Sex describes the biological characteristics used to identify individuals as either male or female, and can be measured by examining an individual’s sex chromosomes, gonads and external reproductive organs. Gender refers to the sociocultural characteristics associated with being either male or female, and is frequently divided into gender identity and gender role.
The sex chromosomes that an individual possesses (usually XX among females and XY among males) begin the process of sexual differentiation. Among females, the Müllerian duct system develops into the female reproductive system, whereas in males the presence of the SRY gene causes the development of the Wolffian duct system, which in turn develops into the male reproductive system. Organisational hormones in the womb influence the developing foetal brain and body, usually causing the foetus to develop in a male-typical or female-typical fashion.
The process of sexual differentiation does not always proceed in a typical manner. A number of conditions can arise due to sex chromosome atypicality, such as Klinefelter’s syndrome or Turner’s syndrome. Alternatively, a variety of atypical conditions, such as 5-alpha reductase syndrome or congenital adrenal hyperplasia, can arise due to the action of organisational hormones. The appropriate course of treatment for many individuals born with such conditions is contentious.
The second important time for the development of sex differences, after the prenatal one, is puberty. During this time an individual sexually matures and secondary sexual characteristics develop. Although many people believe there are large-scale behavioural and psychological differences between the sexes, current research suggests that only a limited number of these differences, such as levels of physical aggression, are significant in magnitude.
The current research suggests there is a biological basis to homosexuality. A number of studies have identified a heritable component to both male and female homosexuality, and research is currently under way to identify so-called ‘gay genes’. Organisational hormones also appear to play a prominent role in the development of homosexuality. Research has identified areas of the brain and certain cognitive and behavioural tasks that appear to vary based on an individual’s sexual orientation, particularly among males.