Well, I'm no scientist nor have I run or read about extensive studies, but I think it's hormones/DNA, at least for some. I was assigned female at birth but I've had excessive male hormones, to the point that I was coerced onto a treatment to lower them. Messing with my hormones messed with my gender identity and I became less masculine for a while...that's a bit of an over-simplification as part of becoming effeminate was a culturally thing as well. Anyway, I eventually got off of all hormonal birth control and after a few years, I've reverted back to a more masculine personality/mind. I imagine it's possible that I'm intersexed and they wouldn't know because the extra sex organs are internal. I don't know for sure, but it was like I was on testosterone in high school so I have to imagine that the extra hormones are coming from somewhere.

I just wouldn't be surprised if there weren't more intersexed people out there than realize it. Or at least there is something else that deviates the hormones etc. from "normal".

Also, I've noticed that I generally think like a male. When I read threads on here on gender-polarizing issues, I almost always understand/agree with the men's perspective and can't understand the women's. I also seem to fit into the male sexuality cliches, for whatever that's worth, like thinking about sex all the time and arousal being visual, that sort of thing.

I don't know how much of gender hasn't been forced upon me as people see me as a girl. There are a lot of peer and society pressures to remove all body hair, wear makeup, wear dresses, etc. Especially when I've dated a straight cis man. Couldn't be very masculine at all in that case. So am I more effeminate than if I grew up in a society that didn't care how I presented? I think it makes an impact too...

So to me, gender is probably heavily influenced by hormones and how much you're taken in by social conditioning.