Let's talk about twins.

Twins turn heads wherever they go.

Seeing two identical infants,
children or adults is irresistible,

causing us to peer into baby carriages,

stare into playgrounds,

and even ask rather personal questions,

such as, "Who is older?",

"Who is smarter?",

and, "Who decides
what both of you will do?",

as if everything twins do
is exactly the same.

Society tells us that we all differ
in appearance and behavior.

So when we encounter two people
who look and act so much alike,

it challenges our belief
in the way that the world works.

But we find ourselves intrigued
and drawn into twins' lives,

trying to understand them.

For most of human history,

psychologists believed that we are
largely products of our environment.

But twin research is teaching us

that so many more of our behaviors
than we ever would have imagined

are influenced by the genes.

There are two kinds of twins,
identical and fraternal,

and both are essential in twin research.

Identical twins result
when a single fertilized egg

divides within the first
fourteen days after conception,

and these twins share
all their genes in common.

Fraternal twins share
half their genes, on average,

just like ordinary brothers and sisters,

and they result when a woman
releases two eggs at the same time

that are separately fertilized
by two separate sperm.

We can compare the similarity
of identical twins in running speed,

or in how fast they solve math problems,

to the similarity of fraternal twins.

And if identical twins are more alike,

and they usually are,

this tells us that the genes
play an important role.

Now, most studies use
identical twins raised together,

but studying the rare pairs
of identical twins reared apart

is even better.

Because if identical twins
raised apart are as alike

as identical twins raised together,

this is even more compelling evidence

that genes are important
in our development.

Think about the identical Jim Twins,
Jim Lewis and Jim Springer,

who grew up in different Ohio cities.

They didn't meet
until they were nearly forty.

And they discovered that both twins
bit their fingernails down to the nub,

they both drove light blue Chevrolets,

they both had mixed headache syndromes
beginning in their teenage years,

and they both liked to vacation

on the same three-block
strip of beach in Florida.

The Jim Twins also both
named their sons James Alan.

Now, James is a fairly common first name,

but Alan is a much less
common first or second name.

Both of the twins had worked
part-time in sheriff's offices,

and part-time at McDonald's.

And they loved to scatter love letters
around the house for their wives.

And in a curious twist,

both twins had married women named Linda,

divorced them and married
women named Betty,

(Laughter)

but then one of the Jim twins
divorced Betty and married Sandy.

We know that divorce
is a partly genetically influenced trait,

so you can imagine the worry
on the part of the remaining Betty.

(Laughter)

Now, what about the twins Jack and Oscar?

They had dramatically
different environments,

but even that did not prevent them
from amassing a whole list of similarities

that surpassed
even those of the Jim Twins.

Jack was raised Jewish
by his father in Trinidad,

and Oscar was raised Catholic
by his grandmother in Nazi Germany.

And when they met for the first time,

they discovered that both twins liked
to wear rubber bands around their wrists,

they thought it was funny
to sneeze loudly in elevators,

they both washed their hands
before and after using the toilet,

and they both read books
from back to front,

and they hated floral centerpieces,

because they said it blocked the view
of the person seated across the table.

How do we explain these similarities?

Well, it could be that reading books
from back to front suggests impatience,

or sneezing loudly in elevators
is a cry for attention,

but regardless,

the fact that we see these similarities
repeated in identical twins

so much more than fraternal twins

tells us that genes do play a role.

Dorothy and Bridget,
reared-apart twins in Great Britain.

They show us that you don't have
to live with somebody to be like them,

you only have to share their genes.

When they met
for the first time as adults,

both twins were wearing seven rings,
three bracelets and a watch.

And the similarities did not stop there.

The twins described themselves
as short-tempered, disciplined and strict.

They liked the same brand of perfume,
they liked the same kinds of books,

They had worn nearly
identical dresses to their weddings,

and carried nearly
identical floral bouquets.

(Laughter)

And they remind me of the identical
reared-apart twins, Mark and Jerry.

