Men and women differ in both their physical appearance, their reproductive function and their behaviour (Kimura, 2000). One viewpoint that has been held for many decades by developmental psychologists is that these differences are due to variations in experience during development which occurs before, during and after adolescence. Recent evidence suggests that the effects of sex hormones on brain organisation occurs so early in life that the environment is acting on differently formed brains in both boys and girls and it is therefore less likely that it is the environment that causes these differences in behaviour (Maccoby & Jacklin, 1991). The biological bases of sex differences in behaviour may therefore be a result of exposure to various sex hormones early in life (Kimura, 2000). Kimura (2000) stated that these biological differences would help to explain why males are more aggressive than females and why they prefer to engage in rough and tumble play and are better at spatial and navigational tasks. Behavioural, neurological and endocrinological studies on both humans and non-humans provide evidence to support this argument.
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Psychologists from different theoretical positions have argued different cases to suggest that there are sex differences in the brain. Ankney (1992) explained these differences in evolutionary terms and claimed that the extra brain weight in male brains (approximately 100 grams) relates to their superior spatial ability. Men have been traditionally thought of as hunters who engage in activities that heavily rely on the manipulation of visuo-spatial features in the environment (Eals & Silverman, 1994). In contrast, women have been viewed as gatherers or foragers and have therefore evolved to have superior visual recognition (Eals & Silverman, 1994). However, evolutionary theories are often too broad. The evolutionary point of view of hunters and gatherers can be used to explain many psychological findings but they are often untestable (Halpern, 1997).
The psychosocial and developmental theories stem from work conducted by Albert Bandura and suggest that sex differences between boys and girls are learnt through experience, observation, imitation and different models of reward and punishment (Slater, Hocking & Loose, 2003). Gender stereotypes are present from birth: boys are encouraged to play with toys which involve visuo-spatial manipulation whilst girls play with dolls. This largely determines whether the child will behave in traditionally masculine or feminine ways (Martin, Carlson & Buskist, 2007).
Money (1995) stated that there were four imperative differences between men and women: men impregnate; women menstruate, gestate and lactate. Money and Ehrhardt (1972) were convinced that human beings are gender neutral at birth and almost all differences between men and women are determined by society and culture. Money and Ehrhardt (1972) believed that sex hormones secreted before and after birth had less effect on brain and behaviour than the sex assignment that takes place at birth which is determined by the anatomy of the external genitalia.
This viewpoint can be challenged when investigating the case of Bruce Reimer. Murphy (2006) explained that in 1966 a set of male twins were born and at 8 months were taken to a clinic for circumcision due to urinary problems. During Bruce's circumcision, an incompetent physician destroyed his penis. Through the media, Bruce's parents learnt about the views of John Money which suggested that their son could be raised as a girl. Therefore, gender-reassignment surgery was performed on Bruce at 23 months old and he was reared as 'Brenda'. However, even soon after the operation Brenda began rejecting girl things and persisted on engaging in more masculine activities. During adolescence, after several suicide attempts, Brenda learnt the truth and refused to carry on life as a female and had gender reassignment to his biologically determined gender. He later committed suicide. This tragic account challenges the views held by John Money that we are gender neutral at birth as 'Brenda' had many complications after the first gender reassignment surgery. It is evident from this account that biological factors should be considered and experience is not the sole determinant of gender.
Maccoby and Jacklin (1991) focused on the biology of sex differences and found that sex hormones which are present before birth sensitise the individual which affects their behaviour throughout childhood and adulthood even though they have similar levels of sex hormones until puberty. This was supported by Levine (1966) who stated that if a female was exposed to unusually high levels of male hormones prenatally, then the administration of androgens in adulthood would masculinise her behaviour. Therefore, Maccoby and Jacklin (1991) concluded that the sex hormones which are present prenatally and perinatally affect behaviour during childhood and affect the way that sex hormones that are later produced affect the individual.
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Other work has explored the differences of the hypothalamus and the thalamus. It is easier to observe these differences in animal brain research. Evidence from rat studies suggests that the sexually dimorphic nucleus (SDN), which is a small area of the hypothalamus containing a cluster of cells, is larger in males than in females and contributes to control of male sexual behaviour (Money, 1995). Kalat (2006) also studied rats and found that a male hypothalamus and a female hypothalamus that has been exposed to extra testosterone cannot generate a cyclic pattern of hormone release.
Evidence for sex differences in the human hypothalamus comes from Swaab, Gooren, and Hofman (1995) who concluded that the differences of the hypothalamic structure might be involved in the development of male identity. Swaab et al. (1995) looked at the SDN and found that there were twice as many cells in young adult men as in women. After the age of 4, there is a decrease in the number of cells produced by the SDN in girls, but not in boys and this is the cause of the sexual differentiation of the SDN. Swaab et al. (1995) did not observe a difference in the SDN cell number between homosexual and heterosexual men which contests Dörner, Docke and Moustafa's (1968) claim that homosexual males have a female hypothalamus based on their study into behaviour in rats. A criticism of these findings comes from Brannon (1996) who stated that gender identity is a complex process that is linked to feelings which is not taken into consideration in these biologically based studies. Also, this claim is thought to be highly controversial.
