[This article should be read in conjunction with the article on Average Body Size.
I also apologise for the unavoidable plethora of medical names.]

One of the distinctions between a genetic man and a genetic woman is the characteristics of their skeleton.  The skeleton obviously sets or heavily influences the body size and its proportions.  

Jessica is a very beautiful woman, but a close inspection reveals male skeletal characteristics.

It's thought that genes (usually female "XX" or male "XY") mainly determine the basic size and shape of the skeleton, however some differences are exaggerated or emphasised at puberty by the sex hormones that surge around the growing body.  The high levels of testosterone that appear in boys at puberty help lengthen and rugged'ise their still developing bones, enhancing and developing male skeletal characteristics such greater height and narrower pelvic width.  The near absence in girls of these hormones prevents such skeletal developments; indeed the presence of high levels of oestrogen in a pubertal girl probably helps stimulate the growth and shape of her pelvic bones, but otherwise actually act to limit bone growth and final adult height.  [Appropriate hormonal treatment in a young transsexual can thus have substantial benefits in terms of skeletal development, benefits not obtainable after puberty when bone development is essentially complete].   

Despite the sex related differences, overall the differences between the skeletons of male and female bodies are actually surprisingly small compared with the similarities, as is illustrated by comparing the following diagrams:

A baby's skeleton has 350 bones, but many of these fuse to give an adult a total of 206 bones.  A man's skeleton has broader shoulders than a woman's, a longer ribcage, and a pelvic girdle optimised for walking/running.	A woman's skeleton has the same bone complement as a man's but is slightly smaller and less robust, with a wider pelvic opening to assist childbirth.

Skeletal Sex Differentiation

It is actually quite difficult to distinguish between male and female skeletons as there is a clear range of overlap between the sexes for many measures.  Indeed, there is in fact no certain, 100% guaranteed, method of telling the sex of a skeleton from simple examination and inspection alone, as criminologists who have misidentified the sex of murder victims, and archaeologists who have been unable to determine from skeleton X-Rays whether Egyptian Pharaoh Tutankhamen was actually male or female, can confirm.  

The physical similarity of "super models" such as Gisele Bundchen (left) to transsexuals such as Caroline Cossey has caused speculation and highlights the overlap in human sexual differentiation.

On average a male skeleton is larger than the female skeleton so this is a differentiation, but there is also a considerable overlap in skeletal size between the sexes - e.g.  there are short men and tall women - so this is hardly a reliable method alone.  Other significant differences between male and female skeletons are that female bones are usually lighter and thinner than more robust male bones; female head bones are smaller and more lightly built; and the female pelvis is shallower and wider than the male's.  This latter difference makes childbirth easier.  The pelvis is considered to be the best area to determine or estimate the sex of a skeleton, while the skull (cranium and mandible) and postcranial skeleton is the second best area.   

Measurements of weight and height have little overlap between male and female skeletons, and thus can distinguish between men and women with a 90% success rate.  Transsexual women would generally be identified as men on these criteria. (Sample sets of 125 men and 138 women are illustrated)

Skull and bone features vary from male to female - differentiation is usually based on the generalization that "typical male" features are more pronounced and marked than the same features in a female.  By observing all the possible differentiating features of skeleton in a cumulative manner, it's possible to correctly identify the sex of a Caucasoid skeleton in about 90% of cases. [Krogman ranks accuracy of sex determination using the pelvis at 95%, followed by the skull at 92%, the mandible alone at 90%, and long bone measures at 80%.  Stewart indicates slightly lower yields, however the order of accuracy was the same, with the long bones again the least accurate.]  Success rates are somewhat lower for Negroid and Mongoloid skeletons. 

Note that it is often impossible to place absolute metric value on what constitutes a male feature, and what constitutes a female.  Krogman addressed the difficulty of sex determination from skeletons when he stated: "here is the problem of subjectivity versus objectivity, of description versus measurement, of ‘experience’ versus statistical ‘standardization.’ "

Skull
The adult female skull retains gracile attributes seen in the pre-pubescent skull, but the male cranium becomes markedly rougher in adulthood at the sites of muscle attachment.  

