A Few Thoughts on Gender

Currently there is no diagnostic test for Gender Identity Disorder, largely because beyond unique physical traits, there is no thought, emotion, or personality trait unique to any one gender.
Medical professionals have three points of criteria.
1: Persistant notion that your thoughts and emotions are of the opposite gender.
2: Persistant belief that you are in the wrong body.
3: Persistant hinderance in the ability to function socially due to gender discomfort.
That works, as I don't believe that anyone honestly answering "yes" to those three questions wouldn't be dysphoric.
But it remains; how does one come to feel that way?
There are said to be three known points of gender, the physical, sexual orientation, and a gut level acknowlegement.
For trans people it is the last one which gives rise to dysphoria.
The mind has a unique knoweldge of the body it is a part of. The brain has a blueprint of this body, and sometimes, very rarely, this blueprint is not of that particular body.
I've heard it likened to phantom limbs experience by amputees, where the brain has a map of the body, and hasn't come to terms with the fact that a part of it is missing.
In the case of Gender Dysphoria, the brain doesn't accept the body as it's own, because it occupies a space different than the blueprint suggests, and it has features not included in the blueprint, and even the timbre of the voice which the brain expects to hear is not the voice that actually exists.

One might argue that physical appearance and pitch of voice are superficial things not actually all that important, but these are valuable things of which we interpret people by.
Personalities come in vast packages, including but not limited to, voice, cadence, body language, body size (a tall person with a similar personality to that of a short person will be percieved differently), and also masculinity and femininity.
Men can be feminine, but because they are anatomically male, are still expected to be men, and the same with masculine women.
However, if a person is dysphoric their personalities are being judged and interpreted against the wrong model.
A feminine man may instead be viewed as "weak", or "fragile", where if he were a woman she would be viewed as "gentle" or "nurturing"; and a masculine woman may instead be viewed as a "controlling" or "cold", or maybe even be given the would-be honorable monicker of "strong, independent woman" whereas if she were a man would be viewed as a "good man", or "strong", or "a leader".
It is by altering the physical representation of the self that we are able to express ourselves fully.

Why were we upset by Elemetary casting John Watson as a girl, Joan Watson? Because Sherlock Holmes and John Watson have an almost historically significant male relationship.
Changing Watson's gender casts the relationship dynamics under a different light, it shifts the motivations, it redistributes everything.
Take a moment to imagine a character you like, what it is you like about them, what their defining characteristics are; now imagine they are the opposite gender. How might their characteristics be conveyed now?
Imagine a female Jay Gatsby, or a male Madame Bovary.
Gender, no matter the person beneath it, is the model by which we interpret an individual's personality.