A conversation with the neuroscience professor at Chicago Medical School and the author of “Pink Brain, Blue Brain.”

Question: What are the differences between boys and girls in the womb?

Lise Eliot: Well, not a lot. You know, actually, we have a lot of data given the number of mothers who undergo ultrasound now. And although I think parents, a lot of parents, believe that boys are more active prenatally, in fact, the existing data don’t really support that. Some studies do see higher activity levels in boys before birth but others see no difference. And we even see higher activities in girls. So we definitely know boys are more active by about two years of age or even one and a half but before birth it’s not quite apparent. Probably the biggest difference between boys and girls we know before birth is that boys do seem to develop a little bit more slowly.

Maybe a week or two less mature by the end of a 40 week gestation. And that actually is a problem for babies who are born premature. It’s pretty well known in the neonatology world that premature boys have much higher risks than premature girls even in terms of basic survival. So something about the Y chromosome or the cascade of hormones that it triggers does seem to slow the male development a little bit that affects boys. But behaviorally there isn’t a whole lot that jumps out at you.

Question: How do boys and girls differ in early development?

Lise Eliot: Yes, in some ways and not in others. And so, it’s not really possible to say, you know, girls mature more quickly than boys. Yes, in some ways they do but not in every way. And, you know, the brain has lots of different circuits, a lot of different abilities, and there’s not a single pattern for male versus female development. Boys definitely are slower to develop in terms of language. This has been known for some time. But to put numbers on it, there’s about a one month difference between girls and boys in terms of the number of words they speak.

So at 20 months of age, a little girl may have a vocabulary of a couple hundred words and a little boy may have a vocabulary of 30 or so fewer words. And then he’ll catch up by a month and so on. In some ways the boys, when we talk specifically about verbal skills, they don’t ever fully catch up. This is one of the gender differences, you know, in verbal skills that we see even in adults. Although this notion that women speak 20,000 words a day and men speak 7,000 words a day is just, sort of, an urban myth that I hope is finally been debunked. The verbal differences and almost all of the cognitive differences are really quite small. A tenth or two-tenths of a standard deviation across the population. So they’re not as large as we tend to magnify them.

Question: How much does biology influence gender differences, and how much does culture play a role?

Lise Eliot: Sure. Well, there’s no question that there are innately programmed differences between boys and girls. Obviously their bodies are different. And some of those same cascades of genes and hormones we know are affecting the brain. Although we’re still years away from knowing exactly which circuit is affected in boys versus girls. But behaviorally we know that boys are more active, as I mentioned. And really one of the strongest differences that is probably hardwired, if you will, is the fascination with moving objects. And boys are definitely more interested in trucks and balls. And every parent will tell you that. And it’s true. In fact, it’s just about the largest sex difference there is.

Only sexual preference itself is actually a larger difference between males and females than this difference in toy preference. It’s much larger than the verbal differences, spatial, even things like empathy and self-esteem which are areas of concern. Those differences are much smaller than the difference in toy choice. Take a three year old boy, a three year old girl, put them in a room with a choice of toys and you will see a pretty clear separation. But what’s interesting, and I like to focus on toy choice because I think on the one hand, it definitely shows us the strong biological roots of sex difference. But on the other hand, as toy choice morphs over the preschool years, we start seeing the role of social influences.

So up to the age of about 12 months, interestingly, boys and girls don’t differ a lot in toy choice. Boys and girls both, it turns out, really like dolls up to about one year of age. And I think that’s because dolls have faces and every baby is riveted by the human face. It’s pretty much their favorite thing to look at, which makes a lot of sense. Boy or girl, you need to focus on faces because those are the people who are going to nurture you. And then what happens is that as children branch out and they get more mobile, when they’re given a choice, you’ll see this truck versus doll distinction about age three. But interestingly, when you follow the same children out to about age five, what we see, at least in Western societies, is that now boys are avidly avoiding anything girl. Even more at age five than age three, they’ll spend only ten percent of their time exploring a doll or a pink tea set. 

But the girls by age five are now splitting their time, 50/50 between the building toys or the Power Rangers and the toy vacuums and toy tea sets. So I think what this shows us is the role of our culture. And certainly children with more open minded parents show more cross gender play than parents who are more discouraging of that. But in addition if you look at what’s happened to girls in the last several decades, they have been told, you can do anything. You can do any career. You can wear anything. You can wear pants or skirts. And sure enough, they have much greater latitude to explore a range of interests. Where as boys, there’s still many strong prohibitions against them wearing anything feminine or doing anything that’s regarded as traditionally feminine. So, you know, it’s culture acting on top of biology which will magnify some differences and minimize others depending on how culturally significant they are.

Question: How can parents prevent children from falling into gender stereotypes?

