a bonobo humanity?

Having had an interesting conversation-cum-dispute recently over the question of male-female differences, and having then listened to a podcast, from Stuff You Should Know, on the neurological differences between the human male and the human female, which contained some claims which astonished me (and for that matter they astonished the show’s presenters), I’ve decided to try and satisfy my own curiosity about this pretty central question. Should be fun.

The above link is to How Stuff Works, which I think is the written version of the Stuff You Should Know podcast, that’s to say with more content and less humour (and less ads), but I do recommend the podcast, because the guys have lots of fun with it while still delivering plenty of useful and thought-provoking info. Anyway, the conversation I was talking about was one of those kitchen table, wine-soaked bullshit sessions in which one of the participants, a woman, was adamant that nurture was pretty well entirely the basis for male-female differences. I naturally felt sympathetic to this view, having spent much of my life trying to blur the distinctions between masculinity and femininity, having generally been turned off by ultra-masculine and ultra-feminine traits and wanting to push for blended behaviour, which obviously suggests we can control these things through nurturing such a blending. However, I had just enough knowledge of what research has revealed about the matter to say, ‘well no, there are distinct neurological differences between males and females’, but I didn’t have enough knowledge to give more than a vague idea of what these differences were. The podcast further whetted my appetite, but writing about it here should pin things down in my mind a bit more, here’s hoping.

I’ve chosen the title of this post reasonably carefully, with apologies for its clunkiness. For the fact is, we still know little enough about our brains. I’ve mentioned humans, but I expect there are gender differences in the brains of all mammals, so I’m particularly interested in that part of the brain that distinguishes us, though not completely, from other mammals, namely the prefrontal cortex.

Here’s an interesting summary, from a blurb on a New Scientist article by Hannah Hoag from 2008;

Research is revealing that male and female brains are built from markedly different genetic blueprints, which create numerous anatomical differences. There are also differences in the circuitry that wires them up and the chemicals that transmit messages between neurons. All this is pointing towards the conclusion that there is not just one kind of human brain, but two. …

Men have bigger brains on average than women, even accounting for sexual dimorphism, but the two sexes are bigger in different areas. A 2001 Harvard study found that some frontal lobe regions involved in problem-solving and decision-making were larger in women, as well as regions of the limbic cortex, responsible for regulating emotions. On the other hand, areas of the parietal cortex and the amygdala were larger in men. These areas regulate social and sexual behaviour.

The really incredible piece of data, though, is that men have about 6.5 times more grey matter (neurons) than women, while women have about ten times more white matter (axons and dendrites, that’s to say connections) than men. These are white because they’re sheathed in myelin, which allows current to flow much faster. On the face of it, I find this really hard, if not impossible, to believe. I mean, that’s one effing huge difference. It comes from a study led by Richard Haier of the University of California, Irvine and colleagues from the University of New Mexico, but this extraordinary fact appears to be of little consequence for male performance in intellectual tasks as compared to female. What appears to have happened is that two different ‘brain types ‘ have evolved alongside and in conjunction with each other to perform much the same tasks. Other research appears to confirm this amazing fact, finding that males and females access different parts of the brain for performing the same tasks. In an experiment where men and women were asked to sound out different words, Gina Kolata reported on this back in early 1995 in the New York Times:

The investigators, who were seeking the basis of reading disorders, asked what areas of the brain were used by normal readers in the first step in the process of sounding out words. To their astonishment, they discovered that men use a minute area in the left side of the brain while women use areas in both sides of the brain.

After lesions to the left hemisphere, men more often develop aphasia (problems with understanding and formulating speech) than women.

While I’m a bit sceptical about the extent of the differences between grey and white matter in terms of gender, it’s clear that these and many other differences exist, but they’re difficult to summarise. We can refer to different regions, such as the amygdala, but there are also differences in hormone activity throughout the brain, and so many other factors, such as ‘the number of dopaminergic cells in the mesencephalon’, to quote one abstract (it apparently means the number of cells containing the neurotransmitter dopamine in the midbrain). But let me dwell a bit on the amygdala, which appears to be central to neurophysiological sex differences.

Actually, there are 2 amygdalae, located within the left and right temporal lobes. They play a vital role in the formation of emotional memories, and their storage in the adjacent hippocampus, and in fear conditioning. They’re seen as part of the limbic system, but their connections with and influences on other regions of the brain are too complex for me to dare to elaborate here.  The amygdalae are larger in human males, and this sex difference appears also in children from age 7. But get this:

In addition to size, other differences between men and women exist with regards to the amygdala. Subjects’ amygdala activation was observed when watching a horror film. The results of the study showed a different lateralization of the amygdala in men and women. Enhanced memory for the film was related to enhanced activity of the left, but not the right, amygdala in women, whereas it was related to enhanced activity of the right, but not the left, amygdala in men.

This right-left difference is significant because the right amygdala connects differently with other brain regions than the left. For example, the left amygdala has more connections with the hypothalamus, which directs stress and other emotional responses, whereas the right amygdala connects more with motor and visual neural regions, which interact more with the external world. Researchers are of course reluctant to speculate beyond the evidence, but as a non-scientist, but as a pure dilettante I don’t give a flock about that – just don’t pay attention to my ravings. It seems to me that most female mammals, who have to tend offspring, would be more connected to the flight than the fight response to danger than the unencumbered males would be??? OMG, is that evolutionary psychology?

It’s interesting but hardly surprising to note that studies have shown this right-left amygdala difference is also correlated to sexual orientation. Presumably – speculating again – it would also relate to those individuals who sense from early on that they’re born into ‘the wrong gender’.

Neuroimaging studies have found that the amygdala develops structurally at different rates in males and females, and this seems to be due to the concentration of sex hormone receptors in the different genders. Where there’s a size difference there appears to be a big difference in number of sex hormones circulating in the area. Again this is difficult to interpret, and it’s early days for this research. One brain structure, the stria terminalis, a bundle of fibres that constitute the major output pathway for the amygdala, has become a focus of controversy in the determination of our sense of gender and sexual orientation. As a dilettante I’m reluctant to comment much on this, but the central subdivision of the bed nucleus of the stria terminalis is on average twice as large in men as in women, and contains twice the number of somatostatin neurons in males. Somatostatin is a peptide hormone which helps regulate the endocrine system, which maintains homeostasis.

What all this means for the detail of sex differences is obviously very far from being worked out, but it seems that the more we examine the brain, the more we find structural and process differences between the male and female brain in humans. And it’s likely that we’ll find similar differences in other mammals.

It’s important to note, though, that these differences, as in other mammals, exist in the same species, in which the genders have evolved to be codependent and to work in tandem towards their survival and success. Just as it would seem silly to say that female kangaroos are smarter/dumber than males, the same should be said of humans. The terms smart/dumb are not very useful here. The two genders, in all mammals, perform complementary roles, but they’re also also both able to survive independently of one another. The amazing thing is that such different brain designs can be so similar in output and achievement. It’s more impressive evidence of the enormous diversity of evolutionary development.