University of Nebraska - Lincoln
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Faculty Publications: Political Science Political Science, Department of
5-1-2005
Are Political Orientations Genetically Transmitted?
John R. Alford
Rice University
Carolyn L. Funk
Virginia Commonwealth University
John R. Hibbing
University of Nebraska - Lincoln, jhibbing@unl.edu
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Alford, John R.; Funk, Carolyn L.; and Hibbing, John R., "Are Political Orientations Genetically Transmitted?" (2005). Faculty
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American Political Science Review Vol. 99, No. 2 May 2005
Are Political Orientations Genetically Transmitted?
JOHN R. ALFORD Rice University
CAROLYN L. FUNK Virginia Commonwealth University
JOHN R. HIBBING University of Nebraska
W
e test the possibility that political attitudes and behaviors are the result of both environmental
and genetic factors. Employing standard methodological approaches in behavioral
genetics—–speciﬁcally, comparisons of the differential correlations of the attitudes of monozygotic
twins and dizygotic twins—–we analyze data drawn from a large sample of twins in the United States,
supplemented with ﬁndings from twins in Australia. The results indicate that genetics plays an important
role in shaping political attitudes and ideologies but a more modest role in forming party identiﬁcation;
as such, they call for ﬁner distinctions in theorizing about the sources of political attitudes. We conclude
by urging political scientists to incorporate genetic inﬂuences, speciﬁcally interactions between genetic
heritability and social environment, into models of political attitude formation.
W
hy do people think and act politically in the
manner they do? Despite the foundational
nature of this question, answers are unfortunately
incomplete and unnecessarily tentative, largely
because political scientists do not take seriously the
possibility of nonenvironmental inﬂuences. The suggestion
that people could be born with political predispositions
strikes many as far-fetched, odd, even
perverse. However, researchers in other disciplines—–
notably behavioral genetics—–have uncovered a substantial
heritable component for many social attitudes
and behaviors and it seems unlikely that political attitudes
and behaviors are completely immune from such
forces. In this article, we combine relevant ﬁndings in
behavioral genetics with our own analysis of data on a
large sample of twins to test the hypothesis that, contrary
to the assumptions embedded in political science
research, political attitudes have genetic as well as environmental
causes.1
Testing this hypothesis is important for two reasons.
First and most broadly, as behavioral scientists we need
to analyze all possible shapers of behavior, not just a
select few. Second, a more complete understanding of
the sources of attitudes and behaviors will help us to
sort through existing puzzles of considerable interest to
political scientists. One example is political ideology.
Why is a reasonably standard left–right spectrum so
widely applicable cross-culturally and over time? The
universal left–right elements of belief systems around
John R. Alford is Associate Professor, Department of Political Science,
Rice University, Houston, TX 77251 (jra@rice.edu).
Carolyn L. Funk is Associate Professor, L. Douglas Wilder School
of Government and Public Affairs, Virginia Commonwealth University,
Richmond, VA 23298 (clfunk@vcu.edu).
John R. Hibbing is Foundation Regents Professor, Department
of Political Science, University of Nebraska—–Lincoln, Lincoln, NE
68588 (jhibbing@unl.edu).
The authors gratefully acknowledge the assistance of Professor
Lindon Eaves and colleagues, particularly for granting us access to
data from the sample of twins known as Virginia 30K. Any errors
in the analysis or interpretation of the data are, of course, solely our
own responsibility.
1 Evidence consistent with an evolutionary theory of political behavior
is found in Brewer 2000, Hibbing and Alford 2004, and Orbell
et al. 2004.
the world and over the decades is difﬁcult for behavioralists
to explain. But if there is a genetic component to
political ideologies, if the constraints on belief systems
come not just from intellectualization or indoctrination
but from something deeper, the concept of ideology
takes on greater meaning and the commonality of ideology
becomes easier to understand.
