Welcome back.

Today we'll continue our discussion of twin studies, which have

played a major role in the field of human behavioral genetics.

Thus far, we've introduced the notion of what a twin study is, an actual experiment

that behavioral geneticists have used to try to

infer the existence of genetic influences on behavior.

And we've also noted a very consistent pattern

that has emerged from those twin studies of behavior.

That pattern is that Monozygotic twins

are consistently more similar than Dizygotic twins.

That consistent difference between the

similarity of Monozygotic and Dizygotic twins.

Could reflect the existence of genetic influences.

Which is the standard interpretation that

behavioral geneticists give to that observation.

Alternatively, we can have that consistent observation

arise because of some failure in the twin method.

Some invalidity in the twin comparison.

So the purpose of the module today is

to really look at two key assumptions that are

made in the twin method to see if they

really support the conclusions coming out of twin studies.

The first assumption is that twins are in somehow

or in some way representative of the general population.

Psychologists study twins for many reasons.

But one of the major reasons is they'd like to generalize findings from twin

studies to the general population, not to condition them just on twins per se.

So are twins really like the non-twin population?

Now that's a question that has an, a, long history in psychology.

For a long time, psychologists have

believed, well at least some psychologists have

believed, that twins are somehow psychologically

vulnerable by virtue of being a twin.

A very important person in developing this

idea is this woman here, Dorothy Burlingham.

She's a rather interesting person.

She's from the famous American family, the Tiffany family, who

own the Tiffany Jewelry Store in New York City in Manhattan.

And, Miss Burlingham studied with Anna Freud, Sigmund Freud's

daughter and learned psychoanalysis in the, the mid 20th century.

And she had always been fascinated with twins,

because she had a pair of twin sisters.

And based on her psychoanalytic training, as well

as her own studies of twins, she came

to the conclusion that identical twins, as they

grew up, had a difficulty in differentiating their

identities from one another.

That they are some way psychologically vulnerable because of having another twin.

This notion that twins, as they developed psychologically have

a hard time differentiating from one another, actually became the basis

of criticisms of early twin studies of psychopathology.

Twin studies of schizophrenia,

which we will look at later.

Here is a quote from a, a critic of the twin studies of schizophrenia,

goes back to, I guess, to 1960, when those studies were first coming out.

And what he believed is that really based on

the Burlingham notion is that twins were particularly vulnerable

to the development of schizophrenia and therefore we really

couldn't trust the results coming out of that research.

He thought that twins had a condition, called Folie a Deux, which

is translated madness by two. And that the twin, being a twin

really made you psychologically or, psychiatrically vulnerable.

Now, the question is, is there really evidence of that?

And the answer to that question is, from

what we know, there isn't a lot of evidence

that twins are psychiatrically vulnerable to developing psychotic conditions.

This is just one example.

This is a large study by Ken Kendler and his Swedish colleagues.

And what they've done, this is based on thousands

of twins, is they've used the Swedish registry system

to look for twins and then to look for

whether or not the twins were diagnosed for various

psychiatric illnesses like schizophrenia, or bipolar or unipolar or depression.

And they compared the rate at which twins were diagnosed with these

disorders to the rates at which non twins in the registry were diagnosed.

And then they report here the ratio of the two.

So if the ratio here is greater than one, then twins

had the, had the disorder at a higher rate than non-twins.

If it's less than one then in the registry

their rate was a little bit lower.

And the point of this here these results here to note

is that the rates don't really deviate much from one at all.

And in fact here are the confidence intervals, none

of the rates deviate at all from one, significantly.

There was absolutely no evidence from this very large Swedish study that

twins, despite what psychologists might have

thought, that twins were psychiatrically vulnerable.

Well they might not be vulnerable to

psychopathology, but maybe their personalities are different.

Is there evidence for that?

Here again, I'm going to look at a single study.

In this case, it happens to be a study I was involved in.

Whereas when we're looking at rates of schizophrenia,

we can divide the rate at which twins

have the disorder by the rate at which

non-twins have the disorder, and get a ratio.

With a quantitative variable like

personality, we're going to measure the differences

between the twins and the non-twins by taking the mean

difference in their personality, and divide by the standard deviation.

This is a very standard statistical measure used in psychology.

It's called a d statistic.

And it's usually thought to be, it would be large

if it, it were, if this number were above 0.7.

Moderate if it were between about 0.3 and 0.7.

And small if it's below 0.3.

And this is a study actually done here in Minnesota,

by a graduate student of mine at the time, Wendy Johnson.

Where she looked at, well there are eleven personality characteristics here, and

what are plotted are the d statistics for both men and women.

Comparing twins to non-twins.

And the point of this figure here, the,

the take home message is that all differences

are very, very small.

Sometimes they're negatives in which case the twins are

scoring lower on that trait than, than the non-twins.

Sometimes they're positive.

