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The biological basis for gender identity, if there is one, is unknown. The basis of biological sex is better understood. A single factor – the steroid hormone testosterone – accounts for most, and perhaps all, of the known sex differences in neural structure (Morris et al., 2004). Testosterone is said to ‘sculpt’ the developing NS by inhibiting or exacerbating cell death and/or by modulating the formation and elimination of synapses. Testosterone masculinizes both the brain and the body, yet experience can interact with testosterone to enhance or diminish its effects on the CNS.

The steps leading to masculinization of the body appear to be consistent across mammals. The Y chromosome contains the sex-determining region of the Y (Sry) gene and induces the undifferentiated gonads to form as testes (rather than as ovaries). The testes secrete hormones to masculinize the rest of the body. Two masculinizing testicular hormones are antimullerian hormone, a protein that suppresses female reproductive tract development, and testosterone, a steroid that promotes the development of the male reproductive tract and masculine external genitalia (Morris et al., 2004). In masculinizing the body, testosterone binds to the androgen receptor protein and then this steroid-receptor complex binds to DNA, promoting differentiation as a male. If the Sry gene is absent (as in females, who receive an X chromosome from the father), the gonad develops as an ovary, and the body, unexposed to testicular hormones, forms a feminine configuration. The genitalia will only respond to testicular hormones during a particular time in development, which constitutes a sensitive period for hormone action: hormonal treatment of females in adulthood is claimed to have negligible effects on genital morphology (Morris et al., 2004).

Contrary to the mainstream hormonal accounts of gender identity, the concept of sexual neutrality at birth, after which infants differentiate as masculine or feminine as a result of social experiences, was proposed by John Money and colleagues (Money and Erhardt, 1972). In the human brain, structural differences have been described that seem to be related to gender identity and sexual orientation (Swaab, 2004). However, the evidence is highly equivocal. Solid evidence for the importance of postnatal social factors in gender identity is lacking. The truth is we simply do not know.