Imperfect Experiment, La Recherche, (France)
Beckwith, Department of Microbiology and Molecular Genetics, Harvard
Joseph S. Alper, Department of Chemistry and the Center for the
Study of Genetics and Public Policy, University of Massachusetts
been referred to as Nature's perfect experiment. They are identical
twins - twin siblings who are born with identical sets of genes.
Psychologists and geneticists claim that a host of human behavioral
characteristics are strongly influenced by genes, basing their
conclusions on studies of identical twins. Newspapers follow with
headlines reporting that everything from intelligence and homosexuality
to religious beliefs and television watching are largely genetically
scientists agree that both genetics and environment are important
in explaining behavior. But, they disagree about the relative
importance of genetics and the environment in explaining the differences
we see among people. They also disagree about whether indications
of strong genetic contributions to behavior are useful in formulating
studies have provided one of the most important arguments for
a genetic basis of human behavior. Identical twins result from
the splitting of a fertilized egg, the zygote, into two separate
zygotes. Since they arise from a single zygote, they are referred
to as monozygotic (MZ) twins. Like MZ twins, fraternal twins,
called dizygotic (DZ) twins, are born at the same time. However,
unlike MZ twins, fraternal twins arise from the separate fertilization
of two eggs. Since MZ twins come from the same fertilized egg,
they are genetically identical. DZ twins, who come from two separate
eggs, are no more genetically similar than are ordinary siblings.
On average, two DZ cotwins (or two ordinary siblings) have inherited
one-half their genetic material in common.
who study identical twins have developed two approaches to measure
genetic influences on behavior. In the first approach, they study
twins all of whom have grown up with their biological parents.
They compare the correlations in the behaviors of identical twins
with correlations found with fraternal twins. If the MZ twins
resemble each other much more closely for a particular behavioral
characteristic than do the DZ twins, they argue that genetic influences
must be important in explaining the relative similarities.
the second approach, researchers study twins who were separated
and placed in different homes at birth or early in their lives.
It is assumed that the environments in the two homes are unrelated
to each other. If this assumption is true, then strong similarities
in the behavior of the separated twins suggests the importance
of genetic influences on behavior.
first sight, the genetically identical make-up of MZ twins appears
to be Nature's gift of a perfect experiment to behavioral geneticists.
Unfortunately, studies based on twins are not as foolproof as
they might seem. In fact, the flaws in these studies are so serious
that research on twins provides little insight into the genetic
origin of the way we behave.
raised with their biological parents
explore some of the limitations of the first twin study approach,
we consider a 1991 study carried out by Drs. Michael Bailey of
Northwestern University and Richard Pillard of Boston University
that examined the concordance for male homosexual behavior among
pairs of twins and non-twin siblings. We choose this study because
it is one of the few that states explicitly some of the methodological
problems encountered in twin studies and makes attempts to overcome
them1. Bailey and Pillard found that among homosexual
males who had an identical twin, that twin was also homosexual
in 52% of the pairs. But, DZ twins showed only a 22% concordance
for homosexuality. A statistical analysis of these data suggested
that the significant difference in the two percentages implies
a strong genetic contribution to male homosexual behavior.
the authors admit, one of their potential problems is "ascertainment
bias." Ascertainment bias refers to a situation in which
the way subjects are recruited for a study leads to a collection
of individuals who are not representative of the general population
exhibiting that behavior. Ideally, to avoid bias, individuals
in any study should be chosen randomly from the population of
interest. In this case, the population of interest is those gay
men who have a twin brother. However, there is no list of such
men. Bailey and Pillard found their twins through responses to
advertisements in gay newspapers and magazines published in Midwestern
or southern cities of the United States.