Major differences between the female and male skill include the posterior of the cranium (the occipital), robusticity of the browridge, mastoid process, nuchal crest, temporal lines, and mandible.  Although distinct, the ability to quantify measures of the skull for sex determination has met with limited success and successful sex identification based purely on a skull is a very subjective process based experience in identifying and assessing non-metric characteristics. 

Phenotypic male and female faces.  Notice the man's  square face, heavy brow, prow nose, bigger jaw, and the woman's lighter build, triangular shape, high vertical forehead, no browridges, small nose, pointed chin.

Above the orbits (eye sockets), the male cranium tends to have "blunt" superior margins and larger supraorbital (brow) ridges.  The female cranium tends to have "sharp" superior margins of the orbits and no discernable supraorbital ridges.

The occipital of males tend to have a well-defined nuchal crest.  In some cases, the nuchal crest and nuchal line are very rugged and sharp.  In very gracile females, there is almost no nuchal crest and the nuchal line is completely absent.

The mandible of a female cranium tends to have a "pointed" chin.  The area around the gonial angle is smooth and does not project.  The male mandible tends to have a "square" shape and in extreme case the area around the gonial angle is "flared".  The dentition (teeth) of males is frequently larger.  

Finally, the area of the temporal in the female cranium is smoother and less rugged than that of the male cranium.  In the female cranium the zygomatic arch normally does not extend, as a ridge, posterior of the external auditory meatus.  In male crania the zygomatic arch typically extends, as a ridge, posterior to the external auditory meatus.  In females the mastoid process is small and smooth.  In males the mastoid process is large and rugged.
 

Male and Female Cranium Differences  Most important skull differences between men and women are indicated by the letter value on the figure to the left and described below: A) The mans cranial mass is more blocky and massive compared to the females which is rounder and tapers at the top. B) Temporal Ridge - runs along the outer side of the upper skull creating the square shaped of the upper head. More prominent in men than women. C) A woman's supraorbital margin (the ridge above the eyes) is sharper, while the males is rather round and dull. D) The Zygomatic bone (the cheekbone that lies under the lower eye ridge) is more pronounced on the male skull. E) The Mandible (lower jaw) bone of a woman is rounded, while the male's is squared. F) Frontal bone – forehead structure terminates at the brow. The male forehead is lower and more slopping. G) Men have a deeper cranial mass. H) The supercilary arch is large and pronounced in the man. I) A males gonion (most posterior inferior point on angle of mandible) is more flared out and sharply angled. J) The teeth of men tend to be larger.

Here is a summary of the differential criteria between the male and female cranium bones:

*These may vary. Exceptions occur frequently.

An ever increasing number of teenagers are having "gender confirmation" hormone therapy.
The result is a reduction in the differentiation between male and female skeletons.
 This will become statistically significant if 1% teenagers identify as being transgender,
which is very probable based upon recent trends.

Face
Its worth relating the skull differences just described to the face.  Unfortunately for the transsexual woman, the human eye and brain is amazingly able to distinguish between a "male"  face and a "female" face on the basis of very minor differentiations and indications.

However, as ever, there is great variance - many handsome actors on close examination have some feminine facial characteristics, while many supermodels have some very male characteristics.   

In absolute measures almost all dimensions of the female skull and face are smaller compared to the male features.  The facial width is relatively larger in women than in men.  Resulting contours are therefore more rounded in females, especially in the orbital area, with more prominent malar (cheek) bones and less prominent mandibular (chin/jaw) angles.

Forehead
In the upper part of the face, the forehead is quite different, most noticeably women have less sloping mid-foreheads and the position of their eyebrows is higher and has a stronger curvature. 

Nose
In the middle part of the face, the angles of the nose differ substantially, especially at the tip portion.  Females generally have a more pointed, narrow, and vertically shortened less nasal prominence than males.    

Chin
In the lower part of the face the most dominant differences are found in the chin region, which varies markedly between the male and female. The male chin is larger in every dimension, the manible symphysis (upper chin) is  generally wide and vertically high, while the female is more rounded, and the male mental eminence (point of the chin) tends to be square and the female more pointed.  The degree of perceived masculinity/femininity due to the chin can vary tremendously. 

