MANOUSH ZOMORODI, HOST:
On the show today - the biology of sex. And just as Molly Webster said, our chromosomes carry our genes in DNA molecules, and our DNA tells our bodies how to express themselves physically, whether we have ovaries or testes, whether we have a vagina or penis. But what about our brains?
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LISA MOSCONI: There are many theories on how women's brains differ from men's brains.
ZOMORODI: That's neuroscientist Lisa Mosconi on the TED stage.
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MOSCONI: I've been looking at brains for 20 years. Pink and blue, Barbie and Lego - those are all inventions that have nothing to do with the way our brains are built. That said, women's brains differ from men's brains in some respects.
ZOMORODI: Lisa studies the connections between neuroscience and biological sex, specifically when it comes to women's health. And hormones are a big part of that.
MOSCONI: A lot of people working in my field, like, in the brain field, do not really look at hormones that much. We don't think about the reproductive organs as something that could potentially affect the brain. And OB-GYNs, they're not equally prepared perhaps to talk about the brain.
ZOMORODI: Hormones allow the brain and the reproductive organs to talk to each other, and that conversation is crucial.
MOSCONI: So from a scientific perspective and biological perspective, DNA really dictates what kind of hormones your body's producing. So still during the prenatal phase when you're basically an embryo developing, these chromosomes really dictate what kind of hormones are circulating in your bloodstream and then inside your brain. So if you have an XX to start with, your brain is going to develop in such a way that optimizes for estrogens. And that starts very early. It's almost three months into gestation where the baby brain is born and then is going to grow more and more. But already then there are growth factors that really promote brain development and development of the body of the child and immediately start populating the baby brain with estrogen receptors. Whereas if you have an XY, then the Y chromosome contains genes that will make your body produce androgens like testosterone. And that means that your brain is going to optimize for androgen receptors. So from the very moment you're born, you're going to have a brain that is really loaded with estrogen and estrogen receptors or with androgens and androgen receptors. So structurally, these brains are a little bit different or biochemically these brains are a little bit different from the moment you're born.
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MOSCONI: Most people think of the brain as some kind of black box isolated from the rest of the body. But in reality, our brains are in constant interaction with the rest of us. These interactions are mediated by your hormones, and we know that hormones differ. Men have more testosterone. Women have more estrogens.
So the brain is connected to the reproductive systems via a network that is called the neuroendocrine system.
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MOSCONI: Our brains and ovaries are part of the neuroendocrine system. As part of the system, the brain talks to the ovaries and the ovaries talk back to the brain every day of our lives as women. So the health of the ovaries is linked to the health of the brain and the other way around. At the same time, hormones like estrogen are not only involved in reproduction but also in brain function. And estrogen in particular or estradiol is really key for energy production in the brain.
I think it's interesting to know how hormones actually work. They're a little bit like a key that needs to open specific locks, and these locks are called receptors. So, for example, in the female brain, there are very specific parts of our brains that are very dense with receptors, and that's where the hormones go. They bind to the receptors. They kind of turn them on. And that generates a million different things, like brain energy, power, you have more immunity, like, more resilience against disease. You have more plasticity. You have more growth. And all these hormones together are responsible for the menstrual cycle in women and they're connected to the ovaries, the gonadal system. And in men, obviously, they're connected to the testis. So these systems are really important and these different structures are connected to each other via the hormones. They flow back and forth every day of our lives. So it's not just reproduction. There are so many things that need to happen inside the brain that are really facilitated by these hormones.
ZOMORODI: More from Lisa Mosconi on how our brains develop differently. We're talking about the biology of sex. I'm Manoush Zomorodi and you're listening to the TED Radio Hour from NPR. Stay with us.
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ZOMORODI: It's the TED Radio Hour from NPR. I'm Manoush Zomorodi. And on the show today, the biology of sex. We were just talking to neuroscientist Lisa Mosconi about the differences between the average male and average female brain.
When it comes to our brains being either male or female, is there actually a range, like, when it comes to the brain and hormones? Because we've been...
ZOMORODI: ...Hearing on this episode about XXY or XYY people.
MOSCONI: Mmm hmm.
ZOMORODI: And earlier we spoke to Emily Quinn, who shared her personal story about how she is intersex, that she is XY...
ZOMORODI: ...But actually, her body doesn't respond to testosterone, only estrogen, which from your description, seems like it would be a response to XX chromosomes. But she has XY chromosomes. So I guess I'm wondering, like, is there a spectrum?