this group of gay men really provide a random sampling of homosexual
male twins in the population? We cannot say for certain, but it
is not unreasonable to suspect that the readers of gay publications
who volunteered to participate in the study represented a special
subset of homosexual males. Such readers who volunteered may,
for instance, be less shy and less concerned than non-participants
about public exposure and are, therefore, willing to involve themselves
in such a study. Further, the fact that they read such journals
in the first place might indicate a greater openness and acceptance
of their sexual orientation than other gay men. Their willingness
to participate in the study and their degree of comfort with their
own gayness may, in turn, reflect an upbringing in a more liberal
and accepting environment. If any of these possibilities is in
fact true, then the study might have excluded a large number of
potential twin participants who would show much lower correlations
with their twin siblings for homosexual behavior. The result of
this ascertainment bias would be an overestimate of roles "homosexuality"
genes play in determining whether or not men manifest homosexual
behavior. When subjects are not chosen at random, it takes little
imagination to envisage potential ascertainment biases for whatever
behavioral trait one wishes to study.
closer look at Bailey and Pillard's data reveals another major
problem that affects many twin studies. Recall that the concordance
for MZ twins was 52% and that for DZ twins was 22%. However, the
concordance for homosexuality in ordinary brothers was only 9%.
But, we know that DZ twins are no more genetically similar than
are brothers who were born at different times. Consequently, according
to a genetic model, DZ twins and ordinary siblings should exhibit
similar concordances. The finding of the strong difference between
the results with DZ twins and ordinary brothers suggests a significant
influence of environmental factors on the development of homosexuality.
is easy to identify environmental differences between DZ twins
and ordinary brothers that might explain Bailey and Pillard's
results. The non-twin brothers, in contrast to the twins, were
born at different times and thus may have grown up in a different
familial and cultural milieu. Perhaps more importantly, they did
not grow up in a "twin environment". Twins exist together
in an unusual and often especially close environment, influencing
each other in untold ways that differ from ordinary brothers.
focus on the comparison between DZ twins and ordinary siblings
instead of on that between MZ and DZ twins, leads us to the exact
opposite conclusion from that of Bailey and Pillard! That is,
the comparison between DZ twins and ordinary brothers suggests
that environmental factors are important in explaining the difference
in concordances between these two types of brothers. In this light
it seems reasonable to go back and ask whether some of the difference
in concordances (52% vs. 22%) between the MZ and DZ twins may
also be due to environmental influences. Critical to such a question
is a fundamental assumption of such twin studies referred to as
"the equal environment assumption."2
to the equal environment assumption, researchers on twins propose
that the degree of shared environments for MZ twins is the same
as it is for DZ twins. This assumption is based on the fact that
each pair of twins, whether MZ or DZ, is raised at the same time
in the history of their families and of their society. If this
equal environment assumption is valid, then the difference in
concordances between MZ and DZ twins would represent a measure
of genetic differences.
is this assumption correct? Are the similarities in environments
of identical twins and fraternal twins effectively the same? And,
if the environments are not the same, does it make any difference
in the behavior of the twins. It seems unlikely that MZ twins
and DZ twins experience the same environments. Identical twins
are so alike that it is often difficult to distinguish one from
the other. As a result, identical cotwins are likely to be treated
more similarly by parents than are fraternal cotwins and, thus,
will experience a much more similar environment within the home.
Even outside the family setting, identical cotwins who look identical
may experience more comparable environments than do fraternal
cotwins. The response of people to the individuals they encounter
depends to a great extent on the physical characteristics of those
individuals, for example, their height, weight, or degree of attractiveness.
The physical similarities in MZ cotwins resulting from their identical
genes may elicit similar responses from other people. These similar
responses may, in turn, result in further increasing the behavioral
similarities between the MZ cotwins. We thus see that the often
strikingly similar behaviors in cotwins can result from both their
identical genes and from their exposure to unusually similar environments.
curious but important semantic debate occurs here where the genes
are exerting their influence through the mediation of physical
characteristics and environmental responses to those characteristics.
The debate is whether to consider such effects as due to the genes
or to the environment. In the technical jargon, these effects
are called "gene-environment covariances."3
We believe that since the covariances arise as a result of the
actions of siblings, parents, teachers, friends, etc., rather
than arising from the direct actions of genes, the effects of
gene-environment covariances should be treated as an environmental
effect. The cultural features of a particular society (or substrata
of society) at a particular time in its history may have a strong
influence on what responses are "elicited" from a society
in response to twins' physical or behavioral characteristics.
