The false biology of manspreading [Updated]

Update Aug. 10, 2017: The text of this blog post has been altered since its original posting. This new text focuses on challenging the claim that manspreading is the result of skeletal anatomy (as claimed by the headline of the article in question), and does not attribute this claim to Prof. McGill.

While drinking my morning coffee, I stumbled on an article “There’s a Reason Some Men Take Up So Much Space When They Sit: In defense of manspreading”. I nearly did a spit-take while reading it. The article’s author, Lou Schuler, suggests after interviewing emeritus kinesiology professor Stu McGill, that men’s legs naturally spread apart when sitting, which is why they take up more space when sitting on public transportation (a phenomenon known as “manspreading”). The idea presented is that biological differences between a male and female pelvis explain these differences in sitting positions.

Hello, I am an expert on pelvic sexual dimorphism. I wrote a dissertation on the topic as it applies to Neandertals. In my research, I regularly estimate sex from fossil and more recent human skeletons based on the sex differences mentioned in the article.

I also think that the claim about pelvis shape affecting sitting position is bullshit.

No manspreading allowed sign from Madrid Municipal Transport Company in 2017. [source].

Manspreading is something that we don’t see cross-culturally (here’s a scholarly article from the 1950s that shows that even back then, knees-together sitting occurred at different frequencies in different cultures). Only a certain type of dudebro or man-man manspreads. Schuler even admitted that after being told that his natural sitting position is knees apart, he still avoids manspreading on the subway because he knows it’s rude.

This means that manspreading is a cultural phenomenon because regardless of one’s anatomy, one can choose not to practice it.

But let’s look at the biology. McGill (in the interview and in the YouTube video linked in the interview) claims that narrow hips cause the ball-and-socket joint of the hip to pinch when sitting with knees together. Side note from me: narrow hips are typically associated with male bodies. McGill notes that the shape of the hip joint varies in humans, and points out that there are (in his words) “genetic” differences between, for example, Polish weight lifters and Asian martial artists, that determine which kinds of movements each will be good at doing (i.e., weight lifting squats versus various martial arts).

Quick aside: As an anthropologist, I was disturbed by the language McGill used because it is similar to that used by people who believe humans can be split into discrete racial categories based on their biology (whether we’re talking genes or pelvis shape). I’m not sure McGill meant to sound like this, but long story short: They can’t. For more on this, here’s a blogpost with some great resources. Now back to manspreading.

If Schuler’s article is right, then it would seem that men, or at least certain men, might struggle to keep their knees together when sitting down because of their bony anatomy, risking manspreading and needing to take up extra space on a subway.

Skeletal anatomy while sitting (a person who does not appear to be manspreading). [source]

As an anthropologist, I luckily know that biology is not quite so simple as “have this skeleton = do this behavior”. The article suggests that a person’s biology determines the behaviors they are capable of doing (for example, martial arts, dancing, weightlifting, or manspreading). Instead there is good evidence that repeated behaviors (which could include martial arts, dancing, weightlifting, or manspreading) train muscles and remodel bone to accommodate those behaviors.

There’s even a term for the effect on bone: Wolff’s Law. Julius Wolff proposed the idea that bones change in response to repeated behaviors in 1892. We’ve been collecting data to support this law ever since.

Julius Wolff, who proposed Wolff’s Law in 1892, with a wolf cap for mnemonic effect (thanks, Jess, for the visual aid). [source]

A classic example of Wolff’s Law is the comparison between the two arms of tennis players (such as those shown below). The genetics are the same because the arms belong to the same person, but the dominant arm will have thicker bone than the non-dominant arm. This is because the bone changes in response to regular activity, such as swinging a racquet at a tennis ball.

Cross-sections of the top, middle, and bottom of a male tennis player’s upper arm. White circles are the humerus bone. Top set is the dominant arm, bottom set is the non-dominant arm. note that the bone is thicker in the top set (dominant arm). [source]

Why isn’t the difference in the figure more pronounced? Because the tennis player also uses (and probably strength-trains) their non-dominant arm. Both arms have some bone-thickening remodeling happening compared to non-athletes, but it happens more in the dominant arm.

What does this mean for pinched hip joints causing men to take up more space than they are allotted on the subway? It means that you can change your muscle flexibility (think of people who take up yoga and are able to bend in new ways) and bony structures (think of the tennis player’s humerus) in response to regular activities by practicing those activities.

So if you get in the habit of practicing martial arts, dancing, weightlifting, or manspreading, your body will adjust accordingly. If you STOP doing one of those behaviors (*cough* such as taking up too much room on the subway *cough*), your body will adjust again. It isn’t immediate, it takes time, but it happens. If you have a narrow pelvis (whether or not you are male) and you regularly sit with your knees together, you will train your muscles and eventually your bones to be able to do so comfortably.

Which means that on both cultural and biological grounds, dudebros and man-mans who manspread on the subway are just being rude.

Postscript: Another article mentioning the McGill interview suggests that sex differences in pelvic shape/manspreading are because men evolved to walk/run/hunt while women evolved to have and hold babies. Hi, paleoanthropologist here, evolution is my jam. And let me tell you, if you think prehistoric humans of any sex were bad at walking, you really don’t understand what our pre-agriculture ancestors did all day. Misunderstanding evolution is no excuse for manspreading.

Update August 9, 2017:

Prof. McGill contacted me and would like me to clarify:

“If you read carefully you will see that I did not comment on male-female differences. Any gender connection came from the context that Lou provided. I commented on hip anatomy and function and disease incidence.”

He’s right — all the parts of the article about sex differences came from the author, Lou Schuler, not from McGill’s quotes.

In any case, my comments still apply to anyone who tries to claim that the sex differences in the human bony pelvis mean men HAVE to sit with their legs spread out into other people’s space, giving them a pass on social etiquette. (And yes, such people have come out of the woodwork since I first posted this). I hope readers enjoyed learning about the plasticity (fancy way of saying ability-to-change) of the human skeleton!