Gender identity refers to the idea of feeling inherently male, female, or neither/both, regardless of biological sex. This idea was first discovered anecdotally and later confirmed scientifically. Evidence supporting it continues to accumulate.
The distinction between gender identity and physical gender (aka "biological sex") has long been noted. Cases of individuals seeking SRS go back over a century, with the likelihood of many additional cases prior to that due to the intense social stigma in this general area. Many historical cross-dressers may in fact have been transgender people attempting to use clothing to alleviate gender dysphoria, in the absence of any effective medical interventions.
It has been clear at least since the late 1990s that gender identity is similarly separate from (though highly correlated with) gender socialization, the gender/sex in which you are raised and taught that you are.
It is also now understood that gender identity is reflected in physical brain structures
Evidence: David Reimer
One landmark case is that of David Reimer, who nonconsensually received SRS and was raised as a girl following irreparable damage to his penis at the age of six months. Unlike transgender patients who seek SRS, he never adapted well to being treated socially as a girl, and age-mates consistently identified him as male, He "de-transitioned" to male at age 15 (having never sought transition in the first place), and finally committed suicide at the age of 38.
Prior to this, the scientific view (also advocated by contemporary feminists) had been that gender identity was purely a matter of socialization: raise a child as a girl or boy, and they will identify as such. This case added to growing evidence that in fact gender identity cannot be socialized, though it was not immediately recognized as such. The scientist who recommended the experiment of performing SRS and raising Reimer as a girl in fact believed initially that it had succeeded, lending further credence to the "socialization" theory, and published papers based on this.
- Conservapedia: "a term used by the Left to give cause to people believing they are a sex other than their biological sex"
RationalWikiredirects to non-binary gender, which is a range of possible gender identities
- 2014-04-15 [L..T] Transgender Europe applauds Malta for naming gender identity "Malta has become the first European state to have gender identity in its constitution following the proposed amendment that was approved yesterday evening in parliament."
- For example, Joan of Arc was burned at the stake merely for the heresy of "cross-dressing", even though she may have only done so for safety reasons. More recently (certainly as recently as the 1970s), popular culture in the US has been extremely harsh to anyone who does not perform gender in culturally expected ways, including but not limited to homosexuality and cross-dressing.
- 2015-01-17 Networks of the brain reflect the individual gender identity: "In a current study, brain researchers were able to demonstrate that the very personal gender identity of every human being is reflected and verifiable in the cross-links between brain regions."
- 2019-04-28 The Tragic Case That Proved Gender Can’t Be Assigned "In the 1960s, the theory of social conditioning was a dominant force in psychology. Researchers were exploring the ways that the world could shape our identities, and gender was one piece of the puzzle. Many psychologists believed that gender was learned – in other words, that it was just one more behavior sculpted by social influence."