Each person has 23 pairs of chromosomes (in each cell of the body), each carries hundereds of genes containing instructions about physical and behavioural characteristics.
One paire of chromosomes are called sex chromosomes because they determine an individuals sex.
There is usually a direct link between an individuals chromosomal sex and their exetrnal genitalia (vagina or penis) and internal genitalia (ovaries or testes).
Hormones in Gender Development
Chromosomes initially determine a persons sex but most gender development is actually governed by hormones. These are both produced prenatlly and in adolesence. Hormones influence the development of genitalia and/or affect the development of the bain, both of which influence gender behaviour.
Development of genitalia
Normally external genatalia are in accord with genetic sex (e.g. genetic male develops a penis). However in some cases a genteic male embryo is exposed to too little male hormone and the result is that the newbprn appears externally to be female.
Conversely, genteic females may be exposed prenatally to relatively large doses of male hormones (for example when pregnant mothers have been given drugs containing male hormones). The result is ambiguous genatalia. Such individuals are usually identifies as female at birth and most are content with this gender assignment, although research such as Berenbaum and Bailey, 2003 has indicated that they are often interested in male-type activities and tomboyish, presumably because of the influence of the male hormones.
AIS- Androgen Insensitivity Syndrome
Androgens, such as testosterone, are male hormones. Some XY individuals have an insensitivity to such hormones, i.e. their bodies tissues do not not respond to the effects of the hormone. In extreme cases the consequence is that no external male genitalia develop. Such individuals are usually identified as girls at birth and raised as girls, although some are identified and raised as boys.
One classic case of AIS was reported in one family from the Dominican Republic (Imperato-McGinley et al., 1979). Four children in the Batista family were born with external female genetalia and raised as girls. The large amounts of testosterone produced during puberty caused their male genetalia to appear. These children were genetically XY but had not developed male genetalia because of an inherited gene that caused androgen insensitivity.
It is interesting to note that the girls seemed to accept their change of sex without difficulty. One explanation offered is that they had never really taken on the feminine role because a number of their relatives had had similar experiences, and therefore they exected to eventually become boys.
Sex Differenecs Explained
Male and female brain are different in many ways. For example, girls generally appear to be better at social skills (such as emphathising) than boys and ar emore talkative, less good at spacial navigation and so on. Geschwind and Galaburda (1987) were the first to suggest that such sex differences may be caused by the effects of testosterone levels on the developing brain. Male brains are exposed prenatally to more testosterone than female brains and this leads to the development of a masculinised brain. If the brain of a genetic female is exposed to testosterone prenatally the effect may be to masculinise the brain. This may explain why David Reimer 'felt' he was a boy even though he was raised as a girl, and can also explain why girls prenatally eposed to male hormones grow up to be tomboyish.
Effects of testosterone on brain development have been confirmed in non-human animals, for example, Quadango et al. (1977) found that female monkeys who were deliberately exposed to testosterone during prenatal development later engaged in more rough and tumblr play than other females and were more aggressive.
Dr John Money proposed that gender is learned- as long as the child is young enough so that they can be socialised to be a boy or a girl, biological sex didn't matter. He used the case study of David Reimer as evidence to support this theory. David started life as Bruce Reimer and had an identical twin Brian. They were both circumcised at 6 months but unfortunately Bruce's operation was botched and his penis was all but burned off. Dr Money suggested Bruce have his penis and testes removed so he looked like a girl and raised as 'Brenda'.
Over the next 10 years the twins visited Money regularly as he took great interest in the case as it allowed him to test his theory. This was a unique natural experiment, a normal XY male raised as a female, with a identical twin borther as a control. Money claimed that the gender assignment had been a success and advertising that this was the new way forward for many intersexe cases.
However during adolesence despite 'Brenda' recieving female hormone treatments, inside he/she was in turmoil as she had always been an outcast in schooland became increasingly depressed. When her parents told her the truth she was "relieved", her difficulties in being a girl were becauses she wasn't a girl. She reverted back to her true sex and called heself David, he had operations to remove the breasts and further operations to construct a penis. He married and raised three step children. Therefore David's case seems to suggest that, at least in some case, biological sex is the primary factor in ones sense of gender.
The Importance of Genetic Factors
Money and Ehrhardt, 1972 claimed that biological sex was not the main factor in gender development. Money argued that sex of rearing was much more important and recommended that intersex individuals, such as David Reimer could be successfully raised as either a boy or a girl. However his famous case study of David Reimer did not support this. This outcome has ben further supported by subsequent research, for example that of Reiner and Gearhart (2004)who studied 16 genetic males born with almost no penis. Two were raised as males, the remaining 14 raised as females, and of these 8 re-assigned themselves as males by the age of 16. Such research suggests that biological factors have a key role in gender development.
Genetic sex does not match external gentalia in cases of abnormal hormone exposure and even hormones do not produce a simple formula for establishing gender. The eventual outcome of each individual is what appears to be a complex and unpredictable combination of genes, hormones, sex of rearing and socialisation. The outcome is described as 'unpredictable' because there do not appear to be any simpler rules. For eample, Congenital Adrenal Hyperlasia (CAH) occurs when XX females have prenatally high levels of male hormones resulting in varying degrees of male genatalia. Research appeares to indicate that whatever gender is assigned at birth seems to be accepted by some individuals but not others. Thus gender development is in part biologically determined (nature) but experience, personal qualities and socialisation (nurture) also have a key role.
Much of the evidence in this area comes from case studies or small samples of abnormal individuals. Such research is a problem as it contains many problems, the main one being the lack of generalisability from abnormal individuals to the wider 'normal' population.
A further criticism of the research is that intersexes may be more vulnerable to social influences than 'normal' individulas because their biological ambiguities mean they have to search harder for clues to their identity, and this is another reason why this research may lack generaslisability.
An interetsing slant on the issue of biological determinism is shown in a dilema faced by the International Olympics Committe. Since 1968 it has tested the genetic sex of all athletes and excluded all but XX females and XY males, which meant that AIS individuals couldn't compete. In 1991 there was a ruling that genetic sex would no longer determine entry to the games; individuals are now excluded from women's events if they are obviously physically male (Brown, 1992). In other words genetic sex no longer determines gender.