The scientists who have studied this behavior have pretty well settled in with an explanation that far more often than not this behavior is a genetic predisposition. In other words, the predisposition is not expressed unless there is something that triggers it. We all have various predispositions built into us. Whether we are right or left handed is actually a predisposition that is activated when we are a baby just learning to grasp things. If right handed you may not be able to do much with your left hand, but if you lose your right hand then the brain transfers as much as possible over to the left hand because of the predisposition nature of handedness.

Thus, with a predisposition such as identifying with the feminine gender when male or masculine when female may take many, many years before something happens that actually activates it and it becomes a disposition. Once activated, situations we encounter are dealt with in accordance with what we perceive to be the proper mode. Thus, it can come and go and be expressed in many different ways, but once activated apparently the brain just flips back and forth in different modes that are perceived as appropriate for the situation. But in some, the "alternate" mode becomes so dominant that transitioning becomes the solution that produces the best comfort and that, according to this explanation, is the source of transexuals.

In the science, it is all thought to be the same process coming from the same source which is a complex of genes that operate together to result in this behavior pattern as being normal in that person; as normal as being right or left handed. At least that is what the current thinking is regarding the cause of this phenomenon we are a part of. There is a great deal of evidence in support of this including identified genetic markers, brain structures and various morphological and anatomical features we have that are indicative of the deeper predisposition that has genetic roots. But it is still a predisposition and therefore variable in its expression. Thus, essentially every description that has been provided to this point in this thread can easily be fitted into the predisposition model. Each person is normal for them and the way things are supposed to work in them as a result of having an activated predisposition to behave this way.

It sure works for me and I am very well educated in the sciences and before retiring, I worked in science for over 40 years. But it has brought a great deal of comfort and confidence to me to know where this probably came from and that there is real scientific evidence to support it and not just conjectures with little verifiable evidence. The struggle, for each of us, is the matter of exactly how do we live with it in the context of the culture we live in? Unfortunately, the science doesn't tell us how to do that. We have to discover that for ourselves. And that, I think, is why forums like this exist and are so useful to many of us.