Lise Eliot: Yeah. I should say as a parent with teenagers and tweens now, it is true our influence only goes so far. But certainly when they’re very young, parents do have a lot of influence and the kinds of play things you bring home will determine what sort of skills your child exercises. So, we’ve always known that building toys are great for developing three dimensionality and coordination and hand eye coordination and spatial skills. But something like 88 percent of Lego sets are sold to boys or as gifts for boys. And so here’s something that parents can do, is to find building toys that appeal to girls. I know, like, playing wooden blocks are less gendered and girls may enjoy more.

There used to be more pink Legos, as I call them. I haven’t, you know, when I’ve looked recently I haven’t seen as many options. I think we see this in general that marketers have figured out that gender really sells. And children do have a strong, strong psychological need to categorize themselves as male or female. It’s one of the first concepts, first categories, that they learn in infancy. They know the difference between a male voice and a female voice. They know the difference between mommy and daddy. And as soon as they understand their own gender, which happens between two and three years of age, they feel very determined to live up to that and express it. So given a choice, given a Toys R Us, two different aisles, they will make those choices more and more clearly. Peers also, obviously, reinforce this.

So, you know, to the extent that parents early on can influence which peers your kids are playing with that can have a role. I talk in the book about research on siblings. There’s pretty good evidence that older siblings shape younger siblings’ interests and even abilities in things such as athletics. Girls with older brothers play more sports and are interested in sports. And boys with older sisters, at least according to one study of twins with – Actually – Well, can we scratch that? Yeah, so boys with sisters, in this case it was a study with twin sisters, were more verbal than boys with twin brothers. It’s presumably who you’re talking to early on will shape the amount of verbal practice you get, which we know is very important for wiring up language circuits.

Question: Does an overabundance of toy choice contribute to gender separation?

Lise Eliot: Yeah. I think that, in some ways, things have gotten more gender segregated just because kids are given so much freedom of choice. From the toys to the clothes to the music to the books, you know, it’s almost become the case that there’s girl books and boy books now. Which, you know, is – I don’t know how J.K. Rowling pulled it off but if we could have more of that literature that crosses over. A theme of the book is that, obviously, I feel that the two sexes have a lot to learn from each other. We do tend towards different strengths and the more time that we spend apart, we reinforce our traditional strengths and the less we learn from each other. And so, to the extent that families can get brothers and sisters working and playing together. And teachers can get boys and girls working together. I think that it’s generally beneficial. Appreciating that children do have a strong need to identify with their same sex groups and have activities that are single sex. But we want both.

Question: Does a single mother raising a child have an effect on their gender identity?

Lise Eliot: There’s been so many kids raised by single moms now that their gender identity, even sexual orientation, does not even to seem to be at all affected by whether you’re raised by one parent or two or gay or straight parents. And that is presumably because the peer group is much more important in gender identity. Now it is possible for parents to switch a child’s gender identity but it has to happen very early. And that sounds kind of freakish but it actually happens in certain situations where if a child is born with a severe birth defect that basically affects the whole lower abdomen and they don’t develop any identifiable genitalia and traditionally, if surgeons had to, sort of, create something, it was easier to create a girl’s body.

And so there is a population of XY individuals with a normal X chromosome and normal Y chromosome who should be boys but were raised from birth as girls. And the interesting thing about this population, and they’ve been studied now by researchers at Columbia and another institution, is that about half of them are perfectly happy with their female identity and the other half have shown signs or actually begun the process of switching roles in adolescence or adulthood; switching to the male identity. So what does this tell us? It tells us that, yes, genes and hormones are important for gender identity but they’re not decisive.

That rearing can also play a very important role and it does play an important role. For example, those girls with Congenital Adrenal Hyperplasia who were exposed to very high levels of testosterone, absolutely their behavior is more masculine in certain ways like physical aggression and interest in trucks and balls but their gender identity is very solidly female. They regard themselves as girls. They may be like tomboys but they do not switch gender or very rarely. So, you know, parents are important but really culture it’s much bigger than just a family. Children are socialized by the gender roles that they’re exposed to among their peers and society at large.

Question: How similar are opposite sex twins?

Lise Eliot: Yeah, so I do talk a lot in the book about opposite sex twins because they have been a favorite research subject for people interested in the role of prenatal testosterone. We definitely know from other animals, and most of the work’s been done on rats and mice, that females who gestate next to males – These other species have large litters of eight or ten pups and researchers can actually compare a female that grows up next to two males versus a female that grows up next to two females and so on. And you can almost do a dose response of the amount of prenatal testosterone exposure because that’s what’s crossing from one fetus to the next. And it’s effects on certain features of behavior, particularly sexual behavior, when they hit at puberty.

