It’s so much more fascinating than just that!

So the most well-known of these structures is the bed nucleus of the striae terminalis. The size and number of a specific type of neuron in that region is highly predictive of the gender of the individual, with very high reliability. Postmortem, you can count these neurons and be very close to sure what the sex of the dead person was.

Not only do trans individuals have the structure of the gender they identify as, but there were several interesting controls done to double-check.

First, they looked both at trans people who had undergone different stages of reassignment and compared them to trans people who had not initiated treatment at all. There was no difference, so HRT, etc, is not the cause of the structural differences.

Then they went to a sample of men who were sufferers of specific types of penile/testicular cancers that required a large number of feminizing hormones during the treatment process. Still no change in the striae terminalis.

And then they went back a couple years later and repeated the same experiment, with the same result.

This was all done close to 20 years ago. We’ve known this for a long time.

Fun addendum: there are a couple types of penile cancer that, sadly, requires the penis to be excised. In 70% of these cases, the men involved experienced phantom penile sensations (phantom limb syndrome). So what’s the rate of phantom penile sensations in MTF post-operation trans people? Almost 0%.