Good question. Not sure there is a clear answer yet. From a scientific perspective, it appears that there are both biological and cultural factors at play. Some evidence suggests that transgender identity has genetic or hormonal roots, but its exact biological correlates are unclear.
We don't always have scientific explanations for reality. I'm bisexual, and well aware that scientists have spent a good amount of time trying to identify the scientific explanation for sexual preference. We haven't been able to do that yet, but that doesn't mean that I don't exist or that my sexuality is invalid.
So you admit its nothing more than a belief then.
Good. Because I don't share that belief and so far science has pretty much disproved all this stuff about it being biological in nature.
Indeed I am aware that my sibling has been through genetic tests, brain scans and hormonal tests and they have all come back saying nothing abnormal.
They did however spend an extra ordinary amount of time online, to the point that they failed a degree because they stayed up all night chatting to people in north america.
Given that science INCLUDES the effect of socialisation and the effect this has on both the brain and our development, I think I'll stick to this scientific evidence until its been disproved. Rather than merely dismissing an entire field of science as not being relevant and being stuck in this belief that it MUST be biological in nature out of hand.
I find this very bizarre that this is done so often. Its almost as if there is an adversion to the idea that our lives might be significantly mapped out and influenced by the world in which we live. I do wonder if its because if we see it, we might be forced to tackle it and our own ingrained prejudices which we have been socialised to obey. The feeling of challenging yourself is a rather uncomfortable when you are confronted with it. Its almost easier to believe this notion of a biological lottery where something has gone wrong and as humans we were powerless as to when and where it strikes.