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[personal profile] ossamenta posting in [community profile] archaeology
Written as part of the Frequently-Or-Not-So-Frequently-Asked-Questions project (itself part of the annual Three Weeks for DreamWidth), for the prompt "I've often wondered how archaeologists determine the sex of a skeleton - I know there are differences in the pelvis, but what else is taken to be a clue? I'm also curious about how accurate it is, and whether anyone's done any studies on that."

Sex estimation of skeletons is one of the fundamental methods of bone analysis, and, yes, lots of studies have been done on that. Essentially, there are three ways of sexing (human) skeletons: shape and relative size of the bones, grave goods associated with a particular sex/gender, and DNA testing. This post is primarily dealing with skeletal sexual characteristics, but the other methods will get a brief mention.

Skeletal sex estimation is based on biological differences between cis* men and -women. Identification of trans individuals normally occur when the skeletal markers differs from grave goods associated with that sex - however, whether those persons were transgender, third gender or genderqueer is very difficult to establish. Even for individuals with clearly marked sex, their gender is not necessarily identified by either sexual characteristics or by grave goods.

There are two kinds of sexual characteristics: primary and secondary. Primary are the genetic markers and sexual organs. The secondary characteristics are the ones that normally develop in puberty, such as wider pelvis for women and extra muscle mass for men. Archaeologists mainly deal with secondary sexual characteristics. These, and this is important, are not binary, but exist in a fluid scale from hyper feminine to hyper masculine. Usually five categories are used: female, female?, indetermined, male? and male.

The main skeletal sexual characteristics that archaeologists use are skull, limb bones and pelvis. The sexual dimorphism is most pronounced in the skull and the pelvis. Men generally have more marked eyebrow ridges, rounder top of the eye socket, larger mastoid process (bone lump behind the ear, where muscles to the clavicle and sternum attach) and larger muscle attachments on the occipital bone (the back of the head, where the neck muscles attach). The muscle attachments at the chin and the rear corner of the mandible are also generally more marked in men, making the hypermasculine mandible very angular. Irritatingly for archaeologists, older women often have more robust features on the skull, and can therefore be mis-sexed as male. Indeed, there is a paucity of older women in skeletal assemblages from Medieval Europe. This has commonly been associated with a high mortality rate in child birth for adult women, but may actually be an example of mis-sexing the older ones.

Sex estimation markers on human female and male skull
Female and male skull. 1: eyebrow ridge, 2: mastoid process, 3: occipital bone

Generally, mens’ limb bones are larger and have more marked muscle attachments. There are, for example, formulas to calculate the sex on the diametre of the femoral head (an element which has good survivability). However, the degree of sexual dimorphism can vary between populations, and there may be differences regarding race as well. Race in skeletal remains is AFAIK much more discussed in the US than in Europe, and I will be leaving that discussion for any US trained archaeologists/osteologists who are reading this and feel up for it. As with anything that is highly variable between populations, a single measurement cannot be used to determine an individual’s sex. If the measurement is very small or very large, one usually assume female or male respectively, but then you need to have the experience to know that that measurement is to the extreme.

The problem with using muscle attachment size as a sexual indicator is that this can vary a lot between populations depending on differences in work load as well as genetic differences. For example, female skeletons from the Scandinavian Mesolithic period (c. 10000-4000BC) are more robust than male skeletons from later periods, which is important to remember if you go from analysing an Iron Age population to a Mesolithic one. Individuals which look male at first glance may actually be female. (added an anecdote to create data :-))

However, the most important sexual characteristic for archaeologists is the pelvis. Muscle attachments may vary in size depending on work load, but all women have in common the ability to bear children (ignoring any infertility issues etc), and their pelvis is shaped differently from mens’: lower and wider. Since it’s a bit awkward to put together the two halves of the pelvis and the sacrum to see this, archaeologists normally use the angle of the sciatic notch and the angle of the left and right pubic bones to estimate sex. On women, these are sort of the same as the angle between the thumb and index finger. For men, you use the angle between the index finger and middle finger.

Sex marker on female and male pelvis: sciatic notch Sex marker on female and male pelvis: sub-pubic angle
Left: Sciatic notch on female and male pelvis. Top right: sub-pubic angle on male pelvis. Botton right: sub-pubic angle on female pelvis.

The sexual characteristics discussed above are not valid for sexing child skeletons, since they depend on differences formed during and after puberty. The pelvis actually consist of three different bones which fuse together first at puberty. Children are normally not sexed, but if there is a substantial population, they can be sexed on (permanent) tooth size. This follows the same principle as metric sexing on adult skeletons: men are on average slightly larger than women. In some populations the sexual dimorphism is so small you can’t accurately tell boys and girls apart, and in some you can see two bell curves. You need to use the same tooth for the entire sample, so it’s better if they’re still attached to the jaw/skull, since they need to be correctly identified.

Since the sex differences are fluid (see above), there will usually be some skeletons in a population that can’t be sexed by using skeletal sexual characteristics. They are then classified as unknown sex or indetermined. Perhaps there are gendered grave goods that can give us a hint of the individual’s possible sex. Or, if we’re dealing with a post-medieval grave there may be a plaque from the coffin with name and death date upon it. This latter case is the optimal way of estimating sex of archaeological remains (always, of course providing that the plaque matches the person in the coffin). It has been used to check osteological ageing and sexing methods on 18th century skeletons from the crypt in Christ Church Spitalfields, London.

Using grave goods to estimate sex is not a good method, since we can’t always say with certainty that the grave goods were the dead person’s belongings. They could have been gifts from descendants for the use in the afterlife, or symbols of the family’s wealth. Also, not all items were only used by one sex. Furthermore, grave goods indicate gender, and not sex per se. For cisgendered persons it potentially works, but only if our binary gender assignation of items are the same as that population’s. Where we’re dealing with extremely acidic or sandy soil, which is terribly bad for bone survivability, or cremated bones, this may be the only possiblity for sexing.

DNA can also be used for sexing skeletons of children and sexually ambiguous adults. Here the main problem seem to be contamination from modern DNA and poor survival of the original DNA. It’s also a rather expensive method, so it’s mostly used for specific research questions.

And finally, just remember that it’s always more complicated once you go into the details…

*: cis = having a gender identity that matches the sex one was assigned at birth. Opposite to trans gender.

Cross-posted to [personal profile] ossamenta and [community profile] fonsfaq


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