So it all came to life

in a dark bar in Madrid.

I encountered my colleague
from McGill, Michael Meaney.

And we were drinking a few beers,

and like scientists do,

he told me about his work.

And he told me that he is interested

in how mother rats

lick their pups

after they were born.

And I was sitting there and saying,

this is where my tax dollars are wasted,

on this kind of soft science.

And started telling me

that when the rats, like humans,

lick their pups in very different ways.

Some mothers do a lot of that,

some mothers do very little,

and most are in between.

But what's interesting about it

is when he follows these pups
when they become adults,

like years in human life,

long after their mother died,

they are completely different animals.

The animals that were licked
and groomed heavily,

the high licking and grooming,

are not stressed.

They have different sexual behavior.

They have a different way of living

than those that were not treated

as intensively by their mothers.

So then I was thinking to myself,

is this magic?

How does this work?

Us geneticists would like you to think

perhaps the mother had
the bad mother gene

that caused her pups to be stressful,

and then it was passed
from generation to generation.

It's all determined by genetics.

Or is it possible that something else

is going on?

So in rats we can ask this question

and answer it.

So what we did is
a cross-fostering experiment.

You essentially separate the litter,

the babies of this rat, at birth,

to two kinds of fostering mothers,

not the real mothers,

but mothers that will take care of them,

high-licking mothers
and low-licking mothers.

And you can do the opposite
with the low-licking pups.

And the remarkable answer was,

it wasn't important

what the gene you got from your mother.

It was not the biological mother
that defined this property

of these rats.

It is the mother that
took care of the pups.

So how can this work?

I am an a epigeneticist.

I am interested

in how genes are marked by a chemical mark

during embryogenesis, during the time
we're in the womb of our mothers,

and decide which gene will be expressed

in what tissue.

Different genes are expressed in the brain
than in the liver and the eye.

And we thought, is it possible

that the mother is somehow

reprogramming the gene of her offspring

through her behavior?

And we spent 10 years,

and we found that there is
a cascade of biochemical events

by which the licking and grooming
of the mother, the care of the mother

is translated to biochemical signals

that go into the nucleus and into the DNA

and program it differently.

So now the animal can
prepare itself for life.

Is life going to be harsh?

Is there going to be a lot of food?

Are there going to be a lot
of cats and snakes around,

or will I live in an upper
class neighborhood

where all I have to do
is behave well and proper

and that will gain me social acceptance?

And now one can think about
how important that process can be

for our lives.

We inherit our DNA from our ancestors.

The DNA is old.

It evolved during evolution.

But it doesn't tell us if you are going
to be born in Stockholm,

where the days are long in the summer
and short in the winter,

or in Ecuador, where equal number
of hours for day and night

all year round.

And that has such an enormous amount
on our physiology.

So what we suggest is perhaps
what happens early in life,

those signals that come through the mother

tell the child

what kind of social world
you're going to be living in.

It will be harsh, and you'd better
be anxious and be stressful,

or it's going to be an easy world,
and you have to be different.

Is it going to be a world
with a lot of light or little light?

Is it going to be a world
with a lot of food or little food?

If there's no food around,

you'd better develop your brain
to binge whenever you see a meal,

or store every piece of food
that you have as fat.

So this is good.

Evolution has selected this

to allow our fixed, old DNA

to function in a dynamic way

in new environments.

But sometimes things can go wrong.

For example, if you're born
to a poor family,

and the signals are, "You'd better binge.

You'd better eat every piece of food
you're going to encounter."

But now we humans
and our brain have evolved,

have changed evolution even faster.

Now you can buy a McDonald's

for one dollar.

And therefore, the preparation

that we had

by our mothers

is turning to be maladaptive.

The same preparation that was
supposed to protect us

from hunger and famine

is going to cause obesity,

cardiovascular problems,
and metabolic disease.

So this concept that genes
could be marked by our experience,

and especially the early life experience,

can provide us a unifying explanation

of both health and disease.

But is true only for rats?

The problem is, we cannot
test this in humans,

because ethically, we cannot
administer child adversity

in a random way.

So if a poor child develops
a certain property,

we don't know whether this is caused

by poverty

or whether poor people have bad genes.

So geneticists will try to tell you
that poor people are poor

because their genes make them poor.

Epigeneticists will tell you

poor people are in a bad environment
or an impoverished environment

that creates that phenotype,
that property.

So we moved to look

into our cousins, the monkeys.

My colleague Steven Soomey