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Did you know that 15 minutes of stress
are enough to change the sex

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of some regions of the brain?

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From the male form to the female form
or from the female form to the male form?

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I didn't know that either,
but when I discovered this fact

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it transformed the way I was thinking
about sex and brain.

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It all started about four years ago

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when I decided to give a course
on the psychology of gender.

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I stayed home for almost a year
and read books and scientific papers

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on the development of men and women
from the moment of conception.

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As a neuroscientist I was,
of course, interested

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in the relation between sex and brain.

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I soon found out that many scientists,
just like most of us,

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believe that there are
male brains and female brains,

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and that this is the reason
for the fundamental differences

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between men and women.

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According to a very popular version
of this story,

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the female brain
has a large emotion center

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and a large communications center;
it is hard-wired for empathy.

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The male brain, on the other hand,
has a large aggression center,

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and a large sex center;
it is hard-wired for building systems.

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Maybe you thought of something else
but this is a scientific theory after all.

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(Laughter)

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This is a very popular theory or
very popular story, because it gives us

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a very simple explanation
to the world we live in.

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It explains why women are
more sensitive and emotional

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and men more aggressive and sexual.

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Why most teachers are women
and most engineers men.

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Already in the womb, the story continues,
the huge surge of testosterone

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transforms the brain of the male fetus

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from the default female form
to the male form.

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So boys are born with a male brain
and girls are born with a female brain.

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Whereas the notion of
a male brain and a female brain

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fits well the popular view of
men from Mars, women from Venus,

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it does not fit scientific data,
which tell us that men and women

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are remarkably similar.

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In the past 50 years or so,
over 50,000 papers have been published

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on sex differences in psychology.

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I haven't read them all,
even a year is not enough,

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but others have and their conclusion
based on this enormous number of studies

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is that men and women are highly similar

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in almost everything
psychologists can measure.

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Intellectual abilities,
cognitive abilities,

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emotional abilities,
personality characteristics,

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interest and attitudes.

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There are only a few domains in which
consistent sex differences are found.

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For example,

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men are on average, not all of them,
but on average, more aggressive

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than women.
So we call aggression a masculine trait.

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Women are on average
more compassionate than men,

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so we call this the feminine trait.

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But also in these domains
men and women are highly similar,

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and the differences are very small.

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Even more important, we now know
that each of us is a unique mosaic

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of both masculine and
feminine characteristics;

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we are not all masculine
and are not all feminine.

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I guess you already knew that,
not about me, but about yourself,

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that you have a feminine side
and a masculine side.

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I will come back to this image
of ourself as a unique mosaic

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of masculine and feminine characteristics
a little later.

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But now let's go back to the brain.

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Already at the end of the 19th century

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scientists discovered a difference
between the brains of men

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and the brains of women.

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They discovered that the brains of men
were on average larger

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than the brains of women.

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Some scientists took this as evidence
to support and explain

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the then widely held belief
that men are smarter than women.

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One scientist even went as far as stating
that women, because of the small brain,

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do not have the intellectual and academic
skills necessary for academic studies.

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If this logic sounds familiar,
this is not an accident.

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This is the same belief we started with,

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that men and women
are fundamentally different

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because men have male brains
and women have female brains.

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However this is an old version
of the myth, so it sounds absurd.

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Today, when women
not only go to universities,

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but outnumber men
at all level of academic studies,

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it sounds ridiculous
that scientists could have believed

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that women could not go to universities
because their brains

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were on average
smaller than the brains of men.

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Now don't get me wrong, women’s brains

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are still on average smaller
than men brains.

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What has changed is not
the size of the brain;

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what has changed is social norms
and laws that prohibited

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and disencouraged women from study.

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In the more than 100 years
that have passed

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scientists continue
to discover differences

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between the brains of males and females
in both humans and animals.

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For example, compared to men,
women have a thicker cortex,

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a higher proportion of grey matter,
and a lower proportion of white matter.

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Compared to women,
men have larger ventricles.

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These big holes you see in the brain.

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(Laughter)

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I see that some of you
that were very happy to discover

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about the fact
that men have bigger brains than women

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are not really happy with this discovery.

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And I can understand this
because if you believe,

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like the scientists
from the 19th century,

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that the size of the brain matters,

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then indeed you will be embarrassed
to discover that your bigger brains

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come together with the bigger --
what shall we call them? Empty spaces?

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But I want to convey the message
that this is all nonsense;

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men do just fine
with their larger ventricles.

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(Laughter)

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Just as women do just fine
with their smaller brains.

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We now know of hundreds of differences
between the brains of men and women.

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Not just differences in the size
as I've just shown you,

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but also differences
in the microanatomy of the brain

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and I'll show you an example shortly.

