Posts Tagged ‘sex differences’
what’s on my mind, and in my brain?
The mind is certainly a very mysterious organ, I reflected,.. about which nothing whatever is known, though we depend upon it so completely.
Virginia Woolf, A room of one’s own, 1928
ah yes, it all makes sense now…
So there’s still plenty to learn about the mind, and maybe calling it the brain is only giving us a false sense of the matter (and I’m thinking of ye olde ‘what’s mind, it doesn’t matter, what’s matter, never mind’ jibe), though we’ve made great neurophysiological strides in recent decades. But having just read Virginia Woolf’s thoughts on the position of women almost a century ago, and being old enough to remember texts like ‘Women are from Venus, men are from Mars’, which sought to ‘explain’ and make the best of the pigeonholes the author presumably believed in, I’ve decided to have another quick look at the current expert views on the neurophysiological and hormonal differences between the sexes.
What I’ve found is that it’s still a contested issue. When I last reported on it, I found myself very happy to accept that there are statistical differences between male and female brains, but no categorical differences. That’s to say, both male and female brains vary widely, and it’s reasonable to say that the differences within each gender are as great as the differences between them. Another striking way to think about it is to say that, were you to hand a still living but completely disembodied human brain (just imagine!) to a trained and experienced neurologist, they’d be unable to say categorically that it was M or F.
Well, the first website I’ve come to disputes this claim. It’s from PNAS (often fondly vocalised as ‘penis’, which may or may not be relevant) and it’s a short essay with only one author, Marek Glezerman. My initial sense of it is that he misses the point, and seems disturbingly emphatic. To give an obvious example, the title of the piece is “Yes, there is a female and a male brain: Morphology versus functionality”. In his opening paragraph (but the essay only has two paragraphs), Glezerman summarises the conclusion he disagrees with, a conclusion I based my own essay on years ago:
The authors conclude that brains of women and men are not dimorphic and not categorically different, as are the genital systems of the two genders, but resemble more an overlapping mosaic of specific functional regions and therefore cannot be distinguished as male and female brains.
Reading this made me wonder, and I thought back to the night before – ahhh, the night before – when I spent time at a well-frequented pub full of individuals, male and female, well beyond the first flush of youth. It occurred to me that there wasn’t a single person there whose sex I would feel mistaken about. Many of the men, and none of the women, were balding, bearded and paunchy. Some did have breasts, I admit, that could’ve competed with the females, but I doubt if they’d have managed the same expression, so to speak. And though there was a lot of variety in the voices, it was easy enough to distinguish males from females in that characteristic. Of course there were also differences in dress, mannerisms and choice of drink, but those could be put down to ‘culture’ and dismissed. Even so there might be enough evidence on display to suggest a categorical difference – a morphological difference – traceable to the brain and hormones.
So, what did Glezerman mean, exactly, by ‘morphology versus functionality’? Well, here’s a long, but essential quote from his essay.
Whenever the terms “female brain” and “male brain” are used, the intention should be functional and not morphological, qualitative and not quantitative. Functionally, brains of women and men are indeed different. Not better, not worse, neither more nor less sophisticated, just different. The very brain cells differ chromosomally. The male brain is exposed to a completely different hormonal environment during intrauterine life than the female brain. The available scientific data as to the crucial effect of testosterone on the developing male brain is overwhelming.
Glezerman provides references for his claim about testosterone and its effects, a subject of great interest to me, but I’ll leave that for another essay. But one wonders if this isn’t a storm in a teacup. Going back to my pub reference, of course there were differences within the sexes – some males seemed more ‘feminine’ than others, whatever that may mean, and some women more ‘masculine’. This may again be a matter of hormone expression rather than personal choice, or a complex combination. I find it fascinating that male hormone levels (i.e testosterone) are dropping in the WEIRD world, a matter of concern to some, but not me…. oh, but that’s for that other essay, or did I already write that one?
PNAS has a reply to Glezerman’s essay, which I’ll now focus on. And I should note how polite and civilised these scientific disputes are: far from the world of social media. This response is even shorter that Glezerman’s little essay (I’ll bet that was by design!), so I’ll reflect on it here, passage by passage.
As Marek Glezerman (1) rightly points out, there are differences between females and males in brain and behavior. Glezerman overlooks, however, the fact that such differences may be different and even opposite under different environmental conditions. That is, what is typical under some conditions in a brain composed of cells with an XX chromosomal complement residing in a body with low levels of testosterone, may be typical under other conditions in a brain composed of cells with an XY chromosomal complement residing in a body with high levels of testosterone.
