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PROFESSOR: OK, good afternoon.
Today's lecture is going to be on child development.
How they go from infancy and head towards adulthood.
Next Thursday we'll talk about development after infancy and
into adulthood and over age.
And this amazing thing about babies and how do they get
going, and how do scientists discover from babies and young
children how their minds are fundamentally
different than adults.
And how your mind was fundamentally different than
it is as an adult.
And then the last thing will be a dangerous experiment,
where my wife will come over with my two-year-old and my
four-year-old and we'll try to do a developmental
demonstration in front of you.
But I should warn you that live children
have their own will.
So we'll see how that goes, okay?
All right.
So one thing that people are struck by babies is that
babies are cute.
And it's not just--
It's thought to be not just fun, but kind of a really
important biological thing.
We talked about motherese before.
Singsong talk to babies that help them learn a language.
There's something about adults finding babies cute that makes
them want to help babies.
Parents and friends--
The cuteness is thought to be an evolutionary reward for
dealing with a baby.
And people have said, well what is it that's a little bit
different about a baby's face than an adult?
They have rounder heads, they have foreheads that protrude
more towards--
that slope back more.
They have larger eyes proportionately.
Smaller jaw bones proportionally.
And people use the term neotony to describe this kind
of infant proportions, or properties, of faces, which
are universal.
And, of course, a lot of people discover this in
advertising or cartoons.
And so here's an interesting example of how Mickey Mouse
has, over the years, from the original version of Mickey
Mouse to the current super cute, super tested, super
Disney version of Mickey Mouse, has become increasingly
neotonized.
And how much the cartoonists literally know that, or just
have kept saying, what makes cuter, and cuter, and cuter,
and ended up with a lot of the same proportions in Mickey
Mouse's face that make infants so cute and desirable.
We don't really know.
But the world kind of feels this.
Even adult research-- there's a little funny line of
research-- that says adults who look more baby-faced get
treated a little bit different socially.
So here's Leonardo DiCaprio.
And baby-faced adults in experiments are perceived as,
just by their faces, as more naive, more honest, more
helpless, more kind, more warm.
People think, this is going to be a nice warm person.
Guessing from faces they're using the cues of neotony.
Other studies have found that people who are baby-faced are
statistically more likely to be found innocent in cases of
intentional wrongdoing.
So where they're trying to prove if somebody on purpose
tried to do something bad, they go, not
that cute little person.
But there's no difference if it's cases that involve
negligence.
Something where you should have done something.
Because we know babies aren't super responsible
for how they behave.
And they're less likely to receive votes in elections.
So people have done--
I won't even tell you this-- but people have done all kinds
of analyses of faces that win elections in the US
and around the world.
And there's two dimensions that people look for.
What looks like somebody's empathetic and what looks like
somebody's competent.
So you want babies when it's fun, but when you have to
figure out how to balance the budget that doesn't seem like
a baby task.
So it's kind of funny.
And a lot of voters go by instincts as well as by facts
or stated values.
So in the development of infants into adults, the
classic debates about how our mind grasps the world have
played out, yet again, in a powerful way.
You know, the nature-nurture debate.
What's in our genes and gets us going?
What's acquired from the environment?
Philosophers who say that knowledge starts with innate,
or genetic, or more evolved capacities.
Other ones who've emphasized that we pick it up from the
world empirically as measuring scientists.
So, for example, Aristotle wrote, "There's nothing in the
intellect which was not first in the senses." that's the
empirical view.
It's out there in the world and you solve it.
And starting with a blank slate.
And, in fact, the behaviorists believed in this with a
vengeance in psychology.
So Watson wrote, "Give me a dozen healthy infants, well
formed, and my own specified world to bring them up.
And I'll guarantee to take any of them, one at random, and
train him to become any type of specialist I might select.
A doctor, lawyer, artist, merchant, chief, and, yes,
even beggerman and thief.
That it's all the environment and a blank slate.
And who really knows?"
But infants are born with some incredible things, an we'll
talk about them today.
One of them is this experiment from Andy
Meltzoff, which is this.
That it seems like within moments of birth, infants are
equipped with a desire to imitate adults.
A desire to imitate.
And why people have been really impressed by this is,
if you have that in your genes and in your nature, it's a
good way to learn about the world is to imitate people you
see around you.
It's kind of a driving principal.
I'll imitate their language, their social mores.
