I personally think the answer is "both, for different people." That is, I think there's more than one cause, and so one person may be primarily defined by nature while another owes a lot to nurture/life experience (this is essentially what I think about sexual orientation, as well, since it's the only thing that explains how some people know since they're kids and others shift or expand preferences in adulthood). I was not trans until I was in my early 30s, so for me personally, any explanation that says it was set at birth and you had to know as a little kid doesn't work. I don't know what made me this way - I wish I did, just for curiosity's sake! - but whatever it was, it wasn't hardwired in by nature... for ME. For others, it absolutely was.