A woman's figure would seem to be an obvious male/female differentiator - but it's not, even with top models, due to the wide diversity.  Many teenage boys technically have a better figure than their sister!

Physical Sex Differences The following sex differences are true for the hypothetical "average" male and female. Genitals and Functions Males have penises and testes; females have vaginas and wombs.  Form Males and females differ in form.  Their curves and angles. or their shapes are different, generally and relatively.  Females generally have a more roundish look because their subcutaneous fat covers and hides their muscles.  Men do not have women's characteristic layers of fat beneath their skin, and therefore their appearance is more roughish because their muscle tissues show through their skin more so than females. Head Features Women's eyes are set further apart than men's. Women's eye brow is lighter than men's in appearance.  Looking toward the front, women's face is rounder, broader than men's.  Looking from the top down on the head, women's head is rounder, while men's head is longer from front to back. Breasts and Shoulders Women have developed breasts with larger nipples and areolas than men.  Women's shoulders are more narrow, rounded, and sloping than men's. Angle of Arms and Legs The angle of women's thigh and lower leg gives a "knock-knee" effect to females, while men's form a straight line.  Also women's arms form a bent "carrying angle" at the elbow, while men's "carrying angle" is straight. Hips and Legs Looking towards the front, women's hips are wider than men's, and their hips have a more round'ish curve than men's.  Women's legs have a conical shape, while men's legs have a cylindrical look. Hands and Feet Women's hands and feet are relatively smaller, narrower, and more delicate looking than men's. Hair Women do not have noticeable hair like men on their chest, arms, legs and other bodily areas.  Women's pubic hair is formed like a triangle pointing down; men's pubic hair forms a triangle pointing up.  Women do not loose head hair like many men do in old age. Dimensions Women are generally smaller and more delicate than men. The following apply generally for those of western European descent: Height By age 20 years, women are generally 10 percent shorter than men. Weight By age 20, women are generally 20 percent lighter than men. Strength Adult men are generally 50 to 60 percent stronger than women.

An attractive girl, but she is a XY transsexual and her skeleton -including her pelvis -  reflects this past.

Pelvis
The pelvic girdle is formed by the sacrum, coccyx, and the two coxae.  A coxa is formed by the fusion of three bones, the ilium, ischium, and pubis, which meet in the acetabulum or hip socket.  At the back each coxa is attached by strong ligaments to the sacrum (base of spinal cord), and in front to each other at the pubic symphysis joint. This joint allows only slight bending movement, but it softens and becomes more flexible in a female giving birth.  [Note: Other names for the pelvic bone are innominate bone and coxal bone.] 

Factors contributing to the overall shape of the pelvis are constrained by both the demands of bipedal locomotion, as well as those for perpetuating the human species - principally in relation to the requirements of childbirth.

It is usually impossible to distinguish between the pelvis of a boy and a girl before puberty.  Thereafter - of all bones - the pelvis shows the greatest sexual differentiation and is thus a primary tool for identifying male vs female skeletons post-puberty.

Exaggerated images of female and male lower torso's

On average, the adult male pelvis is much heavier and narrower than that of the female.  In comparison the female pelvis is broad and shallow, the geometry is designed with a greater outlet for passage through its bony openings of a baby's head and shoulders during birth.  The female pelvis is also less massive and more delicate and its muscular impressions are slightly marked.  

A triangular shaped pubis with a broad medial aspect and no evidence of a ventral arc is a characteristically male pattern.  The female pattern for these features is a rectangular pubis, pronounced ventral arc, and sharp, narrow medial aspect of the ischiopubic ramus.  According to Bass, the presence of a ventral arc is the most diagnostic of the female pubic features. 

A calculation of supposedly ideal female proportions

In the female pelvis the ilia are less sloped, and the anterior iliac spines more widely separated; hence the greater lateral prominence of the hips.  The pelvic inlet of females is larger and has a greater absolute circumference. The body of the pubis is longer, thereby increasing the size of the pelvic outlet.  These characteristics all reach a maximum in a woman's mid- to late-20's - just before her fertility starts to decline significantly.