Here's what Meltzoff did.
Sticks out his tongue, opens his mouth, purses his lips.
And here's the infant, one day or a couple days old, tongue
out, mouth open, lips pursed.
Now, I should warn you about one thing.
These are the greatest hits pieces of that research.
People debate about how perfectly this
happens, all right?
So I was frustrated when my infant came home and I did all
this stuff and she would not instantly do everything she
was supposed to do.
It's a statistical thing.
Because when you see these pictures, they're showing you
the best moments.
They have to defend that it happens above chances
statistically.
But it's not like every expression you
make they copy you.
And that's why there's still some debate about exactly
what's going on.
But they definitely, on average, imitate an adult
within a day or two of birth for completely arbitrary--
as far as we think--
expressions.
So it's a very powerful mechanism for learning that
appears to be in place.
Because there's two ways that we could imagine genetically
programmed learning.
One is that you know the content of it already.
But another way is that you just have stuff
that makes you a learner.
So being an imitator is a general way to make you a
learner without being very specific to the content.
So we talked a little bit about this already, but
psychologists are unbelievably brilliant--
I think, I'm really blown away-- about how they can
figure out when an infant knows and doesn't know, or
young child knows or doesn't know.
And they do this indirectly by things like looking time.
And you've seen this.
I'll give you an example or two.
And you'll see some more examples.
As a measure of what the baby knows, or
the young child knows.
And, of course, if they look longer at something it could
be because they prefer it.
And, also, if it's based on a memory task--
like you saw something, you see it again--
that would tell you there's some sort of discrimination .
They can tell things apart based on memory.
And then, sometimes, people will push this memory thing
with habituation or familiarization, so if you
show something to a baby over, and over, and over again, they
get sort of tired of it.
And at that moment they'll change the stimulus.
And they'll see whether the baby gets newly attentive.
Now, if it's something that the baby doesn't grasp, they
won't get newly attentive.
You know, nothing's changed in their mind.
If it's something the baby understands, then they'll say,
wow, something interesting is happening.
And they'll look longer.
So, here's an example.
Baby getting bored because they're showing something over
and over, and something new, oh!
That's the kind of things that people measure.
And we're doing it loosely here.
They measure it pretty carefully.
I should tell you, the people who do this kind of research--
they'll film it, they'll show it to unbiased people like
you, maybe judges.
OK, to say, how long is the baby looking?
Where's it looking?
You don't know where the baby should be looking at or not.
So researchers take care to make it not just that you're
convinced in your heart of hearts that
they're looking more.
They film it.
They show it to people who don't know what's going on.
They try to make it a blind procedure in many ways, the
better research.
So how do we know things?
Even very young infants look selectively at novel objects.
If something's new, even at one day, if you show them
checkerboards with different size squares, same size, same
size, same size, wait a couple seconds, smaller squares.
They look more.
This shows you they saw the difference in the squares.
They even remember, somehow in their mind, the difference
between the squares.
Because when you switch it they start looking more.
Something interesting has happened based on perception
and memory.
Another thing that we've mentioned--
I'll just remind you-- is that infants, almost from birth,
seem to love to control, or provoke, their environment.
So we already mentioned to you that 20 month olds prefer a
mobile that responds to their bodily movements.
They like things that they control.
If they take a two-month-old, and they put a string on their
wrist by which they can control a showing of Sesame
Street, they kind of like that.
And if you stop Sesame Street, they show anger on their face.
Hey, I was in charge here, who has the remote control now.
And four and five month olds, a little bit older, remain
angry when the video comes back on because that's not
what they wanted.
They wanted to control the presentation of the video.
So infants have this apparent craving to explore and feel
like they have some control over their environment, at an
age when they don't control a lot because of their limited
physical capacity.
In the first three or four months, infants explore the
world a lot by their mouths.
That's why people worry to not leave around things that they
will put into their mouth and swallow, that are dangerous.
Because their visual and auditory systems, and their
system with their hand, are slowly developing.
By five or six months around the world, they start to
explore with their hands and their eyes around them.
As their motor system gets more mature.
As their visual system gains acuity.
And we already know that infants use social cues to
guide exploration.
One of the big changes this happened in the last 10 years,
I'd say, in infant studies or child studies, has been a lot
of appreciation about the intersection between cognition
and social interaction.
There used to be like two different worlds of people who
studied language.
