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Let's talk about twins.

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Twins turn heads wherever they go.

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Seeing two identical infants,[br]children or adults is irresistible,

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causing us to peer into baby carriages,

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stare into playgrounds,

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and even ask rather personal questions,

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such as, "Who is older?",

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"Who is smarter?",

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

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as if everything twins do[br]is exactly the same.

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

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

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it challenges our belief[br]in the way that the world works.

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

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trying to understand them.

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For most of human history,

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psychologists believed that we are[br]largely products of our environment.

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But twin research is teaching us

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

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are influenced by the genes.

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There are two kinds of twins,[br]identical and fraternal,

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and both are essential in twin research.

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Identical twins result[br]when a single fertilized egg

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divides within the first[br]fourteen days after conception,

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and these twins share[br]all their genes in common.

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Fraternal twins share[br]half their genes, on average,

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just like ordinary brothers and sisters,

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

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that are separately fertilized[br]by two separate sperm.

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

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or in how fast they solve math problems,

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to the similarity of fraternal twins.

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And if identical twins are more alike,

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and they usually are,

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this tells us that the genes[br]play an important role.

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Now, most studies use[br]identical twins raised together,

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but studying the rare pairs[br]of identical twins reared apart

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is even better.

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Because if identical twins[br]raised apart are as alike

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as identical twins raised together,

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this is even more compelling evidence [br]

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that genes are important[br]in our development.

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Think about the identical Jim Twins,[br]Jim Lewis and Jim Springer,

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who grew up in different Ohio cities.

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They didn't meet[br]until they were nearly forty.

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

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they both drove light blue Chevrolets,

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

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and they both liked to vacation [br]

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

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The Jim Twins also both[br]named their sons James Alan.

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Now, James is a fairly common first name, [br]

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

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

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and part-time at McDonald's.

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

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And in a curious twist,

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both twins had married women named Linda,

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divorced them and married[br]women named Betty,

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(Laughter)

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

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We know that divorce[br]is a partly genetically influenced trait,

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

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(Laughter)

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Now, what about the twins Jack and Oscar?

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They had dramatically[br]different environments,

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

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that surpassed[br]even those of the Jim Twins.

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Jack was raised Jewish[br]by his father in Trinidad,

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

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And when they met for the first time,

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

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they thought it was funny[br]to sneeze loudly in elevators,

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

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and they both read books[br]from back to front,

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and they hated floral centerpieces,

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

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How do we explain these similarities?

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

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or sneezing loudly in elevators[br]is a cry for attention,

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but regardless,

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the fact that we see these similarities[br]repeated in identical twins

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so much more than fraternal twins

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tells us that genes do play a role.

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

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

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you only have to share their genes.

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When they met[br]for the first time as adults,

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

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And the similarities did not stop there.

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

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They liked the same brand of perfume,[br]they liked the same kinds of books,

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They had worn nearly[br]identical dresses to their weddings,

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and carried nearly[br]identical floral bouquets.

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(Laughter)

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

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Mark and Jerry grew up[br]in different cities in New Jersey.

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

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They were both volunteer firefighters,

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but their signature quirk was[br]holding a can of Budweiser beer

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with their pinkie finger underneath.

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And you can see that they also[br]carried huge key rings

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that they attached to their belts.

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Belts had huge buckles on them.

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They both threw their heads back[br]sharply when they laughed.

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They were six foot four,[br]bald and heavily mustached,

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and when I took them out to dinner,

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I discovered they had[br]a shared passion for Chinese food

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and for steaks served extremely rare.

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

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My colleagues and I[br]affectionately called them that,

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because when they met for the first time,

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they discovered that they laughed[br]uncontrollably with each other,

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and with nobody else.

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And they had the same[br]crooked pinkie fingers,

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the same disinterest in politics,

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and they drank their coffee cold,[br]black and without sugar.

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These twins had had a first[br]miscarriage in their first pregnancy,

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followed by two healthy boys[br]and a daughter.

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That may not be so surprising,

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because female physiology[br]may impact the sex of our children,

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and in this case, the physiology[br]was perfectly matched.

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Shortly after they met,[br]they concocted a drink called Twin Sin,

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

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My colleagues and I[br]thought it was a little curious,

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but the twins thought[br]it was just delicious.

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I finally want to mention

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

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One pair from the city,[br]one pair from the country.

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We don't know how this happened,

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but early on, in the premature nursery,

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one newborn twin[br]was accidentally exchanged

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with one newborn twin in the other pair.

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

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when in fact, they were[br]completely genetically unrelated.

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When they were twenty-five,[br]the truth was discovered,

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and the real pairs were reunited.

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I went down to Bogotá to study them,

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and I discovered that the personalities[br]of the reunited twins

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aligned almost perfectly.

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In one case,

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the twins were outgoing,[br]gregarious, risk-taking,

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and in the other case,

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they were introverted,[br]a little cautious, a little restrained.

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

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but seeing them repeated[br]in identical twins,

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more so than in fraternal twins,

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gives us a genetic perspective[br]on human development.

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Now, we need to think seriously[br]about these findings,

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because they have huge implications[br]for how we raise our children

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

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our goals, and our dreams.

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Now, in my experience,

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parents of one child[br]are environmentalists,

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and parents of two children[br]are geneticists.

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And I say that

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because parents of two children[br]realize rather quickly

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

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And that's because children[br]come into the world

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with their own genetic predispositions

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that lead them towards certain activities[br]and places and opportunities.

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Parents have to have realistic[br]expectations about each child.

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Two dancing parents[br]may not have a dancing child,

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

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genes get reshuffled in each generation.

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You know, I tell parents,

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

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Because each child evokes[br]certain treatments and certain responses

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from each of their parents.

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

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

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

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encouraged gently to do so.

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In this way,

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

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And I also want to acknowledge[br]the teachers and the mentors

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and the managers who work[br]so closely with twins and with others,

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and affect their lives.

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As a psychology professor, [br]

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I identified a very promising[br]undergraduate student,

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one who showed[br]real ability, talent, and drive.

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

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and he lacked guidance and opportunity.

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I worked with this student[br]on a number of projects,

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and today,

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

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Back to our twins.

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Twins are not just[br]mere objects of fascination.

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Just by being themselves,[br]just by acting naturally,

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they give science a powerful tool

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for understanding genetic[br]and environmental influences on behavior.

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And in this way,[br]they tell us about our humanity,

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why we are the way that we are,[br]and how we got that way.

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Thank you.

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(Applause)