ATTITUDE FORMATION
Debates concerning the source of political attitudes revolve
primarily around the question of whether early
childhood factors have lasting relevance or whether
these factors tend to be overwhelmed by more proximate
events. Survey responses to political items presumably
reﬂect attitudes and are thought to be a
combination of longstanding “predispositions” and
more recent “off-the-top-of-the-head” considerations
(Zaller 1992, chaps 1–3; also see Converse 1964). Alternatively,
an “on-line” pattern of processing could
allow new incidents to ratchet affect one way or another
from previously existing summary locations (see
Lodge, McGraw, and Stroh 1989). Regardless, proximate
forces include recent conversations and experiences,
question-wording, priming from previous questions,
and a variety of similar factors. Predispositions,
on the other hand, are thought to be a “distillation
of a person’s lifetime experiences, including childhood
socialization and direct involvement with the raw ingredients
of policy issues” (Zaller 1992, 23).2
Great
interest exists in determining the relative clout of the
early as opposed to the late environment but no interest
has been displayed in determining the relative clout of
environmental as opposed to genetic variables.
A parallel conclusion applies to research on individual
attitudes rather than survey responses generally.
For example, the consensus among those who study
tolerance is that the extent to which individuals are
tolerant hinges on a combination of “antecedent conditions
and contemporary information” (Marcus et al.
1995). Antecedent conditions, in turn, are believed to
2 To his credit, Zaller (1992) goes on to acknowledge a possible role
for “inherited” traits in shaping predispositions (23).
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Are Political Orientations Genetically Transmitted? May 2005
be shaped by “personal circumstances” such as “family,
neighborhood, region . . . and early group experiences”
(Marcus et al. 1995, 5; for more on the importance of
long established proclivities, or antecedent conditions,
see Stouffer 1955). Typically, no role for geneticallyinduced
tendencies is considered (for an exception, see
Monroe 2004, chap 6).
More broadly, the literature on political socialization
has long revolved around the question of the effects
of early as opposed to late environmental forces.
Early political socialization researchers (e.g., Easton
and Dennis 1969, Greenstein 1960, Jennings and Niemi
1968, and Searing, Schwartz, and Lind 1973) and the
authors of The American Voter (Campbell et al. 1960)
presented arguments and evidence supporting the primacy
of early events. Later researchers, however, questioned
the value of early childhood socialization and
provided evidence that judgments about more recent
conditions and occurrences can dramatically alter preferences
we might have held as children and adolescents
(see, e.g., Fiorina 1980; for good summaries of the
debate over the relative importance of early and late
environmental events, see Cook 1985; Merelman 1986,
and Sears 1989). In the last 50–60 years, the emphasis
in the literature has gone from personality studies
(Adorno et al. 1950; Eysenck 1954; Laswell 1930), to
ideological and childhood socialization studies, to the
effects of media frames, perceptions of current conditions,
and other types of contemporary information. In
fact, for the past couple of decades research on political
socialization has been suffering through a “bear
market” (Cook 1985), and studies of personality, while
experiencing a remarkable comeback in psychology
(for an introduction, see Wiggins and Trapnell 1997),
have been largely absent from political science since
McCloskey’s (1958) work in the 1950s on the conservative
personality. Thus, political science debates
concerning the source of political attitudes and behaviors
have been over timing, over whether attitudes
and behaviors are primarily shaped early in life or by
more proximate occurrences. Conspicuously absent is
consideration of the possibility that certain attitudes
and behaviors may be at least partially attributable to
genetic factors.
MODERN BEHAVIORAL GENETICS
But what is the physical process by which a genetic
allele could shape a political attitude? If there is any
connection at all, is it not that the effect is so small
that it can be safely ignored? And even if this is not
the case, in light of potentially troubling normative
implications such as biological determinism, is it not
best to ignore relationships between genes and social
behavior? It is difﬁcult for many outside the biological
sciences to understand how it is even possible for genes
to inﬂuence behavior, so a brief discussion is in order.