The size of the pelvis varies not only in the two sexes, but also in different members of the same sex, and does not appear to be influenced in any way by the height of the individual.  As a result, women of a short stature tend to have obviously very broad hips. 

Sacrum
As a rule, the female sacrum is wider and flatter than that of the male, permitting a greater outlet for the birth canal.  However, this is a rather subjective observation, and should only be used in conjunction with other techniques of sex determination.

The following diagrams compares the male and female pelvis, notice the differences in the pelvises shown below, especially the much wider pubic arch and birth canal (aka pelvic outlet/inlet) in the female.  Also, the ischia spines are less pronounced and significantly further apart in a female - in order to allow the head of a baby to pass through.  

Anterior (front) View

Superior (Top) View

Superimposing diagrams of the adult female and male pelvis at the same scale demonstrates the additional width of the later.  As result the average transwoman has narrower hips than the average cis-woman.  This can be visually disguised (see below left) by means of silicon hip and buttock implants, but one serious problem remains...

Ava Glasscott age 30, - just after having SRS.  Height 5' 10", Weight 145 lbs, Figure 34-27-38, Cup size DD. The reasons for her subsequent success as a female model (despite her age) are self evident.

The wider pelvis of a woman results in the femur (upper leg bone) joining the hips to the knee at a different (less vertical) angle than in men; that’s why women walk slightly differently to men.  Most notably women have a slightly greater swinging of the hips and hold their arms out a little more for balance and to keep space with their wider hips.  The differences are very subtle but  evolution has made us all ultra-sensitive at detecting male vs female characteristics, and a male rather than female gait can be a give-away at a distance.  Some transwomen put a lot of time and effort in to "walking like a woman" but it's not a natural motion for their skeleton, and is difficult and even uncomfortable to sustain.  Also, when consciously trying to trying to adopt a female gait, it's all too easy to exaggerate this and arrive at a 'cat walk model' or 'gay man' motion that actually attracts unwanted attention.

Born John Matheny, Las Vegas showgirl Jahna Steele has the slight skeletal build common in women and rare in men.  Her height (68 inches, or 173 cm) is actually above average for a woman, but this PR photo disguises that.

Long Bones and other Bones
In the adult skeleton, the long bones in the legs and arms can be examined and differences in the diameter of the humeral head, femoral head, and bicondylar breadth used for sex determination, and sites of attachment for the deltoid and hamstring muscles analysed for degree of robusticity.  Interestingly, and rather unexpectedly, the average female upper arm bone (humerus) is 1" (2.5 cm) longer than the male upper arm - making it one of the most visually obvious signs of a male or female to an informed observer.  The short upper arm makes it easier for males to throw footballs and lift heavy objects (less moment arm), the longer upper arm makes it easier for females to hold and suckle babies.

Finger length ratio has become a widely known form of sex differentiation.  Most men have an index finger (digit 2, or 2D) shorter than their ring finger (digit 4, or 4D), whilst most women have an index finger that is as long or longer than their ring finger.  Significant deviation from a 2D to 4D ratio of 0.98:1 ratio is a fairly strong indicator of natal sex.

It's believed that the difference in finger length is due to the action of "male" androgen hormones on the skeleton of a developing foetus.  This seems to be confirmed by the fact that suffers of Androgen Insensitivity Syndrome (who are genetically XY male, but not affected by Androgen) often have a statistically female digit ratio, whilst women who suffer from congenital adrenal hyperplasia (which raises their androgen levels before birth) often have a statistically male ratio.

 

A young transsexual woman.  On initial impressions her petite frame indicates female, but her large hands and longer ring finger give a strong hint. Patrícia Araújo (born Felipe Silva de Araújo) shows off legs that have made her a top female model in Brazil.

