Did you know that 15 minutes of stress are enough to change the sex
of some regions of the brain?
From the male form to the female form or from the female form to the male form?
I didn't know that either, but when I discovered this fact
it transformed the way I was thinking about sex and brain.
It all started about four years ago
when I decided to give a course on the psychology of gender.
I stayed home for almost a year and read books and scientific papers
on the development of men and women from the moment of conception.
As a neuroscientist I was, of course, interested
in the relation between sex and brain.
I soon found out that many scientists, just like most of us,
believe that there are male brains and female brains,
and that this is the reason for the fundamental differences
between men and women.
According to a very popular version of this story,
the female brain has a large emotion center
and a large communications center; it is hard-wired for empathy.
The male brain, on the other hand, has a large aggression center,
and a large sex center; it is hard-wired for building systems.
Maybe you thought of something else but this is a scientific theory after all.
(Laughter)
This is a very popular theory or very popular story, because it gives us
a very simple explanation to the world we live in.
It explains why women are more sensitive and emotional
and men more aggressive and sexual.
Why most teachers are women and most engineers men.
Already in the womb, the story continues, the huge surge of testosterone
transformed the brain of the male fetus
from the default female form to the male form.
So boys are born with a male brain and girls are born with a female brain.
Whereas the notion of a male brain and a female brain
fits well the popular view of men from Mars, women from Venus,
it does not fit scientific data, which tell us that men and women
are remarkably similar.
In the past 50 years or so, over 50,000 papers have been published
on sex differences in psychology.
I haven't read them all, even a year is not enough,
but others have and their conclusion based on this enormous number of studies
is that men and women are highly similar
in almost everything psychologists can measure.
Intellectual abilities, cognitive abilities,
emotional abilities, personality characteristics,
interest and attitudes.
There are only a few domains in which consistent sex differences are found.
For example,
men are on average, not all of them, but on average, more aggressive
than women. So we call aggression a masculine trait.
Women are on average more compassionate than men,
so we call this the feminine trait.
But also in these domains men and women are highly similar,
and the differences are very small.
Even more important, we now know that each of us is a unique mosaic
of both masculine and feminine characteristics;
we are not all masculine and are not all feminine.
I guess you already knew that, not about me, but about yourself,
that you have a feminine side and a masculine side.
I will come back to this image of ourself as a unique mosaic
of masculine and feminine characteristics a little later.
But now let's go back to the brain.
Already at the end of the 19th century
scientists discovered a difference between the brains of men
and the brains of women.
They discovered that the brains of men were on average larger
than the brains of women.
Some scientists took this as evidence to support and explain
the then widely held belief that men are smarter than women.
One scientist even went as far as stating that women, because of the small brain,
do not have the intellectual and academic skills necessary for academic studies.
If this logic sounds familiar, this is not an accident.
This is the same belief we started with,
that men and women are fundamentally different
because men have male brains and women have female brains.
However this is an old version of the myth, so it sounds absurd.
Today, when women not only go to universities,
but outnumber men at all level of academic studies,
it sounds ridiculous that scientists could have believed
that women could not go to universities because their brains
were on average smaller than the brains of men.
Now don't get me wrong, women’s brains
are still on average smaller than men brains.
What has changed is not the size of the brain;
what has changed is social norms and laws that prohibited
and disencouraged women from study.
In the more than 100 years that have passed
scientists continue to discover differences
between the brains of males and females in both humans and animals.
For example, compared to men, women have a thicker cortex,
a higher proportion of grey matter, and a lower proportion of white matter.
Compared to women, men have larger ventricles.
These big holes you see in the brain.
(Laughter)
I see that some of you that were very happy to discover
about the fact that men have bigger brains than women
are not really happy with this discovery.
And I can understand this because if you believe,
like the scientists from the 19th century,
that the size of the brain matters,
then indeed you will be embarrassed to discover that your bigger brains
come together with the bigger -- what shall we call them? Empty spaces?
But I want to convey the message that this is all nonsense;
men do just fine with their larger ventricles.
(Laughter)
Just as women do just fine with their smaller brains.
We now know of hundreds of differences between the brains of men and women.
Not just differences in the size as I've just shown you,
but also differences in the microanatomy of the brain
and I'll show you an example shortly.