Genes provide instructions for the production of proteins,
which are built and identiﬁed by a speciﬁc combination
of amino acids (which in turn are constructed
from complex organic molecules). As such, each protein
has a chemical sequence that then interacts with
other chemicals in the body, sometimes reacting directly
with these other chemicals but often serving as
enzymes that facilitate but are not themselves altered
by chemical reactions. If a gene coding for a particular
enzyme is absent, the chemical reaction it is meant to
enhance will occur with much less efﬁciency. For example,
a gene for the enzyme tryptophan hydroxylase-2
(Tph2) facilitates production of the neurotransmitter
serotonin in the brain, but a certain form of this gene
(which varies from the standard form by a single amino
acid) produces about 80% less serotonin and people
with this mutant allele appear to be signiﬁcantly more
likely to suffer from unipolar depression (Zhang et al.
2005).
Still, the connection is rarely so simple that a given
genetic allele can be seen as causing a certain behavior.
More typically, ﬁndings in modern behavioral genetics
reveal the effect of genes to be interactive rather than
direct, let alone determinative. To provide one illustration,
in humans there is a gene on chromosome 17
involved with serotonin reuptake (5-HTT). As is often
the case with genes, 5-HTT has a long allele and
a short allele. Mice have a parallel gene, and in that
species the short form had previously been connected
to listless, depressive behavior. Scientists were eager to
determine if such a correlation between the short form
of 5-HTT and depression was present in humans. In a
long-term study of the health records of nearly 1,000
New Zealanders whose 5-HTT alleles were known, it
was found that major episodes of depressive behavior
were not much more prevalent among those with the
short form. But then the researchers combined genetics
and the environment; speciﬁcally, they interacted each
subject’s 5-HTT allele with the number of high-stress
events (romantic calamities, bankruptcies, deaths of
loved ones, etc.) experienced in that individual’s life.
They found that those who had a high number of such
events and who had the short form of 5-HTT were
signiﬁcantly more likely to display behaviors associated
with depression compared to either those experiencing
few high stress events or those with the long form who
suffered through a comparably large number of highstress
events (see Caspi et al. 2003).
In this particular case, genotype did not make people
behave a certain way; rather, it inﬂuenced the extent
to which their behavior was contingent on the
environment—–and this pattern likely will apply to all
sorts of other human activities. Whether the behavior
of interest is depression, cooperation, fear response, or
susceptibility to drug addiction, some people are more
sensitive than others to particular features of their environment,
and genetics, far from determining behavior,
inﬂuences its sensitivity. Genetics makes the mood
of some people far more dependent on the extent to
which their lives have been beset with difﬁculties and it
likely makes some people’s political attitudes far more
contextually dependent than others. In other words,
the connection between genes and attitudes may not
involve speciﬁc attitudes as much as the ﬂexibility of
those attitudes (Is abortion always wrong, or does it
depend?). The issue is not nature versus nurture but
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the manner in which nature interacts with nurture (see
Marcus 2004 and Ridley 2003).
MONOZYGOTIC AND DIZYGOTIC TWINS
The process of identifying in the laboratory the precise
genes responsible for given human behaviors
(especially those behaviors that do not have corollaries
in lab-friendly animals such as mice) is extremely
challenging. Fortunately, even without identifying the
genes responsible, it is possible to compile information
on the matter of most concern to social scientists:
the extent to which attitudes and behaviors have
a genetic component. The relevant procedures center
on comparisons of monozygotic (MZ; frequently but
erroneously called identical) twins and dizygotic (DZ;
fraternal) twins.
MZ twins develop from a single egg, fertilized by
a single sperm, and share an identical genetic inheritance.
DZ twins develop from two separate eggs, fertilized
by two separate sperm, and are in effect simply
two siblings that happen to be born simultaneously. As
such, DZ twins share the same average of 50% of genetic
material as do any two biological siblings. It is this
ﬁxed ratio (two to one) of genetic similarity between
MZ and DZ twins, and the contrasting average equivalence
of environment inﬂuence, that provides most of
the power of twin designs. It is important to appreciate
that the assumption of environmental equivalence is
one of equivalence across types of twins, not across
pairs of twins or across twins within a given pair. For
example, there is undoubtedly at least some variability
in parental socialization across siblings, even those of
identical age, but across multiple twin pairs the assumption
is that this variability is essentially equal for the
MZ and the DZ pairs.
This assertion that the effect of genetics is measurably
distinct for MZ and DZ twins, while the effect of
the environment is either equivalent or at least randomly
distributed around equivalence, is crucial to everything
that follows from twin research. It is important
therefore to raise and consider the criticisms of this
fundamental assumption. The arguments come in two
essential varieties. The ﬁrst is that MZ twins, genetics
aside, experience a more similar environment because
they are treated more similarly than are DZ twins. This
would seem particularly telling for childhood socialization,
where, for example, parents might show less of a
tendency to treat MZ twins as individuals compared
to DZ twins. The second is that MZ twins, genetics
aside, interact with each other more throughout life
than do DZ twins. This would seem to be of particular
importance for adult socialization, where closer adult
contact between MZ twins might lead us to expect a
greater degree of environmentally induced similarity
than we would see for the more distant DZ twins.
Both caveats have been subject to sustained and varied
investigation and neither has been found to hold up
under empirical scrutiny. The argument of more similar
treatment fails on several fronts. Parents frequently
miscategorize their twins (DZ twins are often believed
by their parents to be MZ twins) and the differential
correlation persists in these instances of miscategorization.
In other words, the degree of correspondence between
MZ twins surpasses that of DZ twins even in the
large subpopulation of twins thought by their parents
to be MZ twins (Bouchard and McGue 2003; Bouchard
et al. 1990; Plomin 1990). The contention that MZ
twins have closer or more frequent contact than DZ
twins turns out to be at best irrelevant. The correlation
between the frequency of contact between twins
and the similarity between twins on all attitudinal and
behavioral variables tested, including conservatism, is
slight and actually negative (Martin et al. 1986). In
other words, twins in greater contact with their cotwins
are not more likely to share the same attitudes and
behaviors, so even if MZ twins have more contact than
DZ twins, this contact is not the cause of any elevated
correlations. But the most powerful refutation of both
of these criticisms comes in recent studies utilizing MZ
and DZ twins raised apart. These studies uniformly validate
MZ and DZ differences found in earlier studies
of twins raised together. Arguments about the relative
degree of shared environmental effects between MZ
and DZ twins simply offer no credible explanation if
the twins in question have been raised apart (Bouchard
1998; Bouchard et al. 1990). In effect, this naturally
occurring, if uncommon, condition provides precisely
the sort of laboratory control that we would want in an
experimental setting.3
Other evidence against the exclusive environmental
argument is that the empirical results suggest MZ twins
reared together are often less likely to share behavioral
traits with their twins than are MZ twins reared apart,
presumably because of extra efforts to establish distinct
identities when the twins live together. In addition, as
adult MZ twins living apart age, they tend to become
more, not less, similar (Bouchard and McGue 2003), a
ﬁnding that is difﬁcult to reconcile with the belief that
only the environment matters. Interestingly, this precise
effect is predicted in an early landmark criticism of
behaviorism and the conditioned response research on
animal behavior that formed its empirical core. Over
time, substantial anomalies began to accumulate in this
research pointing toward a primacy for some nonenvironmental
behaviors. Breland and Breland (1961)
summarized this tendency with the phrase “learned
behavior drifts toward instinctive behavior” (684).
Given the genetic differences and environmental
similarities of the two types of twins, for any trait
that is partly heritable the tendency for MZ twins to
share that characteristic should be stronger than the
tendency for DZ twins to share that characteristic.
In contrast, characteristics that arise purely from the
environment, whether shared by the twins, as would
typically be the case for parental socialization, or not
3 To explain this ﬁnding, opponents would need to argue that adoption
agencies are more likely to place MZ twins in similar homes
than they are to place DZ twins in similar homes. In fact, information
on twin zygosity is typically unavailable to those making placement
decisions, and even if it were available, it seems highly unlikely that
it would factor into their decisions.
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Are Political Orientations Genetically Transmitted? May 2005
shared by the twins, as would be the case for many
adult experiences, should not generate any signiﬁcantly
different patterns when we contrast MZ and DZ twins
(see Eaves, Eysenck, and Martin 1989 and Plomin et al.