The set of environments available to the twins depends on society
and the people who interact with the twins; it is not produced
by the genes of the twins. In some cultures overweight people
are the subject of scorn and teasing, in others they are admired.
Consequently, these potentially changeable features of the interactions
of twins with their family, society and culture are more appropriately
treated as environmental effects. The similarities in behavior
and personality in MZ twins arising from similarities in the environment
are not directly caused by these genes; they are directly caused
by the environmental similarities. Clearly, treating gene-environment
covariances as an environmental effect decreases the apparent
role that genetic differences play in explaining behavioral differences.
contrast to our proposal, most behavioral geneticists treat gene-environment
covariances as genetic effects. They follow the example of University
of Minnesota Psychologist Thomas Bouchard who argues :"[Identical]
twins tend to elicit, select, seek out, or create very similar
effective environments and, to that extent, the impact of these
experiences is counted as a genetic influence."4
We leave it to the reader to decide.
evidence is there that the shared environments of MZ twins contribute
to the similarities in behavior? Clearly, as we see from Bouchard's
statement about gene-environment covariance effects, he believes
that such effects do exist for those environments elicited by
the twins. A number of studies that confront this issue have been
carried out to test the equal environment assumption.5,6
In principle, comparison of fraternal twins who look most alike
or have even been mistaken for identical twins with those fraternal
twins whose physical appearance is not very similar might shed
light on this question. Under certain assumptions, this mistaken
classification of DZ twins by parents and doctors should lead
to a familial environment that is the same for MZ twins as it
is for these particular DZ twins. The few studies done in this
area have obtained mixed results.2
have also attempted to assess the degree of equal treatment and
upbringing that identical twins received and whether that similarity
was correlated with their similarity in behavior. These studies
rely on the memories of children or parents or both. Clearly these
measures are not very reliable and, perhaps as a result, different
studies of different behaviors have reached different conclusions.
recent papers cast further doubt on the equal environment assumption
by suggesting that environmental influences occurring early in
life play an important role in the development of behavioral traits
in identical twins. Dr. Bernie Devlin and coworkers of the University
of Pittsburgh and Carnegie-Mellon University showed that conditions
within the womb may have a substantial effect on the concordance
of subsequent scores on IQ tests for identical twins.7
Furthermore, Drs. Elisabeth Spitz of the Université de Metz and
Michèle Carlier of the C.N.R.S. in Orléans have found that identical
twins who develop in a single chorionic sac in the womb (monochorionic
MZ twins) show different concordances for certain aptitudes when
compared to identical twins who develop in separate sacs (dichorionic
MZ twins).2 When the researchers corrected the calculations
to take into account the high proportion of monochorionic MZ twins,
they found that genetic differences could no longer account for
the differences in performance on an IQ-type test.
Daniel O'Loughlin and coworkers of the University of Texas observed
that the greater incidence of premature births among twins makes
it difficult to extend specific conclusions drawn from the study
of twins to conclusions about the role of genes in behavior in
the overall population.5 The resultant lower birth
weights and other complications associated with prematurity could
result in enhanced similarities in twin development compared to
ordinary siblings. Because of these developmental problems affecting
twins, it may be unwarranted to conclude anything about the role
of genes in behavior from genetic studies of identical twins.
summary, there is not a large body of evidence regarding the equal
environment assumption. Until comparatively recently, researchers
in behavioral genetics ignored the complex environmental and developmental
factors that affect estimates of the importance of genetic differences.
As a result, few of the studies that support one or another perspective
on the importance of these factors have yet to be replicated.
The rather belated recognition of these problems, the paucity
of studies, and the contradictions between the studies that have
been published have led Spitz and Carlier to conclude, with regard
to the equal environment assumption, "...il est évident qu'un
consensus n'existe pas."2
second class of twin studies requires finding identical twins
who, at an early age, were separated from their families and placed
in separate homes. These studies are affected by many of the same
difficulties that beset the research on twins who have been raised
by their biological parents. Ascertainment of subjects for these
studies presents a serious problem. Except in those cases where
twins are identified through a national registry, such as exists
in Denmark, the twins involved do not represent a random sample.