But overwhelmingly there are very, very minimal

differences between twins and non-twins in personality.

No differences in psychopathology.

No differences in personality, well what about cognitive ability?

Something that we'll be coming back to a little later in this course.

This is another study that was involved in, in Denmark.

Where again using the registry system, it turns that, Danish students

at the age of 15, they're in ninth grade at that time.

Take a cognitive assessment, a cognitive achievement

test and what we've compared here are

large sample of non-twins, singletons to a

large sample of twins on this standardized assessment

of their scholastic achievement.

And what you can see is that the means are absolutely

the same and even, I think, the standard deviations are the same.

The distributions look virtually identical.

So if we look at present day evidence there's very little

difference in twins and psychopathology rates,

in personality, or in cognitive ability.

There are differences between twins and non-twins, but

they're, they don't really arise in the major

domains that we'll focus on in this course.

Where differences arise, they have to, they have

to do with things related to prenatal stress.

So for example, twins are, not unexpectedly, more

likely to be born with low birth weight.

They're more likely to suffer traumas associate, associated

with, with birth event, things like cerebral palsy.

They're somewhat more likely to

be left handed.

But the type of things that we're going to look at in this

course, there really is no evidence

that twins are fundamentally different from non-twins.

Most behavior geneticists feel fairly confident

in generalizing from twins to non-twins.

The second assumption, was called the equal environmental similarity assumption.

What behavioral geneticists would like to

do, is, from the observation that monozygotic

twins are more similar than dizygotic twins, to conclude it's

because monozygotic twins are genetically more similar than dizygotic twins.

But there's also another possibility, maybe the monozygotic

twins are environmentally more similar than the dizygotic twins.

The assumption that they are not more environmentally similar is called the

equal environmental similarity assumption and it's

really a key assumption in twin studies.

Is it valid?

Well one concern that twin researchers have is that there, it might

actually be invalid from the very

earliest stages of development, prenatal development.

If we recall back to when we introduced monozygotic and dizygotic twins.

I made the point that 2 3rds of

monozygotic twins are monochorionic: they share a single chorion,

they share a single placenta, they share a single circulatory system.

All dizygotic twins are dichorionic, they have two

placentas, as well as 1 3rd of monozygotic twins.

Maybe when we're comparing monozygotic to dizygotic twins, what we're seeing is

not the effects of genetics, but the effects of sharing a placenta prenatally.

Is that a possibility?

Unfortunately, there's not a lot of data to answer that question.

And the reason is it's a little bit difficult to determine after the fact

if there was one or two placentas,

someone actually needs to examine the placental tissue.

And that's not routinely done in twin births.

But there are some studies we can look at.

And I'm going to just mention one here from Belgium.

It's a beautiful study of this.

And in this case what they're looking at is cognitive ability.

But it's, I think, although it's a single study, I

think it's fairly illustritive of the findings in this area.

And what they did is, they measured, again, we're

going to measure twins' similarity here in terms of a correlation.

No similarity is zero.

Perfect similarity is one.

And what I've plotted here for three different measures of

general cognitive ability, we can just look at total IQ.

The, the, the pattern is the same for all the three.

Is what's plotted here is the correlation for the monozygotic monochorionic twins.

Those are the 2 3rds of monozygoric, monozygotic twins who

share a placenta, versus the dizygotic twins correlation for IQ.

They always share. They always never share a placenta.

They have two placentas.

And you can see that right, these twins are more similar than those twins.

Is that genetics? Or is it sharing one

placenta, versus having two placentas?

Well, the key observation clearly is the dichorionic monozygotic twins.

Do the dichorionic monozygotic twins look like the green bar?

In which case it's a chorion effect.

It's not genetics. Or does they look like a blue bar?

In which case we conclude that it's genetics and not the placenta.

Well here are the results when we fill in for the dichorionic

monozogyte twins.

Which you can see is the red bars look like the blue bars.

In general when twin researchers have looked at chorion, what you see is that

what really matters is the genetic similarity, not sharing a placenta.

So the prenatal factors don't seem

to invalidate the equal environmental similarity assumption.

But what about postnatal factors?

Originally when

critics of twin studies were concerned about this issue,

the thing they turned to first was physical similarity.

You've seen some pictures of monozygotic twins now, and dizygotic twins.

And of course, monozygotic twins, like these men here, are strikingly similar.

And there's a belief, at least among some psychologists, that

our, our experiences in life are tightly controlled or influenced by

how we look.

If you ran into these men who might look to you a little bit scary, you might

react to them in a much different way than if you saw someone who look less scary.

And the notion is, which is captured in this quote

here by a a very famous psychologist Richard Nisbett from

University of Michigan is that, really, the environments of these

men might be much more similar than two random individuals

by virtue of them looking very similar, and then people reacting

to them in a similar way, and creating similar psychological environments.

