Let’s say that traits include whether I am tall or short. Ok. Linguistically, we can say “I am tall” describes a trait I have. We can say, " I am white." Again, describing a trait I have. But we are using these words as adjectives.
We can say, “I am a male” and “I am male,” describing my biological sex.
If we want to describe me exhibiting some behavioural trait of mine by cultural gender regarded as feminine, we cannot say, "I am female " or “I am a female”. Male and female already applies to our biology. We need to use masculine and feminine. At any time a man can be behaving as a masculine male, or a feminine male. And a woman as a feminine female, or a masculine female.
But a biological female cannot also be a feminine male; nor can a biological male also be a masculine female. A male-female hybrid is an hermaphrodite.
Physically altering a male body to include female parts creates an hermaphrodite, not a female. And physically altering a female body to include male parts creates an hermaphrodite, not a male.
This is the only thing in this debate from either side that makes sense to me because the “‘gender’ is a word meant to normalize the idea of no sex-differences” narrative seems like it’s addressing air.
I think this is just grammar. It sounds weird to say “I am a tall”. We either say “I am tall” or use it as a modifier, like “I am a tall person”. In British English when referring to take out food they might say something like “Let’s have a Chinese” when wanting to order Chinese food, but to our American ear’s this sounds really weird because thats not how we phrase it.
But gender is one of those words where its both a descriptor, but also an identity. We do the same thing with American. Its both normal to say “I am an American” and “I am American”.
Identity and behavior can contradict each other. A pacifist could lose their temper and hit someone.
A biological male cannot be a biological female and vice versa, but a biological male could have a female gender identity and a biological female could have a male gender identity. They could also have a gender identity that falls somewhere in between male and female or one that is fluid. “Feminine” and “masculine” are really used as gender identities… they tend to be used in regards to the gender expression. So you could be a cisgender woman who acts masculine but still identifies as a woman and therefore is not transgender.
You can’t be a hermaphrodite unless you have both testicles and ovaries. There is no surgery that make you a hermaphrodite.
I’m not sure how you would test it… maybe they have. My understanding is that this conclusion is based on decades of research and interviews with transgender people. I’d suggest you go here and start checking out the references and external links at the bottom of the page: