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So it all came to life

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in a dark bar in Madrid.

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I encountered my colleague
from McGill, Michael Meaney.

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And we were drinking a few beers,

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and like scientists do,

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he told me about his work.

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And he told me that he is interested

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in how mother rats

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lick their pups

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after they were born.

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And I was sitting there and saying,

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this is where my tax dollars are wasted,

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on this kind of soft science.

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And started telling me

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that when the rats, like humans,

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lick their pups in very different ways.

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Some mothers do a lot of that,

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some mothers do very little,

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and most are in between.

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But what's interesting about it

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is when he follows these pups
when they become adults,

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like years in human life,

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long after their mother died,

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they are completely different animals.

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The animals that were licked
and groomed heavily,

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the high licking and grooming,

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are not stressed.

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They have different sexual behavior.

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They have a different way of living

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than those that were not treated

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as intensively by their mothers.

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So then I was thinking to myself,

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is this magic?

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How does this work?

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Us geneticists would like you to think

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perhaps the mother had
the bad mother gene

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that caused her pups to be stressful,

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and then it was passed
from generation to generation.

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It's all determined by genetics.

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Or is it possible that something else

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is going on?

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So in rats we can ask this question

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and answer it.

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So what we did is
a cross-fostering experiment.

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You essentially separate the litter,

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the babies of this rat, at birth,

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to two kinds of fostering mothers,

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not the real mothers,

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but mothers that will take care of them,

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high-licking mothers
and low-licking mothers.

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

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And the remarkable answer was,

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it wasn't important

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what the gene you got from your mother.

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It was not the biological mother
that defined this property

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of these rats.

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It is the mother that
took care of the pups.

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So how can this work?

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I am an a epigeneticist.

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I am interested

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in how genes are marked by a chemical mark

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

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and decide which gene will be expressed

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in what tissue.

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

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And we thought, is it possible

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that the mother is somehow

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reprogramming the gene of her offspring

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through her behavior?

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And we spent 10 years,

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and we found that there is
a cascade of biochemical events

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

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is translated to biochemical signals

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that go into the nucleus and into the DNA

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and program it differently.

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So now the animal can
prepare itself for life.

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Is life going to be harsh?

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Is there going to be a lot of food?

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

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or will I live in an upper
class neighborhood

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where all I have to do
is behave well and proper

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and that will gain me social acceptance?

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

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for our lives.

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We inherit our DNA from our ancestors.

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The DNA is old.

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It evolved during evolution.

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

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

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

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all year round.

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And that has such an enormous amount
on our physiology.

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

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those signals that come through the mother

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tell the child

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

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

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

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

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

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If there's no food around,

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

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

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So this is good.

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Evolution has selected this

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to allow our fixed, old DNA

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to function in a dynamic way

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in new environments.

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But sometimes things can go wrong.

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

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

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

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But now we humans
and our brain have evolved,

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have changed evolution even faster.

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Now you can buy a McDonald's

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for one dollar.

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And therefore, the preparation

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that we had

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by our mothers

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is turning to be maladaptive.

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The same preparation that was
supposed to protect us

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from hunger and famine

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is going to cause obesity,

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cardiovascular problems,
and metabolic disease.

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

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and especially the early life experience,

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can provide us a unifying explanation

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of both health and disease.

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But is true only for rats?

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The problem is, we cannot
test this in humans,

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because ethically, we cannot
administer child adversity

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in a random way.

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So if a poor child develops
a certain property,

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we don't know whether this is caused

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by poverty

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or whether poor people have bad genes.

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

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because their genes make them poor.

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Epigeneticists will tell you

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poor people are in a bad environment
or an impoverished environment

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that creates that phenotype,
that property.

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So we moved to look

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into our cousins, the monkeys.

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My colleague Steven Soomey