An even more controversial claim comes from Simon LeVay (1991). LeVay (1991) obtained brain tissue from 41 subjects: 19 were homosexual men who died of complications from the AIDS virus, 16 were heterosexual men, 6 of these also died from AIDS, 10 from other causes and 6 were women, 1 of whom had also died from AIDS. LeVay (1991) found that there were no differences between the INAH 1, INAH 2 and the INAH 4. However, LeVay (1991) found that the INAH 3 is dimorphic with sexual orientation. The nucleus was more than twice as large in the heterosexual men than the homosexual men and the women. However, the size of the INAH 3 may be the cause or consequence of the individual's sexual orientation. The nucleus size may be a result of the homosexual lifestyle, AIDS virus or the treatment. Studies with homosexual women would strengthen his argument. Findings should suggest that the INAH 3 of a homosexual woman would be the same size as a heterosexual man but there has been no research into this at this present time.
The work of LeVay (1991) has been challenged by Byne (1994). Byne (1994) disputed LeVay's findings because his work has never been reproduced so there is no support for his findings. AIDS has been shown to decrease testosterone levels, so it should be expected that those who suffered from that condition would have a smaller INAH 3 and LeVay (1991) only looked at one gay man who did not die of AIDS. Methods he used to measure the size of the neuron clusters have been rejected. Byne (1994) also argued that as LeVay was a homosexual he had an agenda. Byne (1994) believed that gays who support gay rights are more likely to believe homosexuality is genetic which is inevitably what LeVay (1991) was hoping to find.
More research into homosexual and heterosexual dimorphism has found that the suprachiasmic nucleus (SCN) and the anterior commissure (AC) attributes to this difference and both are larger in homosexual brains. Swaab and Hofman (1990) found that the SCN in homosexual men contains 2.1x as many cells as the SCN of heterosexual men. Allen and Gorski (1992) examined 90 postmortem brains from homosexual men, and heterosexual men and women. The findings showed that the AC in homosexual men was 18% larger than in heterosexual women and 34% larger than in heterosexual men. This difference may underlie differences in cognitive function and cerebral lateralisation amongst these three groups. It also argues against the idea that a single structure causes or results from a homosexual orientation. However, Lasco, Jordan, Edgar, Petito, and Byne (2002) examined the cross-sectional area of the AC in postmortem material from 120 individuals, and found no variation in the size of the AC with age, HIV status, sex, or sexual orientation. The critique of the experiments by Swaab and Hofman (1990) and Allen and Gorski (1992) is similar to that of LeVay (1991), the patients died of AIDS which could cause the size differences. Money (1995) stated that it needs to be determined whether the difference originates prenatally and if so, their source can be traced to the DNA of the genome.
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More up to date research by Bailey et al. (1999) does seem to suggest that homosexuality may be heritable. Bailey et al. (1999) conducted several studies using monozygotic and dizygotic twins of the same sex. They found a 52% concordance rate in monozygotic twins and a 22% concordance rate in dizygotic twins. However, Bailey et al. (2004) used an unsystematic way of recruiting participants and their self-selected sample has been criticised.
Recent research comes from Lenroot and Giedd (2009) who focused on the adolescent brain as this is a time when increased differences between males and females predominantly appear including differences in physical attributes and behaviour. Lenroot and Giedd (2009) found that several regions of the brain were consistently different including the basal ganglia, hippocampus and the amygdala. They found that the most consistent sex difference is brain size; males have a 9-12% larger brain. This finding links in with evolutionary explanations of sex differences in the brain. They also suggested that male and female brains use different patterns of activation during tasks that require cognitive ability to achieve the same performance.
It is evident that there are sex differences in different parts of the brain and it is also apparent that these differences can be linked to behaviour, whether it be ability at performing certain cognitive tasks, or something as complex as sexual orientation. Explanations provided by evolutionary psychologists base their claims on extra brain in males but these theories tend to be too broad and untestable. Developmental and social psychologists tend to place too much emphasis on the influence that the environment and experience has on sex differences whilst not really taking biological differences into consideration. The work of John Money with regards to the case of Bruce Reimer highlights the importance of brain biology. His belief that we are born gender neutral was proven otherwise as Bruce rejected living life as a girl. At one point his belief gained widespread acceptance but it is clear from this tragic case that biology is important.
Much of the research in this area has focused on sexual orientation but this is also the area that has received the most criticism and many of the studies are awaiting replication. These findings, along with those of LeVay (1991) provide groundwork for future research in this area.
There are several theoretical views that provide different accounts on whether there are sex differences in the brain and how these differences arise; but what is apparent is that both nature and nurture must occur or development of the brain would not. Perhaps biological factors provide the basic differences within the brain, but social factors add the fine detail to create particular behavioural differences.