For instance, in the very large twin study conducted by Dr. Thomas
Bouchard and his coworkers,4 new subjects were attracted
by media coverage which emphasized the striking similarities between
separated twins. The pairs of twins who volunteered for the study
in response to the news coverage may represent a biased population.
They may, for example, have a particular interest in 'being twins'.
Or they may have responded because of their own striking similarities.
In both cases, these twins might resemble each other more closely
than would a more randomly ascertained subject group.
in the case of twins raised together, the physical similarities
between identical twins raised apart could be important in invoking
similar societal responses. Maternal effects may be important
as well. Not surprisingly, the study by Devlin et al. reported
such effects in both separated twins and twins raised together.7
addition to these problems common to both types of research, studies
of MZ twins raised apart present their own problems. Critical
to this work is the assumption that the separated twins have been
raised in significantly different environments. If the environments
created by the different homes were similar, it would be difficult
to know whether a high concordance for a trait was due to the
similar environments or to identical genes of the MZ twins.
assumption that the environments of separated twins are dissimilar
is not likely to be correct. In many cases, either the parents,
relatives, or the adoption agencies attempt to place children
in environments as similar to that of the original home
as possible. The twins may be placed in the homes of relatives.
They may live in the same town and even attend the same school.
In seeking similar environments, adoption agencies often consider
such factors as socio-economic status, religion, and cultural
interests. We do not know how much these similarities in environment
contribute to similarities in, for example, IQ test scores.
many decades, researchers studying separated twins did not consider
very seriously the possibility that similarities in the separated
environments might affect their conclusions. However, in 1974,
psychologist Dr. Leon Kamin of Northeastern University examined
in detail the data from the major twin studies on the genetic
influences on IQ. He found that, more often than not, separated
twins did in fact grow up in very similar environments.8
As a result of Kamin's work, "...behavior geneticists had
to sharpen their arguments, design new, more careful studies,
[and] obtain fresh evidence," according to researcher Dr.
Neil MacKintosh of the University of Cambridge.9
a response to Kamin's critique, some of the more recent studies
of the genetics of behavior have included methods for evaluating
the similarities of environments into which adopted twins are
placed. These studies use survey instruments that attempt to quantify
available cultural and intellectual household resources.4,10
For example, a count is obtained of the number of books in an
adoptive family's home. It is easy to laugh at such a simplistic
measure. But the problem is enormously difficult. Is it even possible
to develop quantitative measures of the intellectual impact of
a particular family environment which has many intangible components?
We do not know the answer to this question and do not envy behavioral
geneticists in their task.
detailed critical analysis of the environments of separated twins
carried out by Kamin in the 1970's has not been repeated on the
more recent larger twin studies. The data required for such an
analysis is usually not available to other researchers perhaps
because of reasonable concerns about the privacy of the participants
in the studies. Whatever the reason, this lack of access to the
data makes it difficult for us to evaluate the conclusions of
fundamental difficulty with twin studies
than providing "Nature's perfect experiment", the genetic
identity of identical twins creates more problems for researchers
than it solves. We have already noted that the environments experienced
by identical twins may be much more similar than those experienced
by DZ twins and ordinary siblings and that MZ twins face a greater
risk of premature birth than do people in general. In addition,
the genetic identity of identical twins may cause problems in
the statistical analysis of the genetics of behavior.
us suppose that a behavioral trait is influenced by the action
of several genes. If these genes operate independently of each
other, the effects of the genes may be additive and standard statistical
methods can be used for calculating the genetic basis of the differences
in similarity between MZ cotwins and DZ cotwins.
