Transcript for Transgender - Born this way?: Part 4
Reporter: Almost every day someone goes roaring through the hills of malibu. There, a woman in a dune buggy. And there she is again flying her plane over the houses of her children along the beach. A person who spent her childhood loving cars, playing sports and knowing even then she was female. Every trans person asks that question, why? Why me? Why is this in my head? There's got to be something biological to this. For a lot of scientists, one intriguing clue on that question comes from identical twins. We are the twins. Reporter: These are 26-years-old southern girls. We can finish each other's sentences, each other's thoughts, each other's feelings. Reporter: Identical girls, whose parents were once sure they were identical boys. We've chosen not to show their full faces. Like so many other trans people, they're concerned about their safety. But they say, they knew they were being forced into the wrong gender when they were 3 years old. And the best way I can explain it, is you're trying to claw your way out of this coffin that just keeps getting dirt put over it. It's a prison. Essentially it's a prison. Reporter: And here's an interesting finding. If you have one identical twin who is transgender, there is a 39% probability the other will be, too. Signaling something is genetic. And consider what happened to these two identical twin boys, who were separated at birth, raised in entirely different homes. And yet still somehow, by the age of 8, they both knew they were in fact girls. Born this way? Born this way, absolutely. Reporter: This is Dr. Jeremi Carswell, Dr. Norman spack of the gender clinic at prestigious Boston children's hospital. They remind parents it's common for little kids to experiment with gender as they're growing up. It doesn't mean anything. But they say a transgender child is different, not just experimenting. Especially as they head toward puberty. They will be insistently certain their anatomy is wrong. I had a 7-year-old who looked at me and said, "God made a mistake and this needs to come off." Reporter: So you're saying that you're not going to talk a child into being transgender. No. No. Reporter: That can't happen unwittingly, accidentally. It just doesn't happen? Everybody I know who has tried to change somebody's gender identity -- they can't do it. Reporter: Why is that? Over the last two years, we've stayed in touch with dozens of experts. Many of whom are increasingly intrigued by the possibility of a biological event in the womb. The brain begins to form early in the first weeks of life. It's two weeks later the genitals begin to take shape. Is it possible that at some point, signals get mixed, the brain gets one gender instruction, the anatomy gets another? One thing is clear, if there's a conflict, it's the brain that will win. But Dr. Spack wants everyone to know the American psychiatric association says it is not a mental disorder to be transgender. But what is the proof of that? I think the proof was that it didn't require psychotropic medication to make it better. But that as soon as the patients were treated in the hormones of the gender that they affirm, they were instantly better. Reporter: As if the brain craves a hormone the body can't produce. Caitlyn Jenner says female hormones almost instantly quieted the inner conflict that caused depression. It's a tough road. It's a tough, tough road. Reporter: And she says since coming out, she's been lucky to have the money to change her appearance -- jawline, her forehead, her breasts. And she has just announced in the book she has become one of the 12% of trans women who go on to get what's called final surgery. It's about what's between your ears and who you are as a person. It's your soul, okay? Reporter: She writes, after much deliberation, I had the final surgery in 2017. The surgery was a success, and I feel not only wonderful but liberated. So all of you can stop staring. But I wasn't less of a woman the day before I had the surgery than I was the day after I had the surgery. Because that did not define who I am as a human being. Reporter: No regrets? None whatsoever. Reporter: Then she issues a kind of social warning, saying this is a topic only a transgender person can bring up, and the rest of us should never ask. So how did you decide to put it in the book at all? This book is about honesty. But that doesn't mean in the future that I have to talk about it, that I'm going to dwell. The media may. But I am not going to dwell on that subject. You know, and I would make a suggestion to all people out there, don't ask the question. It's not an appropriate question to ask any trans person.
This transcript has been automatically generated and may not be 100% accurate.