0:00:20.519,0:00:27.041
Did you know that 15 minutes of stress[br]are enough to change the sex

0:00:27.041,0:00:29.362
of some regions of the brain?

0:00:29.362,0:00:36.911
From the male form to the female form[br]or from the female form to the male form?

0:00:36.911,0:00:40.471
I didn't know that either,[br]but when I discovered this fact

0:00:40.471,0:00:46.606
it transformed the way I was thinking[br]about sex and brain.

0:00:46.606,0:00:48.692
It all started about four years ago

0:00:48.701,0:00:52.243
when I decided to give a course[br]on the psychology of gender.

0:00:52.243,0:00:56.583
I stayed home for almost a year[br]and read books and scientific papers

0:00:56.583,0:01:01.253
on the development of men and women[br]from the moment of conception.

0:01:01.253,0:01:04.068
As a neuroscientist I was,[br]of course, interested

0:01:04.068,0:01:06.981
in the relation between sex and brain.

0:01:06.981,0:01:11.474
I soon found out that many scientists,[br]just like most of us,

0:01:11.493,0:01:14.813
believe that there are[br]male brains and female brains,

0:01:14.813,0:01:18.160
and that this is the reason[br]for the fundamental differences

0:01:18.160,0:01:20.630
between men and women.

0:01:20.639,0:01:23.509
According to a very popular version[br]of this story,

0:01:23.518,0:01:26.338
the female brain[br]has a large emotion center

0:01:26.338,0:01:31.506
and a large communications center;[br]it is hard-wired for empathy.

0:01:31.506,0:01:34.985
The male brain, on the other hand,[br]has a large aggression center,

0:01:34.985,0:01:41.784
and a large sex center;[br]it is hard-wired for building systems.

0:01:41.784,0:01:47.561
Maybe you thought of something else[br]but this is a scientific theory after all.

0:01:47.561,0:01:49.879
(Laughter)

0:01:49.890,0:01:53.962
This is a very popular theory or[br]very popular story, because it gives us

0:01:53.982,0:01:57.490
a very simple explanation[br]to the world we live in.

0:01:57.494,0:02:01.507
It explains why women are[br]more sensitive and emotional

0:02:01.507,0:02:04.090
and men more aggressive and sexual.

0:02:04.098,0:02:09.516
Why most teachers are women[br]and most engineers men.

0:02:09.516,0:02:15.026
Already in the womb, the story continues,[br]the huge surge of testosterone

0:02:15.026,0:02:18.400
transforms the brain of the male fetus

0:02:18.400,0:02:21.972
from the default female form[br]to the male form.

0:02:21.972,0:02:29.212
So boys are born with a male brain[br]and girls are born with a female brain.

0:02:29.220,0:02:32.662
Whereas the notion of[br]a male brain and a female brain

0:02:32.662,0:02:36.982
fits well the popular view of[br]men from Mars, women from Venus,

0:02:36.982,0:02:41.746
it does not fit scientific data,[br]which tell us that men and women

0:02:41.746,0:02:44.895
are remarkably similar.

0:02:44.895,0:02:50.928
In the past 50 years or so,[br]over 50,000 papers have been published

0:02:50.928,0:02:54.227
on sex differences in psychology.

0:02:54.227,0:02:56.976
I haven't read them all,[br]even a year is not enough,

0:02:56.994,0:03:02.621
but others have and their conclusion[br]based on this enormous number of studies

0:03:02.621,0:03:05.556
is that men and women are highly similar

0:03:05.556,0:03:08.887
in almost everything[br]psychologists can measure.

0:03:08.887,0:03:11.765
Intellectual abilities,[br]cognitive abilities,

0:03:11.765,0:03:15.121
emotional abilities,[br]personality characteristics,

0:03:15.121,0:03:17.611
interest and attitudes.

0:03:17.616,0:03:23.283
There are only a few domains in which[br]consistent sex differences are found.

0:03:23.283,0:03:24.864
For example,

0:03:24.864,0:03:28.785
men are on average, not all of them,[br]but on average, more aggressive

0:03:28.787,0:03:33.079
than women.[br]So we call aggression a masculine trait.

0:03:33.079,0:03:36.458
Women are on average[br]more compassionate than men,

0:03:36.458,0:03:39.290
so we call this the feminine trait.

0:03:39.302,0:03:46.063
But also in these domains[br]men and women are highly similar,

0:03:46.063,0:03:49.733
and the differences are very small.

0:03:49.733,0:03:56.343
Even more important, we now know[br]that each of us is a unique mosaic

0:03:56.356,0:03:59.915
of both masculine and[br]feminine characteristics;

0:03:59.915,0:04:05.575
we are not all masculine[br]and are not all feminine.

