Is it innate or does it have a cause?
There is no cause! Trans people identify this way because it is the label that best describes who they are in terms of their gender identity. There are even scientific studies proving that trans people’s brains are just wired that way (Rametti G et al. 2010), in the same way that someone who is cisgender (identifying with their gender assigned at birth) is wired a certain way.
A person’s sense of gender identity forms from a very young age through interaction with close family and other young children. The types of toys we play with and clothes we wear as children help to affirm our gender and, as we grow up, the interactions we have with other children and adults solidifies the social role that society expects us to take up.
A lot of stereotypes, such as blue for boys and pink for girls, play into the (wrong) idea that there is only one right way to be a man or a woman, and this can lead to trans people supressing their feelings in order to fit in. There are trans people of all ages across the world, some of which discovered their identity relatively young and some who didn’t realise they were trans until old age. It’s access to the right information that helps trans people discover their identity and gain the courage to come out.
Rametti G et al. 2010 [https://www.ncbi.nlm.nih.gov/pubmed/20562024]
In this study, the MRI technique called diffusion tensor imaging (DTI) was used to compare the differences in white matter of 18 trans men (who had never been treated with testosterone) to 24 cisgender men, with a control group of 19 cisgender women. They identified white matter in the brain that differentiated the cisgender men from the cisgender women. The results showed that the white matter pattern in the trans men was closer to the pattern of the cisgender men than the cisgender women, proving an inherent difference in brain structure in trans men.