So maybe it's the physical similarity and the

reaction that physical similarity draws from others that

leads twins to be more, monozygotic twins at

least, to be more psychologically similar than dizygotic twins.

Well although that was the initial hypothesis, I can

tell you that there's absolutely no support for that hypothesis.

And the reason we know there's, that there's no support for that

hypothesis is first of all we can look, and what researchers have done

is looked at monozygotic twins who are very physically similar like those

men in the previous slide versus

monozygotic twins who are less physically similar.

And, if physical similarity is important, then

the monozygotic twins who are really physically similar

should be psychologically more similar than

the ones who are less physically similar.

In study after study, that is not the case.

So, physical similarity, per se, does not seem to be an important factor.

Another way twin researchers have looked at this

issue is by looking at what's called zygosity confusion.

Sometimes parents don't actually know the correct zygosity of their children.

They might have a pair of dizygotic twins who look

quite similar and they might then believe that they're monozygotic.

Or conversely, they might have a pair

of monozygotic twins who they think are dizygotic.

And twin researchers have asked, well, if you look at these twins who are, where

parents are misinformed about their zygosity, does

that misinformation lead to differences in their similarity?

And the answer, again, is no.

The monozygotic twins who were thought to be dizygotic look

as similar as monozygotic twins who are known to be monozygotic.

And conversely for the dizygotic twins.

So, physical similarity, per se, also does not

seem to invalidate the equal environmental similarity assumption.

So from this can we conclude that there's nothing to be concerned about here?

That the environments of monozygotic twins are really

not more similar than the environments of dizygotic twins?

Well the answer to that actually is an unequivocal no.

Again, I'm going to highlight this with one study.

Because the findings here are very, very consistent.

I'm going to take a study I was involved in a few years ago not because

it's necessarily the greatest study in the world but because I'm very familiar

with the study and it does illustrate the point I want to make here.

What the study was about is life stress

and there's a lot of psychological literature that indicates

that life stress is an important factor in the

development of psychopathology and in particular things like depression.

Stress can trigger depression.

So it's a, a very important psychological variable, it's a very important

aspect of our environments that

might trigger osycho-, psychopathological events.

In this case, we took a sample of adolescent young adult twins.

And we gave them a life stressor, what's called a life events interview.

And we characterized the interview the

life stressors in three different categories.

Some were familial life stressors.

Things like

has your family had money problems or did your parents get divorced?

Things that happened at the family level.

And among things that didn't happen at the family level, we differentiated between

things that would largely be outside the control of the twin, like whether

or not they got mugged or robbed, versus things that they might have

had some role in, like whether or not they were doing well in school.

Or whether or not

they had lost a job.

And then we asked, well how were the twins similar

on these three different types of life events or life stressors?

And here's the answer.

Here are the correlations again. Remember, zero is no correlation.

One is perfect similarity.

For monozygotic and dizygotic twins, the monozygotic are in blue.

The dizygotic are in red.

Not surprisingly, for things that happened at

the family level, there are no differences

in those experiences between monozygotic and dizygotic twins.

They're both very highly similar in those and they should be.

The equal environmental similarity assumption is validated here.

Right, because they're both equally similar.

The monozygotic are not more similar, than the dizygotic.

But look what happens here.

In these, the monozygotic twins, in things that are outside the

control of the twins, largely, you don't

see greater monozygotic, than dizygotic twin similarity.

But in things that might be somewhat under our

control, how we're doing in school and the stress consequent

to that, or whether or not we're doing well

at work, monozygotic twins are more similar than dizygotic twins.

A very general pattern begins to emerge when we begin to look

at the environments of monozygotic and dizygotic twins.

Environmental factors that are externally imposed, we do not see, like

the familial factors or the independent factors on the previous slide.

We don't see greater monozygotic than dizygotic twin similarity.

Alternatively, environmental factors that reflect our own behavior to some

degree, whether or not we do well in school or

whether or not we have problems in our relationship or problems at work.

We will see greater monozygotic than dizygotic twin similarity.

Does that invalidate?

Does that greater similarity for those type of events invalidate the twin method?

One of the things that really differentiates behavioral

genetics from the other aspects of genetics is this.

That the

environments that we experience, each of us, in

part, we control, or we form those environments.

And we might form those environments, in part, because of our genetic constitution.

When we begin to look at the

heritability of traits like intelligence, or personality

we need to recognize in behavior unlike

when we're looking at medical diseases that

in part what is meant by heritable effects might be our ability to form environments

or place ourselves in experience that leads to the development of those traits.

That there might be genetic factors that actually

the way we go about constructing our experience.

If everything in behavioral genetics rested on the twin method,

I think we would be pretty concerned about drawing strong inferences.

Next time in the next module, we'll take up a different design, adoption studies.

And see whether or not findings from adoption studies really

support the types of conclusions that we're making in twin studies.

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2D: Are Twin Studies Valid?

Introduction to Human Behavioral Genetics

Meet the Instructors