0:04:06.576,0:04:09.310
I guess you already knew that,[br]not about me, but about yourself,

0:04:09.310,0:04:13.930
that you have a feminine side[br]and a masculine side.

0:04:13.941,0:04:17.295
I will come back to this image[br]of ourself as a unique mosaic

0:04:17.300,0:04:20.433
of masculine and feminine characteristics[br]a little later.

0:04:20.442,0:04:23.433
But now let's go back to the brain.

0:04:23.433,0:04:26.123
Already at the end of the 19th century

0:04:26.123,0:04:29.258
scientists discovered a difference[br]between the brains of men

0:04:29.280,0:04:31.280
and the brains of women.

0:04:31.285,0:04:34.957
They discovered that the brains of men[br]were on average larger

0:04:34.984,0:04:37.107
than the brains of women.

0:04:37.107,0:04:41.427
Some scientists took this as evidence[br]to support and explain

0:04:41.444,0:04:46.151
the then widely held belief[br]that men are smarter than women.

0:04:46.151,0:04:51.853
One scientist even went as far as stating[br]that women, because of the small brain,

0:04:51.853,0:04:57.749
do not have the intellectual and academic[br]skills necessary for academic studies.

0:04:57.749,0:05:01.012
If this logic sounds familiar,[br]this is not an accident.

0:05:01.012,0:05:03.754
This is the same belief we started with,

0:05:03.754,0:05:06.503
that men and women[br]are fundamentally different

0:05:06.503,0:05:11.260
because men have male brains[br]and women have female brains.

0:05:11.265,0:05:16.533
However this is an old version[br]of the myth, so it sounds absurd.

0:05:16.533,0:05:19.869
Today, when women[br]not only go to universities,

0:05:19.869,0:05:23.537
but outnumber men[br]at all level of academic studies,

0:05:23.537,0:05:26.901
it sounds ridiculous[br]that scientists could have believed

0:05:26.906,0:05:30.232
that women could not go to universities[br]because their brains

0:05:30.232,0:05:34.091
were on average[br]smaller than the brains of men.

0:05:34.091,0:05:36.144
Now don't get me wrong, women’s brains

0:05:36.144,0:05:38.936
are still on average smaller[br]than men brains.

0:05:38.940,0:05:41.700
What has changed is not[br]the size of the brain;

0:05:41.700,0:05:46.518
what has changed is social norms[br]and laws that prohibited

0:05:46.518,0:05:50.931
and disencouraged women from study.

0:05:51.098,0:05:54.165
In the more than 100 years[br]that have passed

0:05:54.165,0:05:56.851
scientists continue[br]to discover differences

0:05:56.851,0:06:01.701
between the brains of males and females[br]in both humans and animals.

0:06:01.701,0:06:08.873
For example, compared to men,[br]women have a thicker cortex,

0:06:08.877,0:06:15.934
a higher proportion of grey matter,[br]and a lower proportion of white matter.

0:06:15.934,0:06:19.877
Compared to women,[br]men have larger ventricles.

0:06:19.877,0:06:22.575
These big holes you see in the brain.

0:06:22.582,0:06:28.385
(Laughter)

0:06:28.638,0:06:31.307
I see that some of you[br]that were very happy to discover

0:06:31.319,0:06:34.242
about the fact[br]that men have bigger brains than women

0:06:34.256,0:06:38.171
are not really happy with this discovery.

0:06:38.857,0:06:41.471
And I can understand this[br]because if you believe,

0:06:41.471,0:06:43.641
like the scientists[br]from the 19th century,

0:06:43.641,0:06:46.256
that the size of the brain matters,

0:06:46.260,0:06:50.084
then indeed you will be embarrassed[br]to discover that your bigger brains

0:06:50.084,0:06:56.957
come together with the bigger --[br]what shall we call them? Empty spaces?

0:06:56.981,0:07:01.772
But I want to convey the message[br]that this is all nonsense;

0:07:01.778,0:07:05.814
men do just fine[br]with their larger ventricles.

0:07:05.814,0:07:10.195
(Laughter)

0:07:10.195,0:07:15.951
Just as women do just fine[br]with their smaller brains.

0:07:16.896,0:07:22.111
We now know of hundreds of differences[br]between the brains of men and women.

0:07:22.111,0:07:24.861
Not just differences in the size[br]as I've just shown you,

0:07:24.861,0:07:27.304
but also differences[br]in the microanatomy of the brain

0:07:27.310,0:07:29.881
and I'll show you an example shortly.