And you do see slight masculinization of females exposed to more testosterone and vice versa. So people have looked really closely at human opposite sex twins. And there’s a couple of hints that physiologically girls with male twins are slightly masculinized. And some of the best data comes from studies of the ear. We have these funny little sounds called auto acoustic emissions that our ears actually emit in response to an incoming sound. This is actually the basis of newborn hearing tests. So across the country now, almost all babies are screened at birth for hearing loss using these auto acoustic emissions. They can put a little sensitive earphone in there and hear whether the ear is putting out the right amount of sound. Well, girls have larger auto acoustic emissions than boys. Adult women have larger auto acoustic emissions than adult men. It has no relevance to our hearing ability but it’s a reliable sex difference.

And so, I think it’s a couple of studies of opposite sex twins have found that girls with male twins have smaller auto acoustic emissions and are more boy like. There’s that. There’s a finding about the size of the teeth in girls with male twins are a little bit larger. What’s interesting to me is that behaviorally, when it comes to some of this toy choice that seems to be so importantly shaped by biological influences, girls with male twins are every bit as feminine as girls with girl twins. Similarly, when it comes to things like sexual orientation or even, you know, age of menstruation and hormonal things that you think might be shaped by prenatal testosterone, there are no fertility differences between women with males twins and not. So the consensus at the moment is that probably humans are not – Don’t share as much testosterone from one twin to the next as other animals do. But there may be something going on there and if it is, it just proves that culture and learning and rearing can potently override whatever little bit of testosterone exposure these girls get.

Question: Is it more difficult to raise a boy or a girl?

Lise Eliot: Well, again since my kids are getting older I think my experience lives up to the folklore that boys are harder when they’re younger and girls are harder when they’re older. Sorry, honey. Please don’t watch this. Certainly boys’ higher activity level is a challenge. And their climbing things, boys are certainly more likely to get into accidents and end up with stitches and broken bones because they’re more physically active and they’re bigger risk takers. And I think there is an innate component to that. I think there’s pretty good evidence that girls are more fearful than boys. But on top of that, there’s studies that have shown, both on the playground and in a laboratory situation, that parents are more cautious with daughters than sons. And they will permit the risk taking in boys more than girls.

So it kind of – That’s the case where our parenting sort of reinforces this small difference. So, yes I did find – Well you know, I have two boys and they’re quite different actually. One of them, I won’t say which one, was a little more challenging in the earlier years although they’re both just angels now. And then, you know, I think the issues for girls at puberty are very challenging. In our culture and most cultures, they’re hit with the whole appearance and body image thing in a way that boys are not. So when you hit puberty and you start putting on weight, that’s fine if you’re a guy. In fact, that’s good as you get bigger and stronger and taller. It sort of gains status and increases your self esteem. For girls, we know in this anorexic ideal culture, it’s very challenging for girls.

And it turns out, I talk about depression in the book because this is one of the bigger clinical sex differences. Up to puberty, boys and girls are about equally vulnerable to depression, but after puberty and throughout adulthood, women are about twice as vulnerable to major depression. You might think, it must be the hormones. You know, puberty, estrogen and progesterone start cycling but the evidence there is not very good that estrogen levels are responsible for mood disorders. In fact, stronger correlation comes from one’s body image and self esteem. So self esteem is very much shaped by body image and low body image is one of the biggest risk factors for girls in depression and obviously anorexia and bulimia. So the cultural impact at puberty is still harder on girls.

Question: Are girls more hardwired to have deep friendships?

Lise Eliot: Well, the psychological measure that we would look at is empathy or awareness of other people’s emotion. And if you ask a man or a woman how do you feel if your friend’s puppy dies? Or how do you feel, you know, if your mother is sick? Generally, women will express greater empathy. They will say that it affects them harder. But if you test people – But that’s a subjective measure. When we test subjectively, we see a large sex difference in empathy. Presumably that reflects depth of relationships. But if you test people objectively, show them on a video screen images of faces and ask them to identify is that person expressing fear or anger or sadness, whatever, then you see a much, much smaller gender difference in empathy.

And so, you know, I think women are better at recognizing emotion but the difference is not as large as we culturally reinforce. And so girls are raised, I think, or through their peer groups to put greater emphasis on relationships. Men get there usually when they’re involved in a relationship and certainly when they become parents, the depth of emotion is no different. I site some studies in the book about imaging the parent brain after someone becomes a mother or father and you see much bigger differences between men who’ve never been fathers and fathers. Women who’ve never been mothers and mothers than you do between mothers and fathers. So it’s the experience of parenting that lights off these limbic circuits and helps you respond to infants.

Question: Are women better at reading people’s faces and tone of voice?

Lise Eliot: Right. So the ability to read emotion in another person is a reliable difference. Quantitatively it’s in the small to moderate range so it’s about four-tenths of a standard deviation difference between men and women. Again, if a woman – if you ask a woman, "How do you feel when you describe another person’s emotion?" she will probably report a bigger response than a man will. But if you actually test people objectively, the male-female difference is smaller.

