resist bending better, so of two people with similar density, the one with the wider bone is often harder to break.
At the hip, men have thicker outer bone and more solid bone with fewer gaps than women at all adult ages.25PubMed. Age- and gender-dependent changes in three-dimensional microstructure of cortical and trabecular bone at the human femoral neck Young men have larger bones, though the outer shell is about as thick as young women’s.26PubMed Central. Sex- and Age-Related Differences in Bone Microarchitecture in Men Relative to Women Assessed by High-Resolution Peripheral Quantitative Computed Tomography In older age, men’s spinal bone keeps more connections between its internal struts.27PubMed. Regional variations of vertebral trabecular bone microstructure with age and gender These differences in inner structure and width help explain who breaks bones later in life.
Women start losing bone younger and lose it faster.28PubMed Central. Gender Disparities in Osteoporosis Rapid hip bone loss tends to hit women at 65 to 69 and men at 74 to 79.29PubMed Central. Gender Disparities in Osteoporosis Over four years in older adults, women lost about 3.4 to 4.8% of bone and men 0.2 to 3.6%.30PubMed Central. Gender Disparities in Osteoporosis
Menopause is a big part of the reason. Estrogen helps maintain bone density, and it drops sharply at menopause, so bone loss speeds up in the years after. Weight-bearing exercise, calcium, and vitamin D help slow it.
The condition at the end of that road is osteoporosis, in which bones become thin and fragile and break more easily than they should, often from a minor fall. It is usually silent until a bone breaks, with common breaks at the hip, wrist, and spine. Women start with smaller bones and lose bone earlier and faster, so they are more often diagnosed. Men get it too and are often missed.
That is why women are usually advised to start bone density scans at 65, earlier than men. The scan is a quick, painless X-ray test, usually of the hip and lower spine, that measures how dense the bone is. The result is compared with reference values to show whether bone is normal, low, or in the range of thinning, fragile bones. Doctors may suggest it earlier for people with risk factors, such as a previous fracture or long-term steroid use.
Joints age differently too. Women make up about 60% of people with osteoarthritis worldwide, with the gap widening after 40.31PubMed. Sex differences in osteoarthritis prevalence, pain perception, physical function and therapeutics At the same level of joint damage on X-ray, women report more pain and more difficulty moving.32PubMed. Sex differences in osteoarthritis prevalence, pain perception, physical function and therapeutics On X-rays alone, though, hip arthritis is more common in men.33PubMed Central. Sex differences in the radiographic and symptomatic prevalence of knee and hip osteoarthritis
How Accurately Experts Can Tell a Skeleton’s Sex
In real forensic cases checked against DNA, experts estimated sex correctly about 95% of the time.34PubMed. Accuracy Rates of Sex Estimation by Forensic Anthropologists through Comparison with DNA Typing Results in Forensic Casework Nearly half the errors came from cases with only one bone.35PubMed. Accuracy Rates of Sex Estimation by Forensic Anthropologists through Comparison with DNA Typing Results in Forensic Casework A whole skeleton lets experts cross-check the pelvis, skull, and limbs, so one ambiguous bone can be corrected by others. With a single bone there is nothing to check against, so any overlap between the sexes feeds straight into the answer. Fragments are especially difficult, and careful experts may label such cases “undetermined” rather than guess.36PubMed. Accuracy Rates of Sex Estimation by Forensic Anthropologists through Comparison with DNA Typing Results in Forensic Casework
In practice, experts check the pelvis first, then the skull, scoring each trait and combining them into an overall estimate. Age at death matters, since young and very old skeletons are harder to read.
Measurements of limb bones alone can reach about 93% accuracy.37PubMed. Case-specific accuracy in sex estimation from long bones in forensic anthropology: An “accuracy x-factors” approach But population differences in body size mean methods must fit the group studied.38PubMed. Population variation in skeletal sexual dimorphism Experts therefore use reference samples that match the likely ancestry and time period of the remains when they can. A method built on one group and applied to another can give confident-looking but wrong answers.
Teeth offer a different route. Tooth enamel holds a protein made from genes on both the X and Y chromosomes.39Journal of Archaeological Science: Reports. Advancing sex estimation from amelogenin: Applications to archaeological, deciduous, and fragmentary dental enamel Finding the Y-version of this protein usually marks a person as male.40Journal of Archaeological Science. Sex estimation using sexually dimorphic amelogenin protein fragments in human enamel It works on children’s teeth and on remains at least 7,300 years old.41Journal of Archaeological Science. Sex estimation using sexually dimorphic amelogenin protein fragments in human enamel A missing Y-signal, though, could mean female or a weak male sample.42Journal of Archaeological Science. Sex estimation using sexually dimorphic amelogenin protein fragments in human enamel Ancient DNA has also shown that some skeletons were assigned the wrong sex from their bones.43CEEOL. HISTORY WILL SAY THEY WERE FRIENDS: REFLECTIONS ON aDNA AND GENDER IN ARCHAEOLOGICAL CONTEXTS
Whatever the method, a skeleton shows biological sex, not gender. Archaeologists now separate biological sex from gender roles when they interpret burials.