The previous chapters have established the key role played by the gonadal steroid hormones in sexual differentiation of the body, and we have also seen evidence that certain behavioural indices (such as gender role, sexual orientation, etc.) may be related to differential hormonal exposure, these indices presumably reflecting differentiation of the central nervous system. This chapter will thus focus more closely on sexual differentiation of the brain, assessing evidence from brain regions involved in sexually dimorphic behaviours (in animals and humans), and those that are assumed to subserve certain aspects of cognition (in humans).

The avian song centres

In many animal species, individuals show different patterns of behaviour, especially within contexts related to courtship and reproduction. Such sexually dimorphic behaviours are likely to be subserved by neurological differences, perhaps to a greater or lesser degree triggered by hormonal changes. A good example is that of bird song. Nottebohm (1970) described in detail the development of bird song, using as an example the wild chaffinch, a species in which adult males sing but adult females do not. The young male has to learn his songs from adult males in the nearby environment; these songs last around two seconds, and consist of two or three phrases, ending with a complex set of notes. This learning process takes place during the first year. A set of rambling vocalisations called ‘subsong’ increase in complexity, and develop into ‘plastic song’, very similar to the final polished version but lacking full complexity and well-defined phrasing.