2001 for a thorough discussion of the relevant statistical
techniques).
The procedures involved with the twin methodology
are standard fare in behavioral genetics but are
not familiar to most political scientists, so it is appropriate
that we explain the basic terminology, theory,
and technique in some detail. Inﬂuences on an individual
trait, whether it is a political attitude or a physical
characteristic, are typically divided into two broad
groups—–heredity (H) and environment (E). The total
variation in a trait can thus be represented as the sum
H + E. Heredity is the impact of genetic inheritance on
trait variation. In the case of a physical characteristic
such as adult height, this would be the proportion of the
total variation in height across individuals due to the
variation across individuals in the multiple genes that
control ultimate physical height. For any one individual,
the source of this genetic inﬂuence is relatively well
deﬁned, as on average 50% of our genes come from our
mother and 50% come from our father. This leads to
the fact that biological children of tall parents are more
likely to be tall than are the biological children of short
parents, though even for a relatively straightforward
additive physical trait like height, the relationship is
far from determinative.
“Environment” is all of the nongenetic external factors
that inﬂuence trait variation across a population.
These inﬂuences range broadly from the earliest biological
environment of the womb, to the physical environment
of a childhood house, to the social environment
of the adult workplace. In the case of adult height,
some of the obvious environmental factors are prenatal
nutrition, the adequacy of childhood and adolescent
diet, and exposure to chemical agents that can inhibit
growth.
Environmental inﬂuences can be further divided into
two subcategories: the shared environment and the
unshared, or unique, environment. The shared environment
is all of the shared external inﬂuences that
we would typically think of as leading to trait similarity
between individuals. Siblings, for example, might
share similar childhood environments, including similar
parental interactions, a similar physical environment,
and similar nutrition. If the siblings happen to
be twins, they would also share a more similar prenatal
environment.4
In the case of adult height, a shared
environmental factor, such as a regional diet limited in
protein and speciﬁc nutrients, could lead to similarity
in height across the entire population of a region.
The unshared environment is all of the distinctive
external inﬂuences that we would typically think of as
4 However, recent research suggests that the prenatal environment
is so important that it can cause variation even in fetuses inhabiting
the uterus at the same time. Prescott, Johnson, and McArdle
(1999) present evidence that MZ twins sharing the same chorion, the
outermost extraembryonic membrane, are more similar in terms of
personality and cognitive abilities that MZ twins in separate chorions.
leading to trait dissimilarity across individuals. While
much of the early childhood environment, for example,
is similar across siblings, much is nonetheless variable.
Siblings differ in diet, disease exposure, peer inﬂuences,
and a host of other unique experiences. Even twins,
whose childhood environment is made increasingly
similar by virtue of their identical age, are exposed to
substantial unique external inﬂuences. With the shift to
adult life, the share of unique inﬂuences on siblings increases
sharply, as peer, workplace, family, and physical
settings typically diverge.
In the classic political science studies of socialization
(see, especially, Jennings and Niemi 1968, 1991
and Tedin 1974), the focus has been on the correlation
between the attitudes of parents and their children. In
terms of the three sources of trait variability outlined
above, as informative as it is, this design does not allow
for an unambiguous estimation of any of the three categories.
The correlation between a parent and a child
arises from a combination of shared genes, shared environment,
and parental socialization (an indirect form
of shared environment in which the parent’s attitudes
provide a path from the parent’s environment to the
child’s environment), all of which are pressures toward
similarity in parent–child attitudes. The failure of this
parent–child correspondence to reach +1.0 presumably
reﬂects the pressure toward dissimilarity coming
from the unshared environment, but since the genetic
similarity of a parent–offspring pair is only .5, there
is as much genetic dissimilarity as there is similarity.
Thus, trait dissimilarity, like trait similarity, is an undetermined
mixture of genetic and environmental inﬂuences.
