Español Português
"THE 21st-Century BRAIN", Explaining, Mending and Manipulating the Mind, Steven Rose, 2005, pages 81 - 83
Developing Difference: Sex/Gender (2)
What differences do such normal or abnormal patterns make to brain development? As I have said, on average, males tend to have slightly heavier brains than females. On the other hand, female babies tend to be "more advanced" in terms of behaviour and functional capacity at birth than do males.
During post-natal development other tiny differences in brain structure appear. Although the two cerebral hemispheres appear identical in size and shape, there are small asymmetries, and male brains tend to be more asymmetric than female.
There are also claimed to be differences in structure and size in regions of the hypothalamus and corpus callosum. This is not surprisingly, a fiercely disputed area, because of the way in which both similarities and differences can be appropriated (or misappropriated) to claim explanatory power in explaining the persistence of the domination of men in a patriarchal society and I will return to in later chapters.
It is well known that certain traits, such as colour-blindness and haemophilia, can be carried through the female but are only expressed in the male, a consequence statement is that all human brains begin as female, and that some "become masculinised" during foetal development, a "masculinisation" for which a chromosomal difference is necessary but not sufficient.
A key to this "masculinisation" is the hormone testosterone. Popularly, testosterone is the "male" hormone, oestrogen the female.
However, in reality testosterone and oestrogen are produced and responded to by both males and females; it is only the proportions that differ, testosterone being on average present in higher concentration in males. Neither hormone is actually made in the brain, but both can enter through the blood stream, and there are receptors on neuronal membranes in the hypothalamus and other brain regions that recognise the hormones.
What "masculinises" the otherwise female brain is a surge of testosterone production occurring between eight and twenty-for weeks into pregnancy. This is part of the process of differentiation between male and female brains, with characteristic average differences appearing in the distribution of neuronal receptors for the hormones. The existence of receptors in the brain for testosterone and oestrogen produced in other regions also illustrates the importance of the multiple brain-body interactions mentioned earlier on this chapter.
The sex hormones are not the only steroids that affect brain processes and they are also closely chemically related to the brain’s own equivalent of steroid hormones, the neuroesteroids, which act a bit like BDNF (Brain-derived neurotrophic factor) and other growth factors, but which are present prior to birth in different concentrations in male an female brains.
These complex hormonal interactions, occurring even pre-natally, are but one of many reasons why it is not possible simply to "read off" average differences between boys and girls, men and women as "causes by genetic and chromosomal sex differences, and why the relationship between understanding gender has been so tricky.
Such differences are indeed average, and there is considerable variation, which is just part of the problem of attempting to reduce human differences in sex and gender (and still more in sexual orientation) to simple statements about chromosomes, hormones, or any other unilinear "biological" measure.
During post-natal development other tiny differences in brain structure appear. Although the two cerebral hemispheres appear identical in size and shape, there are small asymmetries, and male brains tend to be more asymmetric than female.
There are also claimed to be differences in structure and size in regions of the hypothalamus and corpus callosum. This is not surprisingly, a fiercely disputed area, because of the way in which both similarities and differences can be appropriated (or misappropriated) to claim explanatory power in explaining the persistence of the domination of men in a patriarchal society and I will return to in later chapters.
It is well known that certain traits, such as colour-blindness and haemophilia, can be carried through the female but are only expressed in the male, a consequence statement is that all human brains begin as female, and that some "become masculinised" during foetal development, a "masculinisation" for which a chromosomal difference is necessary but not sufficient.
A key to this "masculinisation" is the hormone testosterone. Popularly, testosterone is the "male" hormone, oestrogen the female.
However, in reality testosterone and oestrogen are produced and responded to by both males and females; it is only the proportions that differ, testosterone being on average present in higher concentration in males. Neither hormone is actually made in the brain, but both can enter through the blood stream, and there are receptors on neuronal membranes in the hypothalamus and other brain regions that recognise the hormones.
What "masculinises" the otherwise female brain is a surge of testosterone production occurring between eight and twenty-for weeks into pregnancy. This is part of the process of differentiation between male and female brains, with characteristic average differences appearing in the distribution of neuronal receptors for the hormones. The existence of receptors in the brain for testosterone and oestrogen produced in other regions also illustrates the importance of the multiple brain-body interactions mentioned earlier on this chapter.
The sex hormones are not the only steroids that affect brain processes and they are also closely chemically related to the brain’s own equivalent of steroid hormones, the neuroesteroids, which act a bit like BDNF (Brain-derived neurotrophic factor) and other growth factors, but which are present prior to birth in different concentrations in male an female brains.
These complex hormonal interactions, occurring even pre-natally, are but one of many reasons why it is not possible simply to "read off" average differences between boys and girls, men and women as "causes by genetic and chromosomal sex differences, and why the relationship between understanding gender has been so tricky.
Such differences are indeed average, and there is considerable variation, which is just part of the problem of attempting to reduce human differences in sex and gender (and still more in sexual orientation) to simple statements about chromosomes, hormones, or any other unilinear "biological" measure.
"THE 21st-Century BRAIN", Explaining, Mending and Manipulating the Mind, Steven Rose, 2005, pages 81 - 83
No comments:
Post a Comment