it is now suspected that many behavioral traits depend on the
interactions among many genes (as well as interactions with the
environment). That is, the genes may interact synergistically,
together having a far greater effect than would be expected from
adding up the influences of individual genes. For example, suppose
that there are 10 genes that contribute in synergistic fashion
to a particular behavior. If one MZ cotwin has all 10 of the genes,
so will his or her cotwin. However, if one DZ cotwin has all 10
genes, the chances that his or her cotwin will have the same combination
is extremely small, less than one in a thousand. Even if the cotwin
had 9 of those genes, without the synergistic effects of the 10th
gene, the difference in this behavior between the two DZ twins
might be very dramatic indeed. This means that any similarities
in this behavior within a family would be much less likely to
be due to genetics. Thus, a study of this behavior that focused
on identical twins could drastically overestimate the importance
of genetic differences in explaining the behavioral differences
seen in the general population.
order to quantify their results, behavior geneticists use data
from twin studies to calculate what is called the heritability
of a trait.3 There has been an enormous misunderstanding
and misuse of this term, which is in part a consequence of the
closeness in sound and origin of the words heritability and inherit.
The term "inherit" in biology clearly refers to the
inheritance of genes and thus has a deterministic characteristic
to it- we obviously do inherit our genes from our parents. But
the "heritability" of a trait is not, as is commonly
thought, the percentage of the trait that is inherited. The New
York Times committed this error in the headline of their article
reporting the results of the Minnesota Twin Study of Bouchard
and coworkers: "Major Personality Study Finds That Traits
Are Mostly Inherited."11 Nor does heritability
measure the degree to which a trait like height or IQ is genetically
determined in an individual. Rather, heritability is defined as
the proportion of the variation of that trait within a population
living in a specific range of environments that is due to genetic
differences among the people in that population.
rather complex definition describes a term that is used by agricultural
scientists in selective breeding experiments to predict and improve
valuable traits in plants and animals raised or grown under precisely
controlled conditions. Since people are not selectively bred nor
raised in controlled conditions, it is not really possible to
obtain accurate estimates of heritabilities for human behavioral
traits. In the words of Spitz and Carlier, this is because of
"l'extrême difficulté d'interprétation de l'héritabilité
dès lors que celle-ci est estimée dans notre espèce, c'est-à-dire
dans une situation où on ne contrôle ni les génotypes ni les environnements."2
is a consequence of the definition of heritability that even if
one could control the environment more than we can with humans,
an estimate of the heritability of a trait would only be an estimate
true for a particular range of environments and would not tell
us anything about how much or in what direction that trait would
change in a different environments. Furthermore, we are brought
back to the question of whether we can even define environment.
Defining a home or cultural environment is not as simple as quantifying
an agricultural environment in terms of the amount of fertilizer
added to soil, the depth of soil, or the amount of irrigation.
us for now ignore these problems and assume that heritabilities
can be estimated for various traits in people. As a result of
all the technical difficulties that we have mentioned, the heritability
estimates appearing in the literature are probably all overestimates.
For example, Devlin and his colleagues have estimated that including
the effects of maternal environment reduces the estimate of the
heritability of IQ from 60% to 48%. In addition, they note that
their analyses do not "preclude other, unmodelled factors,
such as cultural inheritance and interaction between genes and
the environment, from having important effects on IQ."7
The recent findings from studies on dichorionic vs. monochorionic
identical twins also appear to require a reduction in heritability
estimates for certain traits. If the criticisms of the equal environment
assumption or the concerns about placement of separated twins
have validity, heritability estimates would have to be further
reduced. Finally, as we have pointed out, researchers have often
incorporated the contribution of gene-environment covariances
into the genetic component of heritability estimates.
continuing to ignore the problems in estimating heritabilities,
we now ask: what do these (reduced) estimates of heritability
mean? What utility do these values have? Some researchers and
some of the media believe that high heritabilities of such traits
as IQ have social and political implications. The late psychologist
Richard Herrnstein of Harvard University and political scientist
Charles Murray of the University of Colorado, in their recent
book "The Bell Curve"12 argue that the average
intelligence in the United States is decreasing because lower
class people with lower IQ's are having too many children. They
believe that women from the upper classes should be encouraged
by new social programs to bear more children. Since, in their
opinion, IQ is genetic and therefore unchangeable, they call for
an end to what they regard as futile welfare and remedial education
programs. In the legal arena, debates over genetic contributions
to criminal behavior have caused lawyers and judges in the United
States to wonder whether individuals who commit crimes have free
will and are responsible for their actions.13
such arguments is the assumption that a behavior with high heritability
is fixed and unchangeable. But genetic does not mean fated. Even
for those traits such as a disease caused by the malfunctioning
of a single gene, an appropriate modification of the environment
may be able to completely reverse the effect of the mutant gene.