They would have algorithms and formulas about the language.
And then the social people who'd study how babies were
cute or something, right?
But there's been more and more thought that a deep thing in
us that helps cognition grow, is the social relations that
an infant is drawn to parents and caretakers.
So, for example, a six-month-old, if their mother
rolls or pounds a ball, they tend to copy, they tend to
imitate that action.
They follow the eyes of a person.
Joint attention.
You know, eyes are the windows on the soul.
We know eyes mean a lot.
Where are people putting their attention?
What are they paying attention to?
Infants pick up on this very early on.
And if somebody looks over there they look over there.
If you look at them they know your attention
is focused on them.
And sometimes people will say--
because psychologists love to test everything-- is it just
the way the head is turning?
So if you cover somebody's head they
don't follow it anymore.
They know it has to be the eyes reflecting where the mind
is putting its attention.
It's not the head direction.
It's the eyes.
Here's something, infants who are a little bit better at
following eyes--
either maturing faster or something--
also learn language faster.
They get words faster, and syntax faster.
It's as if part of that social interaction is helping to
drive the language learning.
So that's how these social things and language things
interact very substantially.
And we already said that a 12-month-old will stop at a
visual cliff, but if the mother smiles there they go.
So constant learning that mixes in the
cognitive and the social.
Now the epic name in developmental psychology is
Jean Piaget.
Because prior to him, roughly speaking, in the science of
child development, people just thought children were sort of
boring and dumb.
They need a lot of attention.
Please finish MIT so we can talk with you intelligently.
I guess people were not that interested.
Partly is the world was different in so many ways.
There was no-- practically no--
formal psychology experimentation or thought
about what really children's mental lives were like.
And Piaget--
I have a wrong year there, 1886--
developed--
he himself was a child prodigy, trained in biology.
Very early interest in whether categories are in our heads or
in the world.
Tried poetry.
Worked with Binet on intelligence testing.
Became interested in patterns of errors and successes.
You'll see that in many ways, what he discovered about how a
child mind is different than yours fundamentally, is based
on the patterns of their errors.
And he said, there's two basic things going on.
This is a huge idea.
One is assimilation, you incorporate new knowledge into
existing cognitive structures.
It's basically ways you think about the world.
And new knowledge is added.
That's important.
But he thought the dramatic thing that happens in infancy
and childhood is what he called adaptation.
That the fundamental ways in which you understand the world
are shifted, through development, to accommodate
new evidence.
So there's a wonderful book if you come across it called The
Scientist in the Crib.
But it's kind of a pun.
It's written by scientists who study infants.
But the idea is that, in a sense, every infant is kind of
his or her own scientist--
as you were--
probing the world and discovering the physical and
social laws of the world.
Because they don't know them coming in and they're going to
pick up a lot of them.
They're going to probe them.
But fail to publish the results.
So Piaget postulated that were these big stages, what he
called the Sensorimotor age, zero to two, where the world
is one of what you see and the physical actions.
And there's not much representation of ideas or
lasting thoughts.
And we'll see some examples of these.
Between two and seven, you start to be able to keep a
thought in your mind.
You still have an egocentric prospective.
You see the world from your view but not appreciating the
views of somebody else.
You start to use language.
A lot of flimsy discussion about fantasy and reality.
I could tell you with my own daughter, who you'll meet,
she's still working out whether fairies
really exist or not.
This was chaotic because we went to some skating show
where Tinkerbell was a human dancing, and that totally made
the situation very complicated.
She's still working on that.
Concrete, as you get 7 to 11, organize logical thought.
Still concrete, as opposed to sort of abstract.
And then you move to more adult like cognition, which
we'll talk about next week.
So the idea is from birth to two years, you would love to
have an infant just tell you what do I think the world is
really like.
They have very modest language, of course, so they
can't tell you what they see.
So the thought is they have fleeting, disconnected sensory
impressions and motor reactions.
It's all, what do I see?
How am I responding?
That their focus of thought is centered on actions, not
things or intentions.
And I'll show you in a very concrete way.
That there's no distinction in their mind between things that
are stable in the world and transient events.
No distinction between themselves and the world
around them.
It's all just one big mush until they connect abstract
representations that separate these things out.
So let's talk about object permanence.
I'm going to show you that you have object permanence just by
saying, here's my pointer.
I need a volunteer.
This is so easy that you will not feel scared.