Question: Are boys hardwired more for aggression?

Lise Eliot: Yeah. Physical aggression, probably not surprisingly, is one of the more reliable sex differences. It’s about a half a standard deviation in difference and it shows up pretty early in childhood. But what’s interesting I think, and was surprising to me, is that it doesn’t suddenly change at puberty. I thought, you know, pubertal testosterone, that’s when boys really become physically aggressive. But surprisingly, no. There’s been a lot of research on this by John Archer in England. There’s no sudden change at all. So if testosterone influences aggression, and it probably does, it happens prenatally. In fact, girls who are exposed to higher levels of male hormones before birth because of a particular genetic disorder known as Congenital Adrenal Hyperplasia, these girls have been studied extensively, as you might imagine, to look at the effects of prenatal hormones.

And physical activity and aggression are definitely higher in these girls than in control girls. So there is a reliable difference in physical aggression. Although the interesting thing about physical aggression is that I have this great chart I probably should have put in the book but that boys and girls both decline very dramatically in physical aggression from the toddler years to puberty. Not surprisingly. Every two and three year old kicks and bites and scratches. And both boys and girls learn very well that they’re not supposed to do it. Boys are always do a bit more of that than girls do but they all decline as they’re trained and they learn that physical aggression is not acceptable. Now most women appreciate that there are other forms of aggression.

I think a lot of men appreciate this too when it’s not appropriate to physically aggress, there are plenty of people that aggress verbally. And girls when they hit not even puberty, even before puberty, engage in more, what’s known as relational aggression, where you – It’s sort of the best friend wars of whispering behind each other’s backs and turning friends against each other which is as prominent in girls as physical aggression is in boys. So it’s not really fair to say one sex is more aggressive than the other. One sex is more physically aggressive definitely.

Question: Is the male brain larger than the female brain?

Lise Eliot: Well, overall, if you want a reliable sex difference in the brain, the male brain is about nine percent larger than the female brain and this is true at birth and it’s true throughout life. And, you know, a lot of people want to say it’s a direct link to intelligence differences although IQ differences are – Generally there’s – Most studies find no IQ difference in men and women. But I was really interested in this. I was looking into this nine percent difference. Certainly, the difference in height is about nine percent. Men are about nine inches taller which boils down to about a nine percent difference. But I was trying to find differences in other organs, you know. Our heart, our lungs, our kidneys, and if you go into the scientific literature and you, you know, use the right search engine and you look for sex differences in the brain, you find hundreds of studies.

But if you look for sex differences in the liver, the kidneys of a human, it turns out it’s very difficult to find this data. Nobody’s interested in sex differences in these other organs. I finally was able to find some stuff on the kidney, I think, and the heart. And it’s a very similar difference, about ten to 15 percent, of larger organs in males as compared to females. And so, I don’t know. Maybe men have better hearts and kidneys and lungs than women but I don’t think so. I think it’s just that larger bodies have larger organs, period. Now when it comes to more specific differences in the brain, they’ve got to be there. You know, we know there are reliable behavioral differences and cognitive differences and even some perceptual differences. I think it’s pretty fair to say that neuroscientists have not really put their finger on what the neurobasis for these is.

There was a strong theory based on a couple of studies from the mid-'90s that claimed that when it comes to language and the fact that women are generally a little bit faster or more accurate with verbal sorts of skills, that that could be explained by the use of the two hemispheres. So there was a study from the mid-90s, very high profile, that found that women use both hemispheres for certain language tasks while men tend to use their left hemisphere which is typically the dominant language hemisphere. And I even used to lecture to my medical students on that and the first draft of the book, that was right in there.

But what I’m trying to do is not just rely on single studies but look at what we call meta-analysis which pulls all the data together and this language lateralization difference between males and females has been looked at in about 20 different studies and when you put all the data together, there’s no difference between men and women in the degree of which we use our left hemisphere or both hemispheres in language tasks. So probably because the difference in language skill is so small, you’re just not going to pick it up when you’re, you know, imaging 20 men and 20 women which most of these studies do. The one area that is looking more reliable in terms of understanding sex differences of the adult brain is emotional processing and in particular emotional memory.

A couple of different laboratories have now found stronger activation of the amygdala, in the right amygdala in men, and the left amygdala in women when they are recalling a strong emotional memory. So we’ve seen this by different labs and multiple studies, I think it’s starting to look reliable that men and women are lateralized differently in terms of amygdala activation. And that kind of fits with the verbal spatial division between men and women. If the left hemisphere’s more verbal, women tend to use their left. They may use more verbal strategies for remembering things and males may use more visual spatial strategies in the right hemisphere. So, we’re getting there with that one aspect of cognition but other things remain sort of mysterious.