Phenylketonuria is a disease caused by a single gene mutation
that results in the accumulation of a compound devastating to
brain function. However, if detected by genetic screening techniques
in babies, the potential severe mental retardation can be completely
prevented by placing the children on a modified diet. Thus a trait
(mental retardation), that under one condition manifests itself
fully in the individuals who carry the altered gene, disappears
with a relatively simple change in the environment.
like intelligence are much more complex than single gene diseases.
Today, most behavioral geneticists believe that intelligence is
the result of the action of many genes and the environment. Alfred
Binet, who developed the original IQ test, knew that intelligence
scores can be changed by modification of a child's educational
environment. And with regard to the even more complex questions
concerning genes and free will, we point out that the term environment
includes the environment that an individual makes for him- or
herself. Thus, questions of free will cannot be resolved by relying
on genetic arguments.
estimates based on studies of twins have generated enormous confusion.
Psychologist Scott Stoltenberg of the University of Michigan has
suggested that this is partly due to the fact that the term heritability
has both "technical and folk meanings."14
He goes on to suggest that where confusion is inevitable, it is
the responsibility of the specialist to abandon "the term
in favor of one without a widely understood folk meaning."
believe that the term heritability is so entrenched that it will
be impossible to change it. However, it is not impossible for
scientists and journalists to explain carefully, in each article
they write for the nonspecialist, what heritability means and
what it does not mean. This can be done very briefly, as we do
to conclude this article.
is a property of a population. It tells us how much of the variation
in a population is due to genetic variation. It is often estimated
by means of twin studies. But twins are special; what may be true
for twins may not be true of the general population. Because heritability
is a statistic, it gives no information about the role of genes
or the environment in an individual. And its information about
the population applies only for the existing range of environments.
It tells us nothing about what would happen in a new environment.
Moreover, since for such traits as IQ the heritability is estimated
to be no more than 50%, there are almost certainly existing environments,
that will increase the IQ scores of many people. We may or may
not feel as though we are free to change various aspects of our
personality and behavior. But it is not heritability that stands
in our way.
Bailey M & Pillard R. (1991) A genetic study of male sexual
orientation. Archives of General Psychiatry 48:1089-1096.
Spitz E & Carlier M. (1996) La Méthode des jumeaux de 1875
à nos jours. Psychiatrie de l'enfant 39:137-159.
Falconer D S (1981) Introduction to Quantitative Genetics.
Bouchard T J Jr, Lykken D T, McGue M, Segal N L & Tellegen
A. (1990) Sources of human psychological differences: the Minnesota
study of twins reared apart. Science 250:223-228.
Ainslie R C, Olmstead K M & O'Loughlin D D. (1987) The early
development context of twinship:some limitations of the equal
environments hypothesis. American Journal of Orthopsychiatry
Morris-Yates A, Andrews G, Howie P & Henderson S. (1990) Twins:
a test of the equal environments assumption. Acta Psychiatrica
Devlin B, Daniels M & Roeder K. (1997) The heritability of
IQ. Nature 388:468-471.
Kamin L (1974) The Science and Politics of I.Q. Potomac:Earlbaum
MacKintosh, N J (1995) Cyril Burt: Fraud or Framed? Oxford
University Press, Oxford.
Rowe D C (1994) The Limits of Family Influence: Genes, Experience
and Behavior. The Guilford Press, New York.
Goleman D. (1986) Major personality study finds that traits are
mostly inherited. The New York Times Dec. 1:C1.
Herrnstein R J & Murray C (1994) The Bell Curve. Free
Press, New York.
Blakeslee S. (1996) Genetic questions are sending judges back
to classroom. The New York Times July 9:C1,C9.
Stoltenberg S F. (1997) Coming to terms with heritability. Genetica