Men and women: like chalk and cheese. Sex differences in brain and behaviour.

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.