Sexual behaviour is expressed in the adult animal thanks to the activities of the motor, sensory, central nervous, peripheral nervous and endocrine systems. The activation of such systems is affected by stimuli coming from the external environment, which also includes the social environment, and by internal signals. The various systems also interact with each other in subtle and sometimes not so subtle ways. In this chapter I specifically explore the role of endocrine and nervous systems in the proximate causation of homosexual behaviour, with emphasis on the adult stage of development. Some aspects of the more complex mental faculties of our species will be also considered. The chapter, however, starts with a transitional link with the previous chapter in the form of a review of human endocrine disorders that have been studied in the context of the early ontogeny of homosexuality.

Human endocrine disorders and homosexuality

Most of the medical conditions that have been studied as means of unravelling the endocrinological mechanisms of human homosexuality are considered disorders in need of a cure or at least of containment of their most damaging effects. From this, however, it should not be concluded, as stated in Chapter 3, that homosexuality is a disorder. In fact I argue in this book that it is not. The usefulness of studying such syndromes lies in the opportunity they afford to study those specific neuroendocrinological mechanisms that are common to both the medical condition and also the manifestation of homosexual behaviour. In this way, the causation of homosexual behaviour can be better understood in its more usual, functional context. This is a simple, but very important point that is worth further elaboration. To illustrate the argument by analogy I will briefly discuss the case of human autoimmune disorders.