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Sex differences in brain and behaviour

  1. Nov 4, 2011 #1
    Men and women: completely different. They think differently, act differently, reason differently… it’s a wonder any relationship between them ever lasts. They might as well come from different planets. So the current pop psychological theories suggest. But is there any biological evidence for all of this?

    Whether an individual is male or female in terms of its chromosomes is decided at conception. XX chromosomes for a girl, XY for a boy, and completely dependent on whether the sperm donates an X or a Y, since females only donate Xs. The presence of a Y chromosome means that the foetus will develop testes, whilst two Xs means the gonad becomes an ovary.

    This isn’t it though. After the initial sorting out of chromosomes, hormones come into play. Before these hormones kick-in, the foetus’ brain is neither male or female. It is exposure to different hormones during a critical period during foetal development which creates the difference between male and female brains. Both chromosomally male and female individuals can develop a ‘male’ or ‘female’ brain, if subjected to the right hormones at the right time.

    For the male these hormones are androgens, released from the foetus’ own developing testes, which start producing androgens as early as the thirteenth day of foetal development and carry on doing this until ten days after birth. The importance of these hormones has been shown by the removal of the testes of rats during the first five days after birth. Interrupting the supply of androgen appears to stop the masculinisation of the brain, causing female stereotypic behaviour instead of male stereotypic behaviour. This means the male rats stop mounting nearby females and start wiggling their rump in the air instead). Other studies have shown that rhesus monkeys exposed to androgen, the male hormone, during the critical period of development, like rough and tumble play, and like playing with other androgen-exposed monkeys.

    Differences between the male and female brain extend beyond hormonal factors. Differences in brain cells can be seen in several brain regions: males have more and bigger neurons in one region of the hypothalamic forebrain, for example, and there are also differences in the hippocampus, amygdala and frontal cortex. Studies on rhesus monkey brains also reveal that frontal lobes develop differently in male and female monkeys. Both infant and adult male monkeys who have their orbital prefrontal cortex removed find it difficult to do tests involving spatial discrimination and delayed responses. Infant females with the same part of the brain removed do not have similar difficulties until they are about 15-18 months old. Thus this bit of the brain becomes specialised in spatial learning sooner in males than females. This might also explain why adult males (monkeys and humans too I am afraid) are better at spatial tasks than females.

    In human brains, the different halves of the brain (or different hemispheres) are better at doing different things. The left side of the brain is best at language and serial processing of information, and the right side of the brain is better at nonverbal stuff: three dimensional visualisation, mental rotation, face recognition. However, betweenthe sexes this asymmetry differs. Girl’s brains don’t develop the right side specificity for nonverbal processes as early as boys do. Girls’ brains thus retain plasticity, or the ability to change, for longer than boys’ brains do. This is supported by the greater ability to transfer language abilities to the right hemisphere after damage to the left side of the brain. This extended plasticity also may account for less incidences of disorders related to left-hemisphere dysfunction, e.g. autism.

    Female brains seem to have a lesser degree of specialisation than males, which many have attributed to the larger corpus callosum in females, connecting the two hemispheres. There also seems generally to be less initial development of the brain into two specialised halves in females. Tests on different cognitive abilities consistently show that females are better at verbal fluency, perceptual speed and mathematical calculations, whilst males are better at mathematical reasoning, spatial relationships and target directed motor tasks. These differences are unlikely to be related to any kind of social conditioning, and more likely to reflect physical differences between the brains of men and women.

    Simon Baron-Cohen, Chair of Psychology at Cambridge University, has been researching sex differences in brain and behaviour. Baron-Cohen characterises male and female brains as being respectively better at ‘systemising’ (understanding how systems work) and empathising (understanding what and why people feel). He draws on studies by Servin et al (1999) which show preferences in children as young as one year old for mechanical toys if they are boys, and dolls and emotional play if they are girls. This could, however, be down to social factors. A more convincing study was done on babies of 24 hours old. It found that boys looked more often and for longer at a mobile than at a human face, and girls did the opposite. Baron-Cohen concluded from this that girls are more predisposed to look at the social stimulus rather than the mechanical, and vice versa for boys. From a more biological basis, babies known through amnio studies to have been exposed to more testosterone in the womb made less eye contact after birth in studies of social interaction, than did babies who had been exposed to less testosterone.

    Baron-Cohen links sex differences in the brain to the disorder autism. Autism is a developmental disorder that results in problems in social interaction, problems in verbal and non-verbal communication, and a restricted range of interests. Baron-Cohen is one of the body of psychologists who believes that this disorder is the product of an ‘extreme male brain’. In other words, the symptoms of autism are the characteristics of a male brain taken to their extreme. This opens up an exciting new field of research into autism and autism spectrum disorders, investigating how this extreme type of brain comes about. Baron-Cohen anecdotally suggested at a recent talk that there could even be an extreme female brain at the other end of the spectrum, but that it was unlikely to present any psychological disorder, since an extreme female brain in Baron-Cohen’s terms would be someone bad at systemising, i.e. bad at getting the computer to work perhaps. Whilst there are many people who you can phone if a system breaks down, as Baron-Cohen said, “…if you have difficulty empathising, it’s not clear who you phone…”.

    Extremes to one side, what is apparent is that the popular opinion on differences in male and female psychology seems to be based on a kernel of biological evidence. Some behavioural differences between the sexes can be attributed to hormonal and structural brain differences. Although this doesn’t mean that women should give up on reverse parking and blame their brains, or that men should give up trying to empathise and blame their hormones.
  2. jcsd
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