My name is Dr Nancy Segal.
I'm a psychology professor at California State University, Fullerton.
What got me into twin research initially is that I am a fraternal twin
and always fascinated with the similarities and differences,
mostly differences that my sister and I have.
And I used to wonder as a child
how this could be if we had the same parents and same environments.
And when I began to study psychology
at the high school and graduate school level,
I learned about genetics and how we all came into the world
with predispositions of our own.
And that explained why my sister and I were so very, very different.
Well, I spent many years at the Minnesota study with twins raised apart,
looking at identical and fraternal twins who've been separated at birth.
It's very important to include the fraternals because they're the natural control group.
And let's think about the differences between those twins
before looking at the insights.
Identical twins share all their genes having split
from a single fertilised egg within the first two weeks after conception.
Fraternal twins share half the genes on average.
They result when a woman releases two eggs at the same time
and they're fertilised by two separate sperm,
so they have 50 percent of their genes in common
on average, like ordinary siblings.
Comparing resemblance in identical twins to fraternal twins
gives us a handle on whether or not genetics has an effect.
It would if identical twins are more alike than fraternal
and they invariably are. Studying twins gives us enormous insights
into how we come to be the way that we are.
Twin studies are a natural model for looking at genetic
and environmental influences on behaviour,
and what we are finding is that many more behaviours
than we ever would have thought, do have a genetic component to them.
Genetics is not everything, but does explain a great deal of why we differ
one person to another.
Well, let's start with looking at genetic influences on physical traits.
We find that height and weight have substantial genetic components.
We find that general intelligence has a substantial genetic component,
a little bit less than the sum of the physical traits
like height and weight and brainwaves.
But nevertheless, a substantial genetic component,
as does special mental abilities.
And then we drop down a bit when we get to job satisfaction.
Probably the most surprising findings in the last 20 years or so had been
that things like religiosity, how much you invest
in religious activities and interests and political attitudes
and social attitudes have a genetic component to them.
It's very important to appreciate the genes do not work in deterministic ways.
They work in probabilistic ways they predispose,
but they do not provide the final word.
Genes change in expression just
because you have a gene doesn't mean it will always be expressed.
It takes a certain environment to bring that out.
We all have genes that will be expressed given a particular environment.
Now, with identical twins, they have the same DNA,
but sometimes gene expression can occur in one twin and not the other.
And this can create differences between them,
and these environmental differences that trigger
different gene expression might even start in the womb.
The beauty of identical twins raised apart
is that they share only their genes and not their environments.
So any resemblance between them is tied to their common genes.
We find some amazing similarities in identical twins raised apart.
Many more than we ever would have anticipated,
not just in the more traditional areas like intelligence,
personality, physical features like height and weight,
but in some more unusual habits, such as a pair
of twins who both used to scatter love letters around the house
to their wives and both bit their nails down to the nub
and both explain their same mixed headache syndrome
in exactly the same way as if someone is beating on their head
with a hammer. These are very challenging,
and you can ask yourself, are they due to just random chance?
My answer to that is no, they're not.
And the rarer they are, the more I believe it's somehow tied to their genes
and the way the genes interact with their environments
to produce these kinds of unusual similarities.
I think that the people who find it challenging
or even disturbing are those who don't fully understand the process.
They think the genes work in deterministic ways
when in fact they work in probabilistic ways.
And so it doesn't mean that we're set in stone, we can't change.
It means that we can change.
We can alter environments to make behavioural expression different.
We can work to prevent disease
or to mitigate it, but we can't all be the same.