Sexually dimorphic behaviour is a trait that has developed through evolution. It physically manifests within complex structures within the brain, the hypothalamic uncinate nucleus is an essential part in this. You can never raise a boy as a girl, or a girl as a boy, it's just how biology works. This has been done and been studied in cases such as David Reimer, who was brought up as a girl and ended up comiting suicide in the end. But what these "transgender" people... I mean the ones who everyone talkes about. People who have a normally masculinized external appearence yet claims to be female, or vice versa (who I will here refer to as "transsexuals" for simplicity). These people seem strangely similar to the other people who choose to transition genders contrary to rearing. Studies on animals suggests that the development of neurological dimorphism occurs around the third trimester, while the reproductive system developes quite early on, so theoretically, you could easily have a disconnect between the reproductive system and behavioural sexual dimorphism. Decades of research has been dedicated to finding causes of transsexuality. As with anything, it is hard to fit everyone in the same group. Of course *someone* could just choose to "transition" as a way to fit in or express themselves. Research on the topic is quite expancive, and quite interesting. People have so far been unable to find and prove a psycologically linked cause for transsexality, but of course, that doesnt mean it couldnt happen in an individual. The original reaserch on the transsexual brain was preformed in the 1990s, since then, many follow up studies have been preformed to answer lingering questions. The studies are mainly focused on structures in the hypothalamus, such as the hypothalamic uncinate nuclus. This part of the brain could only be studied throught physical examination. A collection of brains from deceased transsexuals has been gathered over time to be studied, so far, all of the brains, both male and female, strongly more closely resembled the opposite sex to which it was born. https://academic.oup.com/jcem/article-lookup/doi/10.1210/jcem.85.5.6564 https://www.ncbi.nlm.nih.gov/pubmed/18980961