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

transforms 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 utero,
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)