I recently watched the above video of a Demi Lovato song. I like Michelle Rodriguez’s stomach as much as the next guy (OK, perhaps more), but one thing that struck me in particular is that throughout the whole narrative arc Lovato, a 5’3 tall female, beats the crap out of many much larger men. Obviously this is a stylized fantasy, and the trope of “butt-kicking babes” is pretty well established in our culture now that we can slot it into the appropriate schema (Lara Croft?). But, recently I’ve been made aware of the magnitude of the strength differences between men and women, so these sorts of scenes are even more fantastical than were before. It’s almost as strange to me as an episode of Sailor Moon. It starts to violate the need for a “minimally counter-intuitive” scenario which is the criterion for a good realistic fantasy (yeah, that’s an oxymoron!).

The table to the left is from Costs and benefits of fat-free muscle mass in men: relationship to mating success, dietary requirements, and native immunity. I’m not too interested in the evolutionary psychological details at the heart of the paper. Rather, let’s focus on some statistics which are given. The key is to focus on the d column, this is the effect size, which indicates the differences between the means of the two distributions in standard deviation units. The mean ages of the two distributions were the same, 33. So d is naturally 0 for this measure. For height men are 1.75 standard deviations taller, on average, than women. This seems about right. You can see in body fat percentage that women have higher values than men. The d here is negative. It gets interesting once you get to muscles. These are measuring volumes. When it comes to arm muscles the average male has 2.5 standard deviation units more than the average female! I was also surprised by the thigh muscle, as arm musculature differences have always been more salient. Finally, there’s the fat free mass.

Some have pointed out to me before that the standard sexual dimorphism calculation in relation to humans may not be informative in the way we might think. There’s about a 10% size differences between men and women. But as you see in the “fat free mass” row the size difference is much more extreme if you account for the higher body fat of women. This is relevant because fat does not make you strong, it just adds more weight and volume. In terms of upper body muscle mass there’s less than a 10% overlap between the two distributions. The vast majority of men have more muscle mass than all women. 99.9% of females have less upper body muscle mass than the average male. The 61% greater average muscle mass in male upper bodies translates into 90% greater average strength (the respective values for the lower body are 50% and 61%). The authors of the paper note that “The sex difference in upper-body muscle mass in humans is similar in magnitude to the sex difference in lean body mass in gorillas, the most sexually dimorphic primate.” Obviously humans don’t engage in obligate harem building, and males are not totally devoted to agonistic behavior as their raison d’etre. So one should be cautious about extending the analogy too far. But this result will likely surprise many. It surprised me.

I spent a lot of time fixating on numbers above because I don’t beat women. More pointedly, I’ve never hit a woman. That’s not because of the way I was raised by my parents. Though they don’t countenance beating women, they came as adults to this country from Bangladesh, so their attitudes toward violence are more “liberal” in a literal sense than the average America. The culture in which I grew up though affected me more in regards to proper behavior in this dimension (the United States, and more particularly, middle class mores). I have a cousin who was beaten up by her husband several times (for the record, they both grew up in Bangladesh into their adulthood). She’s about 4’10 and he’s 5’8. Though I abhorred this behavior I didn’t have any concrete understanding of what this might have meant. I’ve gotten into fights, but only with guys, and they weren’t ever that much smaller than me. Now I understand better why a 5’8 man should never get violent with a 4’10 woman. The discrepancy is far greater than height would suggest, because the woman has less muscle mass per pound. I have some intuition about this because my wife is about my height and of athletic disposition for a woman, and when she tried to throw down my sorry out of shape ass it was pretty easy for me to prevent her. How is it possible that despite us being the same height, and her being in shape and me not being in shape*, I could still best her? Because I still had more upper body muscle mass due to being a male.

Now, mind you, there are a small minority of women who are stronger than a small minority of men. The statistics above make it clear. But it is very unlikely that in a pairwise interaction the very strongest females will randomly face the very weakest males. In terms of relationships, where domestic violence occurs, it is very unlikely for reasons of assortative mating that the very strongest females will be paired up with the very weakest of males. On the contrary.

There are two reasons I’m posting this. First, I’m assuming most of my male readers have never beaten a woman, so they too lack good intuition about what they might be capable of if they did do such a thing. There isn’t the sort of thing you really want first-person experience of, so scientific research which can gain you some sense of the shape of reality is useful. Second, the general skepticism and rejectionism of biological differences in behavior between the sexes which is now common on the cultural Left can start to bleed into other domains in the most surreal ways. I’ve had friends with science backgrounds who balk somewhat when I attempt to start any discussion about sex differences with the contention that there is a difference in upper body strength. They don’t necessarily even want to concede this without dispute. In these earlier conversations I didn’t know of any research on the magnitude of the difference, it just seemed “common sense.” But perhaps the positive diminution of domestic violence in some sectors of American society has had the side effect that people forget how strong the magnitude of difference in strength is?

Related: Men Are Stronger Than Women (On Average). In which I report that the average German man has a grip strength more powerful than the majority of the woman’s Olympic level fencing team.

* This was in the past, now that I lift my upper body muscle mass has increased considerably.