This is a hormone that has fascinated me. It’s a small molecule that seems to be doing remarkable things. The variation we see in this hormone comes from a number of different sources. One of those sources is genes; many different genes can influence how much testosterone each of us produces, and I just wanted to share with you my fascination with this hormone, because it’s helping us take the science of sex differences one step further, to try to understand not whether there are sex differences, but what are the roots of those sex differences? Where are they springing from? And along the way we’re also hoping that this is going to teach us something about those neuro-developmental conditions like autism, like delayed language development, which seem to disproportionately affect boys more than girls, and potentially help us understand the causes of those conditions.
What I want to talk about tonight is this very specific hormone, testosterone. Our lab has been doing a lot of research to understand what this hormone does and, in particular, to test whether it plays any role in how the mind and the brain develops.
Before I get to that point, I’ll say a few words by way of background about typical sex differences, because that’s the cradle out of which this new research comes. Many of you know that the topic of sex differences in psychology is fraught with controversy. It’s an area where people, for many decades, didn’t really want to enter because of the risks of political incorrectness, and of being misunderstood.
Perhaps of all of the areas in psychology where people do research, the field of sex differences was kind of off limits. It was taboo, and that was partly because people believed that anyone who tried to do research into whether boys and girls, on average, differ, must have some sexist agenda. And so for that reason a lot of scientists just wouldn’t even touch it.
By 2003, I was beginning to sense that that political climate was changing, that it was a time when people could ask the question — do boys and girls differ? Do men and women differ? — without fear of being accused of some kind of sexist agenda, but in a more open-minded way.
First of all, I started off looking at neuroanatomy, to look at what the neuroscience is telling us about the male and female brain. If you just take groups of girls and groups of boys and, for example, put them into MRI scanners to look at the brain, you do see differences on average. Take the idea that the sexes are identical from the neck upwards, even if they are very clearly different from the neck downwards: the neuroscience is telling us that that is just a myth, that there are differences, even in terms of brain volume and the number of connections between nerve cells in the brain at the structure of the brain, on average, between males and females.
I say this carefully because it’s still a field which is prone to misunderstanding and misinterpretation, but just giving you some of the examples of findings that have come out of the neuroscience of sex differences, you find that the male brain, on average, is about eight percent larger than the female brain. We’re talking about a volumetric difference. It doesn’t necessarily mean anything, but that’s just a finding that’s consistently found. You find that difference from the earliest point you can put babies into the scanner, so some of the studies are at two weeks old in terms of infants.
You also find that if you look at postmortem tissue, looking at the human brain in terms of postmortem tissue, that the male brain has more connections, more synapses between nerve cells. It’s about a 30 percent difference on average between males and females. These differences are there.
The second big difference between males and females is about how much gray matter and white matter we see in the brain: that males have more gray matter and more white matter on average than the female brain does. White matter, just to be succinct, is mostly about connections between different parts of the brain. The gray matter is more about the cell bodies in the brain. But those differences exist. Then when you probe a bit further, you find that there are differences between the male and female brain in different lobes, the frontal lobe, the temporal lobe, in terms of how much gray and white matter there is.
You can also dissect the brain to look at specific regions. Some of you will have had heard of regions like the amygdala, which people think of as a sort of emotion center, that tends to be larger in the male brain than the female brain, again, on average. There’s another region that shows the opposite pattern, larger in females than males: the planum temporale, an area involved in language. These structural differences exist, and I started by looking at these differences in terms of neuroanatomy, just because I thought, at least those are differences that are rooted in biology, and there might be less scope for disagreement about basic differences.
I’ve talked a little bit about neuroanatomy, but in terms of psychology, there are also sex differences that are reported. On average, females are developing empathy at a faster rate than males. I keep using that word ‘on average’ because none of these findings apply to all females or all males. You simply see differences emerge when you compare groups of males and females. Empathy seems to be developing faster in girls and in contrast, in boys there seems to be a stronger drive to systemize. I use that word ‘systemizing’, which is all about trying to figure out how systems work, becoming fascinated with systems. And systems could take a variety of different forms. It could be a mechanical system, like a computer; it could be a natural system, like the weather; it could be an abstract system, like mathematics; but boys seem to have a stronger interest in systematic information. I was contrasting these two very different psychological processes, empathy and systemizing. And that’s about as far as I went, and that was now some 11 years ago.
Since then my lab has wanted to try to understand where these sex differences come from, and now I’m fast-forwarding to tell you about the work that we’re doing on testosterone. I’m very interested in this molecule, partly because males produce more of it than females, and partly because there’s a long tradition of animal research which shows that this hormone may masculinize the brain, but there’s very little work on this hormone in humans.