And as more and more differences have been discovered,
the belief that there is a male brain and a female brain
has become stronger and stronger
because everyone was taking for granted the assumption that these differences
were adding one to the other as I've depicted in this figure.
At first this sounded completely logical to me;
there are many differences between the brains of males and females,
so there must be a male brain and a female brain.
But then I read the paper I told you about in the beginning,
the one that found that stress can change the sex of the brain,
and I realized that this logic was flawed.
Let's look at this study together.
In this study the researchers were interested in the effects of stress
on a brain region called the hippocampus.
They measured the density of dendritic spines.
You see here a neuron with its dendrite
and the small red dots are the dendritic spines.
And here we can see an actual dendrite
from a male rat and a dendrite from a female rat,
and I added red arrows so it is easier to detect the spines.
And we can clearly see a sex difference.
Males have have fewer spines compared to females.
Cherish this moment, this is the first time you actually get to see
the sex difference in the brain.
So we can say dendrites in this region have a male form
which is sparse spines and a female form which is dense spines.
There was another group of rats in this study;
these rats were stressed for 15 minutes;
as long as this talk, so you can think what is happening to my brain now.
(Laughter)
And here we can see a dendrite from a stressed male
and a dendrite from a stressed female.
Strangely enough, the dendrite from the stressed male
has what we have just termed the female form, that is, lots of spines.
And the dendrite from the stressed female
has what we have just termed the male form, which is few spines.
So we see that the form of dendrites in this region depends on sex;
it is different in males and in females. But it does not depend only on sex.
Knowing that the dendrite you are looking at is from a female
is not enough to predict the form of the dendrite.
whether you have few spines or lots of spines.
To predict this you also need to know
whether this female was under stress or wasn't.
So though sex is important it is interaction of sex and environment,
in this case stress, that determines the form of neurons in this region.
As you may imagine, I was very surprised by this study.
So I started to look for other similar studies.
It wasn't really hard to find them.
There were studies reporting similar effects of stress
when the stress was experienced in uterus, just after birth and in adulthood.
There also similar effects following other manipulations.
For example, whether the rats were housed individually or in groups,
whether they had things to play with or didn't.
The different manipulations affected many brain regions, not just the hippocampus.
And many features of the brain such as the size, number of neurons
and dendritic morphology as I've just shown.
What was common to all of these studies
was the finding that, whatever the manipulation,
some features of the brain changed their sex and some didn't.
You may be wondering now, "So what is the meaning
of talking about the sex of a brain region,
if we see that simple manipulations can reverse what is male and what is female?"
You are absolutely right, it is meaningless
to talk of the male form and the female form.
It makes much more sense, it's much more reasonable
and rational to use informative terms such as sparse versus dense,
high versus low, long versus short.
But I will continue to use this male-female terminology,
because this will help me make my point clear.
Let's go back now to the male fetus,
and to the testosterone surge that is transforming his brain
from the female form to the male form, and not only one component, stress.
Picture the fetus mother during the long weeks of pregnancy.
She sometimes experiences stress,
not all the time, but sometimes she experiences stress.
Whenever she does some features of her fetus brain change their sex.
So when her boy is born, his brain is a mosaic
of both male and female characteristics.
This mosaic is uniquely his.
Molded by the complex interactions of his hormones
with the environment he has been living in up until now.
The same is true for the female fetus. Her brain is also molded
by complex interactions of hormones and environment,
so that the little girl is also born with the mosaic or brain mosaic
composed of female and male characteristics.
So we see that we are already born
with the brain which is neither male nor female.
It is intersex that is a mixture of both male and female characteristics.
This, or our, intersex brain will continue to change throughout our life
as a result of our unique experiences.
We can now add to the image of ourselves as a unique mosaic
of masculine and feminine characteristics,
our image of our brain being a unique mosaic
of male and female characteristics.
I will close with this.
Many people believe that there are male brains and female brains,
because this belief gives them an explanation
to why men and women are so different,
and why they should behave differently and be treated differently.
I have shown you today that it is meaningless
to talk of the sex of the brain.
Brains do not have sex.
If you must relate to the brain's sex, you can call it intersex,
a mixture of male and female characteristics.
There are no male brains and no female brains.
Therefore, their existence cannot explain fundamental differences
between men and women.
Which is not really a big problem considering the fact
that men and women are remarkably similar.
Be yourself. (Laughter)
Thank you.
(Applause)