Common Misconceptions: Shoulder width, an essay

There are a lot of incorrect beliefs about the differences between male & female bodies that are often used to point out how trans women "can't" pass. One of the most persistent of these in our own community regards shoulder width.

In archeology and forensic anthropology, shoulder-width is not used to determine the sex of the skeleton because those differences are due to soft tissue, no bone. As everyone here likely knows, testosterone leads to disproportionate upper body strength relative to those on estrogen. While there are several differences between male and female skeletons, most of those are in the skull and pelvis. Male rib cages tend to be disproportionately wider. Males also tend to be taller. Beyond those, differences are minor such as differences in bone density and circumference and some extremely minor but technically statistically significant differences in foot proportions. Most of these are attributed to greater muscle development in those who have testosterone and to greater levels of physical activity.

Shoulder width (biacromial width/diameter) is proportional to height & only one paper (published 1970) found a difference in the "biacromial index" (biacromial width/stature x 100) of 22.5 for males and 21.7 for females. This difference is attributed to sex hormone levels, not to skeletal differences due to those ratios converging as men age (biacromial diameter declined, not height, i.e. ligament changes due to lower androgen levels), this study found a female ratio of 22.7 and this found a male ratio of 22.58. This representation of the data show a very slight difference (comparable to the 0.8% difference above), however as you can see from the data, there is considerable overlap & it is far more correlated with height than sex, i.e. male and female points differ primarily in size, not in ratio.

You can use this last link to input your own data if you want to see how you compare. Shoulder width is measured as the distance between each acromion (bony point at the end of your shoulder) with your arms in a particular position, it's worth looking up how before having someone measure it.

Seriously, don't worry about your shoulder width! Like most things, it's primarily about giving HRT time to do its work.

ETA: To clarify, this means that for a 6'6" male and female, you'd expect about 1.5cm difference in biacromial width.