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What you may call the hip or pelvis is actually formed by the joining of ilia, ischia, pubis bones to the sacrum and the coccyx. The shape of the human pelvis is unique amongst primates and part of the complex of anatomical changes which allow us for bipedal motility.

The Male Pelvic Girdle

The Male Pelvic Girdle

Between males and females, the pelvis is significantly different. I’ll review some of the features that should be common knowledge to anyone with a forensic or physical anthropology background. For starters, where the two pubis bones meet, at the lower edges of the two inferior public rami, there is a feature called the subpubic angle. Males have an angle smaller than 90 degrees, while females have a larger one. You need not take a protractor to make this observation. If you got a pelvic girdle with subpubic angle wider than a right angle then you most likely have a female… anything smaller and you have a dude.

But there is more, under the posterior inferior spine of the illium, and above the ischial spine, exists a feature called the greater sciatic notch, a sort of passage way for the piriformis muscle and the sciatic and posterior femoral cutaneous nerves. In females the notch is broader than males. Another feature, the biiliac width, a metric measurement made from the widest point between the two ilia is also a sex determining feature. Relative to their overall body sizes, females have large biiliac widths. The two ilia seem to flare out wider in a female. With a wider biiliac width, comes a wider pelvic inlet or the circumference of the lesser pelvis forms.

The Female Pelvic Girdle

The Female Pelvic Girdle

So why do females have larger pelvic inlet, width between the two ilia, and a larger subpubic angle? That’s because they give birth. A wider pelvis allows for better distribution of the added weight that comes during pregnancy. A wide pelvic inlet allows for more space to hold the baby in utero. Furthermore, passing the largest brain to body size mammal through a narrow pelvis would not only be painful, but poses a serious danger to both the mother and baby during childbirth.

The average female adult has a biiliac width of 28 cm. Certain populations, such as Greek women have biiliac widths of 27.5 +/- 2.29 cm, falling within the average (Steyn et al., 2008). Inupiat women have widths averaging 28.6 +/-  0.2 cm, Finns at 27.9 +/- 0.2 cm (Ruff et al., 2004). But, east Asian populations, such as the Japanese have smaller pelvises, with less variation. The average billiac width of women from Japan is around 27.2 +/- .02 cm (Ikoma et al., 1988).

This all makes sense, east Asian people are on average smaller than white people or people from Africa. In fact, anthropologists have regularly relied on estimating body size and mass from biiliac measurements. The average Japanese woman is 153 cm tall, while European women from Germany or the Netherlands average 166 cm in height. You can see such a distinction when comparing Finnish and Japanese pelvic girdles. Asian newborns babies are also have smaller weight at birth (3.2 kg) compared to white babies (3.4kg). A  white woman with a wider pelvis can give birth to larger white babies.

So what happens when a white man, with big white genes, reproduces with a small Asian woman? Well, Razib pointed out a new study in the American Journal of Obstetrics and Gynecology which reviews the impact of such couplings. The paper, “Perinatal outcomes among Asian–white interracial couples,” documented that 33% of such couples surveyed had caesarean deliveries. The latest NIH data on the caesarean rate in the United States is 30%.

The authors suggest that the reason why such couples have 3% more C-section deliveries is that the smaller Asian pelvis is less able to accommodate babies of a certain size. The Asian-white couples had larger babies, with a median 3.36 kg for Asian-mother/white-father versus 3.21 kg for babies from Asian-Asian couples.

There’s a much larger discussion to be had than just reviewing a review of the anatomy and evolutionary history of such a study. Ever so recently, we hosted yet another post on the anthropology of race, which summarized that, “race does not exist in the world in any ontologically objective way.” If you’ve been a regular reader, you would know I’ve tackled this mantra many times. What could be anymore ontologically objective than such a study?

The nature of an Asian is on average smaller in body size than other humans. Of course there is variation. There are some large Asians, but the are very few. The majority are smaller in comparison to other humans. Studies like this show that Asian-mother and white-fathers produce larger babies and have increased rates of C-sectiond deliveries.

There are serious health issues with C-section deliveries, and thus serious, tangible biologically race related issues when people from two different populations mate and increase their chance of having a C-section delivery. The health issues I mention are the increased childbirth mortality rate. On average 1 in every 10,000 women who gives a natural birth will die during childbirth, while 1 in every 2,500 women who undergo C-sections will die during childbirth. In otherwords, women who give birth via C-section are 4x as likely to die. Furthermore, caesarean deliveries increase the risks for malpresentation, placenta previa, antepartum hemorrhage, placenta accreta, prolonged labor, uterine rupture, preterm birth, low birth weight, and stillbirth in their second delivery. The paper also outlines the higher prevalence of gestational diabetes for such interracial couplings.

That all being said, race is not just a social construct. How we interpret biological differences, such as pelvis size, skin color, etc. are not socially constructed but real observations, made from quantiative analysis. There are distinct anatomical, genetic, even behavioral differences that are not derived soley from stereotypes.

    Michael J. Nystrom, Aaron B. Caughey, Deirdre J. Lyell, Maurice L. Druzin,Yasser Y. El-Sayed (2008). Perinatal outcomes among Asian–white interracial couples in American Journal of Obstetrics & Gynecology 199 (4), (385.e1-385.e5) DOI: 10.1016/j.ajog.2008.06.065
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