Why do you use the term Gender Role Transition rather than Sex Change?
Gender roles are highly influenced by physical limitations and cultural needs. They are also influenced by personal interest. This latter element, personal interest is, highly influenced by sex hormones. Although both males and females produce testosterone and estrogen, men whose testes are working properly, have 10 to 20 times more testosterone in their systems than women do. When you combine that with having a correspondingly larger number of androgen receptors in their brain, the individual is going to need to express all of that in what we generally recognize as masculine behavior. In addition to the brain responding to all that testosterone, muscle mass, both sexual and physical energy levels are also effected in a more robust way. Keep in mind, however, testosterone can also have a negative effect on a willingness to cooperate with one's peers, a dulling of sensitivity, the blunting of emotional lability and lack of interest in nurturing.
Women on the other hand, have very limited amounts of testosterone. What little they have is produced primarily by their adrenals, which aid them in maintaining muscle tone and sexual desire. What women lack in androgens, they more then make up for in estrogen produced primarily by the ovaries. Estrogen, makes an entirely different contribution to the female anatomy by rounding it and giving it the typical female shape and soft look. Along with physical changes, estrogen feeds the female receptors in the brain. The receptors in the brain regulates the complex female reproduction cycle, expands the individuals emotional range, develops tactile and emotional sensitivity, and encourages reproductive desires.