MOSCONI: Oh, for sure, for sure. There's a lot of overlap between different brains and different people. I think what we see as scientists is that on average, like, the average male brain is somewhat bigger than the average female brain.
ZOMORODI: Ah. Isn't that just a stereotype?
MOSCONI: No, it's true.
MOSCONI: Male brains with more androgens, like testosterone, are generally bigger.
MOSCONI: But then there's everything else in between. So yes, it's never black and white. A lot of people ask me, if I give you a brain scan and you just look at the brain scan, can you tell me if that brain belongs to a man or a woman? It was like, no. Of course not.
MOSCONI: Yeah. No, you cannot - you cannot do that.
ZOMORODI: But then how do you know the difference? When's the moment when you're like, oh, yes, this is female, or oh, yes, this is a male brain?
MOSCONI: In my experience, the most defining feature is the change over time...
MOSCONI: ...Starting in the late 40s. So instead of looking at the anatomy of the brain or the volumes or the size of the brain, what really changes is the functionality of the brain. And so women's brains tend to show declines in brain energy levels around menopause...
MOSCONI: ...Whereas these declines are not seen in men of the same age. So if you give me a brain scan and you don't tell me who the scan belongs to, but I can see brain energy levels, let's say, and a person's, like, 45, 48, women are much more likely than men to have lower metabolic activity...
ZOMORODI: In their brains.
MOSCONI: ...In their brains. Yes, in their brains.
ZOMORODI: OK. So as a woman barreling towards her late 40s, that gives me great pause because...
ZOMORODI: ...Essentially what you're saying is that a man my age is pretty much going to have the same brain his whole lifetime. But as a woman, I - my brain is going to go through a change because my body responds to hormones differently. Correct?
MOSCONI: Yes. Yes. And it's not just menopause. I think we tend to - we think of aging as a linear phenomenon. But it's not...
MOSCONI: ...Really linear.
ZOMORODI: Mmm hmm.
MOSCONI: So both men and women go through something called a transition stage. Puberty is a really big one linked to an explosion of hormonal power. Men just, all of a sudden, have a ton of testosterone. Women have enormous amounts of estrogen. And that has really been shown to change the structure, the function and the connectivity of the brain in both teenage boys and girls - right? - in adolescents. And what's interesting is that this remodeling leads to a lot of synapses to be discarded. And synapses are the point of connection between different neurons. And that has been interpreted as an optimization. So the brain is getting rid of all the neurons that it doesn't really need.
ZOMORODI: Ah, cleaning house. OK.
MOSCONI: Yes, exactly. It's like you're doing a nice cleaning. You're like, OK, at this point you know how to tie you shoes, right? So I don't need these neurons anymore. You can do it by yourself.
MOSCONI: But then there are two things that happen to women and not to men that also impact their brains in a big way - pregnancy being a really important one. The brain goes through a remodeling after the baby is born.
MOSCONI: It's very similar - yeah, it's very similar to what happens during puberty. So the brain seems to be actually getting rid of a bunch of neurons and synapses that...
MOSCONI: (Laughter) Well, it's true.
ZOMORODI: I'm laughing because it felt like that. I felt like my - I felt like I had a different head on my shoulders after...
ZOMORODI: ...I had a baby, yeah. For good and bad, you know? And, you know, it really - I really felt like they'd swapped out, like, my thinking process. But what you're saying is, like, that is kind of what happened. The hormones...
ZOMORODI: ...Gave my brain a big, old shower, and I emerged as kind of a little bit of a different person, I guess.
MOSCONI: Yes, absolutely. And so many women tell me the same. But that's an optimization. From an evolutionary perspective, that's a plus. And then women go through another transitional stage, which is menopause. And what we have shown is that these changes, these symptoms, do not start in the ovaries as many people think. They start in the brain.
ZOMORODI: So how much of this is based on genetics? Because it kind of sounds like overall, our genes dictate how our brains will respond to our hormones. Like, is that the right way to look at it?
MOSCONI: It's in part genetic, but also in big part is really...
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MOSCONI: ...Whatever happens after you're born. So all your experiences, the way you live your life, your medical health, all these things really combine to dictate, No. 1, the health of your hormones, No. 2, the health of your brains, No. 3, how these two different systems really work with each other. Our brains are different. They're not better or worse. They're just different. And they're not - these differences just about pink and blue or what kind of toys we like or what kind of jobs we we want to have, it's really - it's not just gender roles. It's also our health.
ZOMORODI: That's neuroscientist Lisa Mosconi. Her latest book is "The XX Brain." You can see her talk at TED.com.
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