I promise.
All right.
Thank you very much.
So you see this?
Where do you think it is now?
OK.
If I was really cool teacher who did
magic it would be gone.
But yes, and you know it's there.
That's object permanence.
If you turn your head you believe it's
probably still there.
But that's going beyond the fundamentals of perception,
that's your knowledge, and representing in your head,
there's something I don't see but I know it's there.
That is not available to a zero to two year old.
So, for example, if they see a rattle they feel a rattle.
But now if they look away, or if you just put a sheet of
paper in front of it--
I'll show you a couple of examples--
they behave as if the rattle no longer exists.
Just like I did in front of you.
If you just cover it up, it's gone as if it doesn't exist.
Because their mind cannot represent what they do not see
right in front of them.
So out of sight out of existence.
So you can show them a toy they love, and until about
eight months, you could do this.
They'll start crying.
They'll protest.
But they won't reach for it, even when it's easily within
their reach.
Out of sight, out of mind.
It doesn't exist.
They can't represent in their mind what they don't see.
And then there's another one.
You'll see a film of this.
But let me tell you this.
Which tells you about actions verses other things.
So imagine you hide a toy under one location, the child
searches there, finds it.
You hide it again in the same location.
You encourage the child to search.
They find it.
They do it two or three times.
Now, in front of the child, you take that same object that
you've shown them like this several times.
They've grabbed several times.
And you simply, in front of them, pick it up
and put it over here.
And they're viewing everything in full view,
just like you did.
What do they do?
When it's their turn to grab they grab here.
The wrong place.
The place where they grab before.
Because what's in their mind is the action they did.
Not the object as an independent thing they can put
in their mind.
Does that makes sense?
And I'll show you several pieces of this.
It's called the A-not-B effect.
I put this A, A, A, A in the front, full view.
Put it over here and they reach over here to get it.
So this is this idea, that in their head is not a toy.
Their head is representing the thing on the right that they
reached for.
Because they don't represent ideas of things.
They represent actions that they've done.
So here's object permanence.
Here's a child sees a toy they like.
You put up, in front of them, this thing and tragically,
it's disappeared.
It's gone out of existence in the world.
Some people have done some sort of NIRS imaging
suggesting that kids who get past this stage are developing
some frontal maturation.
But I think the most compelling things are the
videos of the kids themselves.
So let me show you another film, again very cleverly, the
Renee Baillergon has been one the leaders in this field--
very cleverly showing you what infants grasp and don't grasp.
Not only the brilliance of these experiments.
These infants are really trying to figure out the
world, and coming up with hypotheses that they're
confirming.
And then when they're broken down they're upset with
what's going on.
So let's move on to age two to seven, especially in the early
years, so-called a preoperational stage.
And one of the coolest things, and we'll try this and see how
it goes with some girls in a few minutes, is this idea.
So if you see two glasses filled with equal amounts of
liquid, and you pour one over into the thinner, taller
glass, c, which will have more water in it?
A or C?
The same.
But not if you're a two year old or a four-year-old.
So it's called conservation of liquid.
So, it's kind of fascinating, isn't it?
It's kind of amazing.
If you do this with a three year old it's virtually
guaranteed they'll do this.
All of you did it all over the world.
And, in some sense, as far as we can guess , all throughout
our species' history.
So brilliant ways of showing that babies, or young
children, really see a little bit of a different world than
you and I as adults.
So here's another very powerful area of research,
which is taking perspectives from the view that other
people have thoughts and feelings besides yourself and
how you interact with them.
And people call this idea theory of mind.
The idea that you understand that other people out there
have their own thoughts, their own feelings distinct from
yours, and in the most challenging case, they can
even be false beliefs.
A person can have wrong information in their head and
truly believe it, even when you know it's wrong.
Because you, as an adult, realize that thoughts are one
thing and physical reality is another thing.
But that's not what children think.
They think there's just one reality.
That's why at the very youngest age, they think if
they cover their eyes that you don't know where they are,.
Because they think there's one physical reality only.
And they don't realize there's another prospective
besides their own.
So here's a very clever experiment showing you that at
three months, children code the word in terms of actions.
And by six to nine months, they begin to understand
intentions, that people out there-- animals too but we'll
just focus on people-- have intentions and
goals in their head.
So here's what the experiment they do using this
habituation.
There's two pedestals here, and a person reaches again and
again for a ball.
