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)