Being a person who spreads himself thinly over a wide variety of intellectual topics (i.e master of none), I had to look up XX and XY (remember mate, two kisses female, one kiss male – which is surely typical). What the response (which has three authors) appears to be saying is that what is typical for a low-testosterone female in some conditions, may also be typical for a high-testosterone male under quite different conditions, in spite of the fact that one set of brain cells carries an XY chromosomal complement, while the other carries XX. Not sure if this carries the day though. But to continue:
Such “reversals” of sex effects have also been reported when the manipulation of environmental conditions was done in utero (by manipulating the dam) and the offspring were tested in adulthood (reviewed in refs. 2 and 3). These observations led to the hypothesis that brains are composed of a “mosaic” of “male” and “female” features rather than of only “male” features or only “female” features, as expected of a “male brain” and a “female brain,” respectively (2, 3)
Wasn’t sure what ‘manipulating the dam’ meant, but a dam is a dam, something that reduces or stops flow, so I suppose this was done in non-human test species? Presumably if you’re able to change hormonal conditions in utero via such methods – or by changing environmental/social conditions, as bonobos appear to have done – you will change the mosaic of behaviour. Bonobos can be quite aggressive, but it appears to be more tilted towards the male of the species. Also, the drop in male testosterone is surely due to changed conditions and expectations for males over a relatively short period – for example in the mere century since A room of one’s own was written, but even more so in the past few decades of mechanisation and anti-machismo, at least in the WEIRD world.
Our study (4) is the first to empirically test whether brains are “male” or “female” by assessing internal consistency in the degree of “maleness-femaleness” of different elements within a single brain. We found that brains with both “female-end” and “male-end” characteristics were more prevalent than brains with only “female-end” or only “male-end” characteristics. This was true for both the volume of brain regions and the strength of connections between regions (assessed in a similar way to ref. 5), in contrast to Glezerman’s assumption that “Other imaging methods might have yielded different results.”
This is claiming evidence for mosaic traits in a majority of the brains under study, both for individual regions in isolation and for brain connectivity. All I can say is that this seems eminently plausible, indeed I would’ve expected such a finding. Not sure, of course, what ‘male-end’ and ‘female-end’ characteristics are exactly. There is a question here, though, about what Glezerman meant by ‘other imaging methods’.
To corroborate our analysis of different aspects of brain structure assessed using MRI, we also analyzed brain function, as revealed in people’s behaviors, personality characteristics, preferences, and attitudes. Also here there were many more people with both “feminine” (i.e., more common in females compared with males) and “masculine” (i.e., more common in males compared with females) characteristics than people with only feminine or only masculine characteristics (4).
Behaviour, over time, can affect brain function and brain regions mightily. An obvious case is language, spoken and written, which is a behaviour that has had considerably impact on the brain, as, for example Maryanne Wolf recounts in Proust and the squid. You’d hardly expect those brain regions that have been adapted/co-opted for language production/reception to have been much affected by gender. The same would go for other skills and practices, such as mathematics. As to the different physical characteristics of males and females (my pub observations), how connected are they to our brains? They certainly have much to do with hormones, of which we have at least fifty types, many of which are connected to/stimulated into action by the pituitary gland, which is in turn stimulated by the hypothalamus, but these regions account for a minuscule proportion of the brain.
There is no doubt that sex affects the structure and function of brain cells. However, the fact that sex can affect brain cells does not necessarily entail that the form and function of brain cells are either “male” or “female” nor that the brains comprised of these cells can be divided into two distinct categories. For such claims to be true it is necessary that the effects of sex are dimorphic, resulting in the formation of distinct “male” and “female” types, as well as internally consistent (2, 3, 6).
I think what’s being said here is that just because our brain cells, indeed all our somatic cells, have either an XX or XY chromosomal complement in their nuclei, this doesn’t dictate essential expressed traits – our intelligence, our humour, our physical skills, our bodily needs, and so forth. As this essay suggests, ‘manipulating the dam’ in utero is likely to have a far greater effect on human development than gender does, unless of course you’re born into a culture in which one gender is significantly undervalued. But let’s not go too near that hornet’s nest.
So to the last lines of the reply to Glezerman:
Hopefully, future studies looking at the relations between sex and other systems in which sex differences have been documented (e.g., the immune system, the cardiovascular system) will assess both internal consistency and degree of overlap, to reveal whether the relations between sex and other systems are more similar to the relations between sex and the brain (mosaicism) or to the relations between sex and the genitalia (dimorphism).
And no doubt there will be differences, especially in relation to hormonal levels associated with the reproductive system, but also in those associated with diabetes, the heart and the circulatory system and so forth, but these are not easily predictable based solely on gender. And there’s another problem with fixating on sex differences in a hard and fast way. It’s not exactly coincidental that male supremacists are all for favouring such differences. That’s why the bonobo example needs to be known and promoted far more than is currently the case.
References
https://www.pnas.org/doi/10.1073/pnas.1600791113#core-r2
What do we currently know about the differences between male and female brains in humans?
the male and female brain, revisited