0:07:29.881,0:07:32.737
And as more and more differences[br]have been discovered,

0:07:32.737,0:07:35.963
the belief that there is a male brain[br]and a female brain

0:07:35.963,0:07:38.577
has become stronger and stronger

0:07:38.577,0:07:43.242
because everyone was taking for granted[br]the assumption that these differences

0:07:43.242,0:07:48.539
were adding one to the other[br]as I've depicted in this figure.

0:07:48.539,0:07:52.803
At first this sounded[br]completely logical to me;

0:07:52.803,0:07:56.386
there are many differences between[br]the brains of males and females,

0:07:56.411,0:08:00.726
so there must be a male brain[br]and a female brain.

0:08:00.726,0:08:04.099
But then I read the paper[br]I told you about in the beginning,

0:08:04.099,0:08:07.336
the one that found that stress[br]can change the sex of the brain,

0:08:07.336,0:08:11.453
and I realized that this logic was flawed.

0:08:11.453,0:08:14.433
Let's look at this study together.

0:08:14.433,0:08:18.561
In this study the researchers[br]were interested in the effects of stress

0:08:18.561,0:08:21.356
on a brain region called the hippocampus.

0:08:21.356,0:08:24.316
They measured[br]the density of dendritic spines.

0:08:24.316,0:08:28.710
You see here a neuron with its dendrite

0:08:28.710,0:08:32.461
and the small red dots[br]are the dendritic spines.

0:08:32.461,0:08:34.917
And here we can see an actual dendrite

0:08:34.917,0:08:38.553
from a male rat[br]and a dendrite from a female rat,

0:08:38.555,0:08:42.369
and I added red arrows[br]so it is easier to detect the spines.

0:08:42.369,0:08:44.873
And we can clearly see a sex difference.

0:08:44.873,0:08:49.204
Males have have fewer spines[br]compared to females.

0:08:49.204,0:08:53.066
Cherish this moment, this is[br]the first time you actually get to see

0:08:53.068,0:08:55.395
the sex difference in the brain.

0:08:55.395,0:08:58.745
So we can say dendrites[br]in this region have a male form

0:08:58.745,0:09:04.191
which is sparse spines[br]and a female form which is dense spines.

0:09:04.195,0:09:06.771
There was another group of rats[br]in this study;

0:09:06.771,0:09:09.497
these rats were stressed for 15 minutes;

0:09:09.497,0:09:13.745
as long as this talk, so you can think[br]what is happening to my brain now.

0:09:13.745,0:09:15.865
(Laughter)

0:09:15.872,0:09:19.408
And here we can see[br]a dendrite from a stressed male

0:09:19.408,0:09:22.316
and a dendrite from a stressed female.

0:09:22.327,0:09:25.753
Strangely enough,[br]the dendrite from the stressed male

0:09:25.766,0:09:31.371
has what we have just termed[br]the female form, that is, lots of spines.

0:09:31.371,0:09:33.632
And the dendrite from the stressed female

0:09:33.632,0:09:39.448
has what we have just termed[br]the male form, which is few spines.

0:09:39.449,0:09:44.681
So we see that the form of dendrites[br]in this region depends on sex;

0:09:44.681,0:09:52.331
it is different in males and in females.[br]But it does not depend only on sex.

0:09:52.331,0:09:56.357
Knowing that the dendrite[br]you are looking at is from a female

0:09:56.357,0:09:59.238
is not enough[br]to predict the form of the dendrite.

0:09:59.238,0:10:02.278
whether you have few spines[br]or lots of spines.

0:10:02.301,0:10:04.973
To predict this you also need to know

0:10:04.973,0:10:09.470
whether this female was under stress[br]or wasn't.

0:10:09.470,0:10:14.564
So though sex is important it is[br]interaction of sex and environment,

0:10:14.564,0:10:21.216
in this case stress, that determines[br]the form of neurons in this region.

0:10:21.216,0:10:24.582
As you may imagine,[br]I was very surprised by this study.

0:10:24.582,0:10:27.601
So I started to look[br]for other similar studies.

0:10:27.601,0:10:29.731
It wasn't really hard to find them.

0:10:29.731,0:10:32.998
There were studies reporting[br]similar effects of stress

0:10:32.998,0:10:38.379
when the stress was experienced in utero,[br]just after birth and in adulthood.

0:10:38.385,0:10:41.762
There also similar effects[br]following other manipulations.

0:10:41.762,0:10:46.942
For example, whether the rats were[br]housed individually or in groups,

0:10:46.942,0:10:50.076
whether they had things to play with[br]or didn't.

0:10:50.089,0:10:54.802
The different manipulations affected many[br]brain regions, not just the hippocampus.

0:10:54.809,0:11:00.211
And many features of the brain[br]such as the size, number of neurons

0:11:00.216,0:11:04.311
and dendritic morphology[br]as I've just shown.

