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MELISSA HINES on the role of hormones in brain development and behaviour.
IMPORTANT as extrinsic factors are, individual sexuality has a physical substrate. Researchers interested in the physical basis of sexuality have focused on the role of the sex hormones (androgens). Intersex conditions can further clarify the importance of androgens because in most cases, intersex genitalia are a result of prenatal abnormalities involving androgen. The question is, could these same hormonal abnormalities also lead to intersex brains or intersex behaviour? In non-human mammals, androgens direct sex-linked brain development during early life and exert permanent influences on sex-typed behaviours. So can research tell us whether similar hormonal effects occur in people?
Sexual differentiation in mammals
Sexual differentiation (or the process of becoming male or female) is initiated by the sex chromosomes, with XX producing a female and XY producing a male. However, the main role of these chromosomes is to determine whether the primitive gonads develop into testes or ovaries. From then on, hormonal products of the gonads, particularly the androgen testosterone, determine whether the external genitalia develop along male or female lines.
In humans, at about week 6 of gestation, information on the Y chromosome directs the gonads to become testes, and by week 8 of gestation, they are producing androgens, including testosterone. In the absence of the Y chromosome (e.g. in an XX individual), the gonads become ovaries, which produce little or no hormone prenatally. Therefore, testosterone is several times higher in XY (male) fetuses than in XX (female) fetuses, particularly between weeks 8 and 24 of gestation. During this period, androgens promote penile and scrotal development. In the absence of androgens, the same tissues develop in the female direction, as clitoris and labia.
In most mammals, androgens not only determine development of the external genitalia, but also shape the development of certain brain regions. As a consequence, they produce permanent behavioural changes. Indeed, hormone administration during early development (but not later in life) can sex-reverse the subsequent behaviour of animals, making females behave like males and males behave like females (for reviews, see Goy & McEwen, 1980; Hines, 2004). For example, female rats given a single injection of testosterone on the day of birth will show increased levels of male-typical, rough-and-tumble play...