Behavioral genetic methodologies from twin and adoption studies through DNA analysis will be described and applied to address longstanding questions about the origins of individual differences in behavioral traits.
2D: Are Twin Studies Valid?
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En provenance du cours de University of Minnesota
Introduction to Human Behavioral Genetics
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In Unit #2 the twin study method will be introduced and general findings from twin studies in psychology and psychiatry will be reviewed. The two types of twins, monozygotic (MZ) and dizygotic (DZ), will be described and methods for assessing their similarity will be given. We will also look critically at the limitations of the twin study method and discuss alternative research designs, including adoption studies and the study of reared-apart twins. The importance of convergent evidence from multiple research designs will be emphasized.
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Matt McGueRegents Professor
Psychology
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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