0:11:04.311,0:11:06.947
What was common to all of these studies

0:11:06.956,0:11:10.333
was the finding that,[br]whatever the manipulation,

0:11:10.343,0:11:16.666
some features of the brain[br]changed their sex and some didn't.

0:11:16.666,0:11:20.070
You may be wondering now,[br]"So what is the meaning

0:11:20.070,0:11:22.744
of talking about[br]the sex of a brain region,

0:11:22.748,0:11:28.099
if we see that simple manipulations can[br]reverse what is male and what is female?"

0:11:28.099,0:11:31.513
You are absolutely right,[br]it is meaningless

0:11:31.526,0:11:34.631
to talk of the male form[br]and the female form.

0:11:34.639,0:11:37.146
It makes much more sense,[br]it's much more reasonable

0:11:37.146,0:11:42.299
and rational to use informative terms[br]such as sparse versus dense,

0:11:42.299,0:11:45.829
high versus low, long versus short.

0:11:45.830,0:11:49.265
But I will continue to use[br]this male-female terminology,

0:11:49.265,0:11:53.330
because this will help me[br]make my point clear.

0:11:53.330,0:11:56.458
Let's go back now to the male fetus,

0:11:56.458,0:12:00.136
and to the testosterone surge[br]that is transforming his brain

0:12:00.136,0:12:06.883
from the female form to the male form,[br]and not only one component, stress.

0:12:06.883,0:12:11.862
Picture the fetus mother during[br]the long weeks of pregnancy.

0:12:11.866,0:12:14.030
She sometimes experiences stress,

0:12:14.030,0:12:17.764
not all the time,[br]but sometimes she experiences stress.

0:12:17.764,0:12:25.125
Whenever she does some features[br]of her fetus brain change their sex.

0:12:25.125,0:12:28.691
So when her boy is born,[br]his brain is a mosaic

0:12:28.703,0:12:32.267
of both male and female characteristics.

0:12:32.279,0:12:34.913
This mosaic is uniquely his.

0:12:34.913,0:12:38.390
Molded by the complex interactions[br]of his hormones

0:12:38.406,0:12:42.948
with the environment [br]he has been living in up until now.

0:12:42.948,0:12:46.987
The same is true for the female fetus.[br]Her brain is also molded

0:12:46.994,0:12:50.607
by complex interactions[br]of hormones and environment,

0:12:50.613,0:12:55.689
so that the little girl is also born[br]with the mosaic or brain mosaic

0:12:55.689,0:12:59.792
composed of female[br]and male characteristics.

0:12:59.792,0:13:01.598
So we see that we are already born

0:13:01.598,0:13:05.864
with the brain[br]which is neither male nor female.

0:13:05.864,0:13:12.791
It is intersex that is a mixture of both[br]male and female characteristics.

0:13:12.791,0:13:18.413
This, or our, intersex brain will continue[br]to change throughout our life

0:13:18.413,0:13:22.600
as a result of our unique experiences.

0:13:22.600,0:13:26.759
We can now add to the image of ourselves[br]as a unique mosaic

0:13:26.759,0:13:29.201
of masculine and feminine characteristics,

0:13:29.201,0:13:32.975
our image of our brain[br]being a unique mosaic

0:13:32.985,0:13:36.988
of male and female characteristics.

0:13:36.988,0:13:39.380
I will close with this.

0:13:39.380,0:13:43.930
Many people believe that[br]there are male brains and female brains,

0:13:43.930,0:13:47.259
because this belief gives them[br]an explanation

0:13:47.259,0:13:49.824
to why men and women are so different,

0:13:49.830,0:13:54.510
and why they should behave differently[br]and be treated differently.

0:13:54.510,0:13:57.032
I have shown you today[br]that it is meaningless

0:13:57.032,0:13:59.417
to talk of the sex of the brain.

0:13:59.417,0:14:02.678
Brains do not have sex.

0:14:02.702,0:14:06.527
If you must relate to the brain's sex, [br]you can call it intersex,

0:14:06.527,0:14:09.550
a mixture of male[br]and female characteristics.

0:14:09.550,0:14:13.725
There are no male brains[br]and no female brains.

0:14:13.725,0:14:18.482
Therefore, their existence cannot explain[br]fundamental differences

0:14:18.482,0:14:20.773
between men and women.

0:14:20.773,0:14:23.828
Which is not really a big problem[br]considering the fact

0:14:23.853,0:14:29.259
that men and women [br]are remarkably similar.

0:14:29.259,0:14:32.049
Be yourself.[br](Laughter)

0:14:32.049,0:14:32.920
Thank you.

0:14:32.920,0:14:41.279
(Applause)