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And as more and more differences
have been discovered,

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the belief that there is a male brain
and a female brain

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has become stronger and stronger

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because everyone was taking for granted
the assumption that these differences

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were adding one to the other
as I've depicted in this figure.

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At first this sounded
completely logical to me;

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there are many differences between
the brains of males and females,

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so there must be a male brain
and a female brain.

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But then I read the paper
I told you about in the beginning,

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the one that found that stress
can change the sex of the brain,

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and I realized that this logic was flawed.

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Let's look at this study together.

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In this study the researchers
were interested in the effects of stress

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on a brain region called the hippocampus.

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They measured
the density of dendritic spines.

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You see here a neuron with its dendrite

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and the small red dots
are the dendritic spines.

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And here we can see an actual dendrite

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from a male rat
and a dendrite from a female rat,

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and I added red arrows
so it is easier to detect the spines.

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And we can clearly see a sex difference.

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Males have have fewer spines
compared to females.

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Cherish this moment, this is
the first time you actually get to see

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the sex difference in the brain.

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So we can say dendrites
in this region have a male form

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which is sparse spines
and a female form which is dense spines.

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There was another group of rats
in this study;

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these rats were stressed for 15 minutes;

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as long as this talk, so you can think
what is happening to my brain now.

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(Laughter)

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And here we can see
a dendrite from a stressed male

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and a dendrite from a stressed female.

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Strangely enough,
the dendrite from the stressed male

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has what we have just termed
the female form, that is, lots of spines.

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And the dendrite from the stressed female

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has what we have just termed
the male form, which is few spines.

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So we see that the form of dendrites
in this region depends on sex;

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it is different in males and in females.
But it does not depend only on sex.

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Knowing that the dendrite
you are looking at is from a female

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is not enough
to predict the form of the dendrite.

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whether you have few spines
or lots of spines.

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To predict this you also need to know

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whether this female was under stress
or wasn't.

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So though sex is important it is
interaction of sex and environment,

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in this case stress, that determines
the form of neurons in this region.

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As you may imagine,
I was very surprised by this study.

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So I started to look
for other similar studies.

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It wasn't really hard to find them.

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There were studies reporting
similar effects of stress

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when the stress was experienced in utero,
just after birth and in adulthood.

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There also similar effects
following other manipulations.

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For example, whether the rats were
housed individually or in groups,

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whether they had things to play with
or didn't.

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The different manipulations affected many
brain regions, not just the hippocampus.

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And many features of the brain
such as the size, number of neurons

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and dendritic morphology
as I've just shown.

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What was common to all of these studies

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was the finding that,
whatever the manipulation,

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some features of the brain
changed their sex and some didn't.

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You may be wondering now,
"So what is the meaning

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of talking about
the sex of a brain region,

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if we see that simple manipulations can
reverse what is male and what is female?"

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You are absolutely right,
it is meaningless

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to talk of the male form
and the female form.

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It makes much more sense,
it's much more reasonable

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and rational to use informative terms
such as sparse versus dense,

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high versus low, long versus short.

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But I will continue to use
this male-female terminology,

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because this will help me
make my point clear.

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Let's go back now to the male fetus,

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and to the testosterone surge
that is transforming his brain

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from the female form to the male form,
and not only one component, stress.

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Picture the fetus mother during
the long weeks of pregnancy.

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She sometimes experiences stress,

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not all the time,
but sometimes she experiences stress.

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Whenever she does some features
of her fetus brain change their sex.

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So when her boy is born,
his brain is a mosaic

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of both male and female characteristics.

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This mosaic is uniquely his.

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Molded by the complex interactions
of his hormones

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with the environment 
he has been living in up until now.

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The same is true for the female fetus.
Her brain is also molded

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by complex interactions
of hormones and environment,

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so that the little girl is also born
with the mosaic or brain mosaic

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composed of female
and male characteristics.

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So we see that we are already born

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with the brain
which is neither male nor female.

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It is intersex that is a mixture of both
male and female characteristics.

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This, or our, intersex brain will continue
to change throughout our life

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as a result of our unique experiences.

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We can now add to the image of ourselves
as a unique mosaic

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of masculine and feminine characteristics,

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our image of our brain
being a unique mosaic

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of male and female characteristics.

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I will close with this.

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Many people believe that
there are male brains and female brains,

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because this belief gives them
an explanation

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to why men and women are so different,

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and why they should behave differently
and be treated differently.

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I have shown you today
that it is meaningless

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to talk of the sex of the brain.

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Brains do not have sex.

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If you must relate to the brain's sex, 
you can call it intersex,

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a mixture of male
and female characteristics.

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There are no male brains
and no female brains.

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Therefore, their existence cannot explain
fundamental differences

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between men and women.

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Which is not really a big problem
considering the fact

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that men and women 
are remarkably similar.

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Be yourself.
(Laughter)

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Thank you.

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(Applause)