That study comes as no surprise to me! I knew absolutely by age 4, even without parental support or being allowed to live as a girl. But is was the 1950s and there was NO acceptance of trans - it was unknown! I was diagnosed as delusional, threatened with 'raparative therapy', and subjected to cruel and continuous efforts to get me to accept my assigned sex, none of which had any effect except to make me miserable and, eventually, suicidal. It wasn't until I was 19 that I began to find medical help and 24 before I was able to transition.
I have seen this phrase "a phase" so bloody often and wonder where the numbers come from because I never met anyone with "gender identity disorder" who ever grew out of it! I am glad to see some "proof" of the deep-seated nature of gender identity.