So now, what do you believe the intention is?
To reach for the ball.
You see it 10 times, the baby's losing interest.
Now comes their shocker to get the baby crying, or laughing,
or drooling.
Get the baby's attention that you can
measure by how they respond.
And in some conditions, something goes down, and when
it goes back up the person reaches.
But now they take the same action.
They reach for the left pedestal, but
it's a teddy bear.
Or another case, they reach for the ball in
the opposite side.
Who, depending on their age, sees these two different
changes in the scenario as startling?
The three month olds are startled when the person
reaches for the same object.
The three month old.
Because in their mind, the only thing that was really
happening was reach for the left, reach for the left,
reach for the left.
And now you're reaching for the right.
The action changed.
Because the action is independent of intention.
It's just what happened, what happened, what happened.
They're shocked when the person goes for
the ball over here.
By six to nine months, they're shocked--
relatively speaking--
when the person reaches for the same action but now the
goal is different.
Grabbing the teddy bear.
It's as if they understand the intention of the person was to
get the ball, and these children think the only thing
that's happening is physical action, physical action,
physical action.
So very clever experiment by this habituation and looking
time to show at three months all you see is actions.
And by six to nine months you start to grasp that people
have intentions in their head above and beyond their actions
that govern their actions.
So to make it really tricky, there's this very clever
experiment that makes--
and we'll try it in a few minutes--
this way.
This so-called Sally-Anne problem because it's been so
widely cited and discussed.
So you tell a child this story.
Imagine there's two girls here, Sally and Anne.
And Sally takes a ball and puts it in this basket.
And then she leaves the room.
Anne, who's a little bit of a troublemaker takes the ball
out of here and puts it into this box, and closes the box.
Here's the big question.
When Sally comes back and looks for this ball, where
will she look?
And a three year old says, in the box.
Now if she was out of the room there's no way for her to look
that's in the box.
But for the three year old, there's not
thoughts and reality.
There's just kind of reality.
Does that make sense?
The three year old can't entertain the idea that Sally
has, in her head, wrong knowledge of
where this ball is.
The she couldn't have known where it is and she's going to
act on the knowledge that's in her head.
That's the content of her mind.
The three year old says there's one reality, the ball
is over here.
I know it.
Everybody knows it.
What else is there to even think about?
But that typical five-year-old will say, Sally will look in
the wrong place.
She'll look in the basket.
She'll look in the wrong place compared to
where the ball is now.
Because she believes it's there.
She has the wrong idea in her head.
That ideas can be independent of physical, observable things
in the world.
This is a universal finding.
I'm going to have to change my settings.
Sorry.
Your handout has this.
In practically every country where they've tested this,
three year olds don't have a theory of mind,
five year olds do.
Turns out--
and you could take a guess--
the older sibling you have, the younger you become at
getting a good theory of mind.
Nobody really knows what the answer but what's your guess?
The more siblings you have that are older
than you, the younger--
by some number of months-- people tend to show that they
can understand that people have all kinds of thoughts and
beliefs in their head.
What's your guess?
Yeah.
AUDIENCE: They have gone through versions of the
Sally-Anne problem themselves.
PROFESSOR: They may have been tortured by their siblings.
But on top of that, the intuition is-- and we don't
really know, this is hard to--
they're living in a world, because they have older
siblings who do unkind things to them, who have superior
powers over them physically, they're much more socially--
have to figure out what's going on to survive happily
and make their way in the world.
The big five year old, he or she is dominant, and the two
year old is trying to steer around.
Like, what is does that five year old think?
How will I not get into trouble with that
five-year-old and my mother and my father?
A younger sibling has more social pressures to figure out
what's going on and steer your way around, and not get in
trouble with your older siblings.
We'll come back to autism.
I'll just say a word about this now.
You hear a lot about it.
The current guess is that it's approximately 1 out of every
140 children.
A shockinging increase in numbers to what people
believed it was many years ago.
Mysteriously, there's many more boys
than girls with autism.
Estimates are 4 to 1, or 10 to 1 range, all there.
It's hard to know for sure.
We'll talk more about that.
Autism is defined by three things.
A sort of weakness, or deficit, in social
interaction.
Children with autism, or adults, have difficulty in
social interaction.
In communication.
And they have lots of repetitive behaviors that can
be difficult to help in interaction with others.