Skeletal Feature	Male	Female
LEGS
Femur bone : Diameter Bicondylar width Popliteal length Trochanteric oblique length	greater greater longer longer	lesser lesser shorter shorter
ARMS
Humerus bone : Diameter Length	greater shorter	lesser longer
FINGERS
Index finger (2D) Ring finger (4D)	shorter than 4D longer than 2D	longer than 4D shorter than 2D
BONES (GENERALLY)
Bone weight is :	heavier	lighter
Bone markings  :	more prominent	less prominent

 

Measurements	Female	Probably Female	Indefinite sex	Probably male	Male
Diameter of head of femur (mm)	<41.5	41.5 to 43.5	43.5 to 44.5	44.5 to 45.5	>45.5
Trochanteric oblique length (mm)	<390	390 to 405	405 to 430	430 to 450	>450

 

Height

Height is a good indicator of sex, although a rapid in increase in height over the 50 years due to better nutrient has confused the statistics and age based percentiles.  The following table indicates some examples of the differentiation between average male and female heights in surveys taken since 2003:

Country	Age Group	Average Male Height	Average Female Height	Ratio
U.K. – England	16+	1.754 m (5 ft 9 in)	1.619 m (5 ft 3 1⁄2 in)	1.08
U.K. – England	16–24	1.771 m (5 ft 9 1⁄2 in)	1.644 m (5 ft 4 1⁄2 in)	1.08
U.K. – England	25–34	1.776 m (5 ft 10 in)	1.637 m (5 ft 4 1⁄2 in)	1.08
U.K. – Scotland	16+	1.750 m (5 ft 9 in)	1.613 m (5 ft 3 1⁄2 in)	1.08
U.K. – Scotland	25–34	1.782 m (5 ft 10 in)	1.635 m (5 ft 4 1⁄2 in)	1.09
U.K. – Wales	16+	1.770 m (5 ft 9 1⁄2 in)	1.620 m (5 ft 4 in)	1.09
U.S.	All Americans, 20–29	1.776 m (5 ft 10 in)	1.632 m (5 ft 4 1⁄2 in)	1.09

Summary

There is no definite 100% way of distinguishing a male from a female skeleton, but here is a list of the major criteria to be considered.

Male	Female	Penny Clifford had SRS at about 31 - below the average age for a westerner.  However while obviously a very attractive woman, the lower picture shows the physical problems that face most adult transsexuals due to their already masculinised skeleton.
Height is greater* (average 5ft 10 in)	Height is lower* (average 5ft 4in)
Skull is heavy and rough	Lighter and smoother
Forehead is usually sloping	More vertical
Sinuses are large	Smaller
Cranium is large	Smaller (by about 10%)
Mandible is large and robust	Lighter and smaller
Teeth are large	Smaller
Upper arm is short	Upper arm is long
Ring finger longer than index finger	Ring finger shorter than index finger
PELVIS
Narrow, robust, heavy, rough	Broad, light, smooth
Pelvic inlet is heart shaped	Oval to round shape
Iliac fossa is deep	Relatively shallow
Ilium extends further above the sacrum	More vertical, lessabove the sacroiliac
Angle under the symphysis is usually much less than 90 degrees	Angle is often greater than 100 degrees
Sacrum is long, with a pronounced sacral curvature	Broad, short less curvature
Coccyx points anteriorly	Points inferiorly
Bone weight is heavy	Lighter
Bone markings are prominent	Less prominent
* Average adult height in the "West", amazingly these figures have increased by over an inch since World War 2, and two inches compared to pre-1900.

 

Transsexual actress Claudia Cherriez.  An extraordinarily beautiful woman but still with male skeletal characteristics. e.g. the length of her limbs and the shape of her jaw bone.

Surgical Feminisation of the Male Skeleton

You may now want to read the article on this site about feminisation surgery on the genetic male.

 

Height Analysis

I used to include a link here to a Java applet for comparing your height against male and female averages.  For example, at 5ft 9inches I was boringly average as a man, but apparently only 3% of women were taller - which was slightly hard to believe given all the women that seemed to tower over me every day!  

Unfortunately the applet hasn't been updated and it triggers far too many security alerts to recommend.  If anyone knows of an alternative, please email me at annie.richards@hotmail.com