Mark and Jerry grew up
in different cities in New Jersey.

And they didn't meet
until they were in their early thirties.

They were both volunteer firefighters,

but their signature quirk was
holding a can of Budweiser beer

with their pinkie finger underneath.

And you can see that they also
carried huge key rings

that they attached to their belts.

Belts had huge buckles on them.

They both threw their heads back
sharply when they laughed.

They were six foot four,
bald and heavily mustached,

and when I took them out to dinner,

I discovered they had
a shared passion for Chinese food

and for steaks served extremely rare.

And I also studied Barbara
and Daphne, the Giggle Twins.

My colleagues and I
affectionately called them that,

because when they met for the first time,

they discovered that they laughed
uncontrollably with each other,

and with nobody else.

And they had the same
crooked pinkie fingers,

the same disinterest in politics,

and they drank their coffee cold,
black and without sugar.

These twins had had a first
miscarriage in their first pregnancy,

followed by two healthy boys
and a daughter.

That may not be so surprising,

because female physiology
may impact the sex of our children,

and in this case, the physiology
was perfectly matched.

Shortly after they met,
they concocted a drink called Twin Sin,

that was made of vodka, blue curaçao,
crème de cacao, and cream.

My colleagues and I
thought it was a little curious,

but the twins thought
it was just delicious.

I finally want to mention

two sets of identical twins, males,
born in Colombia, South America.

One pair from the city,
one pair from the country.

We don't know how this happened,

but early on, in the premature nursery,

one newborn twin
was accidentally exchanged

with one newborn twin in the other pair.

So these two sets of brothers grew up
thinking they were fraternal twins,

when in fact, they were
completely genetically unrelated.

When they were twenty-five,
the truth was discovered,

and the real pairs were reunited.

I went down to Bogotá to study them,

and I discovered that the personalities
of the reunited twins

aligned almost perfectly.

In one case,

the twins were outgoing,
gregarious, risk-taking,

and in the other case,

they were introverted,
a little cautious, a little restrained.

Again, we don't fully understand
the reasons behind these similarities,

but seeing them repeated
in identical twins,

more so than in fraternal twins,

gives us a genetic perspective
on human development.

Now, we need to think seriously
about these findings,

because they have huge implications
for how we raise our children

and how we can all make
the best of our abilities,

our goals, and our dreams.

Now, in my experience,

parents of one child
are environmentalists,

and parents of two children
are geneticists.

And I say that

because parents of two children
realize rather quickly

that what works for one child
doesn't work for the other.

And that's because children
come into the world

with their own genetic predispositions

that lead them towards certain activities
and places and opportunities.

Parents have to have realistic
expectations about each child.

Two dancing parents
may not have a dancing child,

because even though each parent
shares half their genes with the child,

genes get reshuffled in each generation.

You know, I tell parents,

"You don't really bring up
your children, they bring you up."

Because each child evokes
certain treatments and certain responses

from each of their parents.

A child with athletic abilities should
be given opportunities to play sports,

an artistic child should be given
opportunities to paint and to draw,

and a shy child should be given
opportunities to speak up more,

encouraged gently to do so.

In this way,

parents can make a huge difference
in the lives of their children.

And I also want to acknowledge
the teachers and the mentors

and the managers who work
so closely with twins and with others,

and affect their lives.

As a psychology professor,

I identified a very promising
undergraduate student,

one who showed
real ability, talent, and drive.

But he was the first
in his family to go to college,

and he lacked guidance and opportunity.

I worked with this student
on a number of projects,

and today,

he is an advanced graduate student
at the prestigious University of Michigan.

Back to our twins.

Twins are not just
mere objects of fascination.

Just by being themselves,
just by acting naturally,

they give science a powerful tool

for understanding genetic
and environmental influences on behavior.

And in this way,
they tell us about our humanity,

why we are the way that we are,
and how we got that way.

Thank you.

(Applause)