And when people test theory of mind in children with autism,
it's typically delayed by five or six years on average, for
those children who can take the test at all.
They have a very slowly developing, very delayed
theory of mind and understanding what's in the
content of the thoughts of other people.
And that's considered one of their biggest challenges.
How they relate to other people.
Understanding what the other person's thinking and feeling.
Just for a couple minutes I'm going to show you a video and
then we're going to try--
So the biggest challenges to Piaget have been this idea
that there's these distinct stages of thought.
That your mind shifts, literally, how it's
constructed and understands the world.
And that people have thought that the process is much more
continuous than Piaget had suggested.
And here's one compelling example of that kind.
So this is the A-not-B experiment.
There's a two food wells, and something of
interest in this infant.
The seven month old looks at the toy here.
It gets covered up.
It's placed in b, and b is one of the two wells.
He continues to look at b when both are covered.
He knows the toy is over there.
And the toy has been placed over and over again in a.
So he gets the A-not-B error.
He has the action.
And even though he sees the toy is placed there.
It's covered up.
He's looking where the toy is.
When it's time to reach he goes to this well.
So this is A-not-B error.
A part of his mind that guides action is stuck and guides
what he reaches for.
But obviously another part of his mind knows perfectly well
where the toy is.
So it's not as if it's all of one, or all of another.
His mind is representing that there's a toy down there, but
the part of his brain or mind that guides his action is
still stuck at this early developmental stage. it's if
he were two ages.
One guiding where his eyes goes.
And one guiding the ultimate action of
where his hands goes.
And as he gets a little older, his eyes and hands will go in
the same place.
OK, last couple of things about how infants are
mathematicians.
Because, of course, we focused on language, or social
thought, but the ideas of counting and, ultimately,
arithmetic or math also come out of children's minds.
And a very clever experiment--
and I'll show you two of them--
that shows even at six months, infants are starting to
prepare themselves to become mathematicians.
And here's how they begin the counting.
They're shown displays of three objects or two objects.
And then they hear two sounds like boom boom.
And where do they look?
They tend to look at the one with two objects.
So this is pretty impressive.
They're counting the two.
They're relating something in sound to something in sight
that's arbitrarily related, but that shares the
numerosity.
And if they get boom, boom, boom, three boom sounds, then
they go, OK, now I like the display with three things.
They understand, in some level, twoness and threeness.
And so you start to have the basis of having numbers.
And not only that, Karen Wynn at Yale has shown quite
cleverly, not only do infants have numbers.
They can do a little bit of subtraction and addition.
So here's the experiment she does.
She places an object in a case.
The screen comes up.
Now comes the second object.
And the hand leaves.
OK.
The screen drops.
And there are two.
One plus one equals two.
And the infant goes, OK, no big deal.
But if the screen drops and there's one, the infant is
startled and looks longer.
And they can do it the other way around.
They can do subtraction as well.
So one plus one equals two, and if there's one left by the
trickery of the kind you saw earlier, they're disturbed
that the addition has not occurred correctly.
Or the subtraction has not occurred correctly.
And there's kind of a debate-- this is the last slide--
and here's the core idea, though, which is this.
That our minds afford us, by evolution, counting up to
about four objects in the world.
And many animals do that as well.
That sort of has to develop a little bit, but we've at lot
running in us.
So that infants can do.
Because, of course, if you show 7-- if you give 77 booms,
they don't look for the display with 77 versus 78.
They can't do that.
So they can operate in the world of 1 to 3 or 1 to 4.
And then they discovered--
there's been a series of studies in tribes, that are
relatively isolated in different parts of the world,
where in their language they only have words for
one, two, and three.
And everything after that is really described as
many, or a whole lot.
They don't have words for five and up.
There's no word in their language.
Because they've never needed it, apparently, in the worlds
they live in.
So they go to them and they have them do
little counting tasks.
And as soon as they get beyond about four they get very
inaccurate.
It's as if the mind naturally can represent about four
things in the object world, four things in the counting
world-- maybe because of that--
and after that everything is a cultural invention of higher
level mathematics.
It builds off one to four.
But you don't have it unless your culture
teaches it to you.
Or your culture invents it.
And there's been brain imaging studies too, suggesting that
counting one to four is very different in the brain than
exact mathematics beyond the number four or five.
That's all culturally learned and invented.
It's based on an innate one to four capacity.