People (by which I mean actual scientists who are qualified to do this) did studies on how people’s brains are formed/shaped. Male and female brains show various slight differences. They are distinguishable. 

Now here’s where it gets interesting. They looked at scans of transgender people’s brains. What do you think they found? 

They found that their brains most closely followed the patterns of the gender that they said they were rather than the gender they were assigned.  

Get that? 

So if a woman was assigned male at birth because she had a penis and they scanned her brain, they found her to have the brain shape expected in a woman. It wasn’t exactly the same but it was clearly much closer to female than to male. 

Once those people started hormone therapy and everything, the brain shape grew even closer to the gender they said they were (the gender that they ARE) until it was pretty much identical. You couldn’t tell the difference except yep, that’s a female brain now regardless of whether she was assigned male at birth. 

It is thought to be this that causes dysphoria in the transgender people who suffer it: since their brain is a female brain (or vice versa) making them female it causes a disconnect between that and what their eyes see when they look at their body. 

So there ya are. Science proves transgender people are the damn gender they say they are.