Our inability to tease apart genetic heritability
and environment, whether shared or unshared, in these
parent–child studies is a direct result of the fact that
there is no measured variation in genetic similarity
across the data set of parent–child pairs (i.e., all biological
offspring share the same average of 50% of
the variable genetic code with each parent).
This inability of standard parent–child observations
to distinguish genetic heritability from parental socialization
(or other features of the shared environment) is
something that has long been understood, but largely
ignored in modern social science. Fortunately, twins
provide a powerful “natural experiment” by introducing
known genetic variation into analyses of the sources
of trait variability. By shifting the focus from the similarity
between parents and offspring to the similarity
between two siblings, we can take advantage of the
fact that some siblings vary in well-known ways in the
degree of their genetic correlation.
POLITICS AND GENETICS: PREVIOUS
FINDINGS AND OUR EXPECTATIONS
Comparisons of the correlations of MZ and DZ twins
on a wide variety of variables have been conducted,
with intriguing results. Using appropriate modeling
techniques including controls for parental traits and
assortative mating, it is possible to partition the explanatory
powers of heredity, shared environment, and
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American Political Science Review Vol. 99, No. 2
nonshared environment on any given variable. These
techniques have been valuable for epidemiological
traits, intelligence, personality, social attitudes such as
those connected to religion, psychological interests,
and behaviors such as risk-taking propensities (for a
thorough review, see Bouchard and McGue 2003). Of
most interest to us are the ﬁndings pertaining to social
attitudes and behaviors. At ﬁrst, researchers were so
conﬁdent that social attitudes were not heritable that
they employed such items as controls. Quickly they
discovered that other controls would have to be found
because most social attitudes consistently displayed
a surprising measure of heritability (see, e.g., Crelia
and Tesser 1996, Scarr and Weinberg 1981, and Tesser
1993).
Political attitudes were never a central focus in this
research stream but many of the patterns found in other
social attitudes should be present for political attitudes
as well, and this assumption guided the formulation
of our expectations. Since the social attitudes tested
to date have demonstrated a strong heritable component,
frequently stronger than attitude covariance attributable
to shared environment, we predict that political
attitudes will also be heavily heritable. Heritability
estimates calculated by previous researchers for attitudes
associated with psychological conservatism are
quite high, while the relevant models typically show
little or no effect for shared environment (the remainder
is likely the result of nonshared environmental
factors). Notably, these ﬁndings come from studies of
twins in settings as disparate as Australia, Virginia, and
Minnesota, and the ﬁndings of the Minnesota study,
utilizing twins reared apart, conform well to the
other studies of twins raised together (for a summary,
see Bouchard and McGue 2003).5
Careful studies of
adopted children conﬁrm the ﬁnding that genetics matter
more than parentally created environment in inﬂuencing
social attitudes and behaviors, personality traits,
and intelligence.6
We further predict that attitudes on political issues
tracking most closely to central personality traits
should be the most heritable since personality traits
are generally heritable and since the heritability of social
attitudes is likely derivative of the heritability of
various personality traits (see Bouchard and Loehlin
5 Conservatism is not unusual in this regard. Rushton, Littleﬁeld, and
Lumsden (1986, 7340) ﬁnd that approximately 50% of the variance
in altruism is the result of “direct genetic inheritance,” with family
environment responsible for 0%.
6 Adoption studies measure the correlation of biological parents and
adopted children where the biological parents have had no contribution
to the rearing (environment) of the child. The most recent adoption
study, utilizing surveys of Korean-American adoptees randomly
assigned to families in the United States, concludes that roughly 75%
of variance in children’s educational attainment is attributable to the
educational attainment of their biological parents, and only 25% is
attributable to the adoptive parents, thus dramatically conﬁrming
the earlier ﬁndings of a substantial correlation between biological
parents and adopted children and a surprisingly paltry correlation
between adoptive parents and children (Sacerdote 2004). This parallels,
with an entirely distinct methodology, the basic ﬁnding of the
twin studies (see Plomin et al. 1997, 1998 and Rhee and Waldman
2002).
2001 and Eaves, Eysenck, and Martin 1989). For example,
one of psychology’s “Big 5” personality traits is
general “openness” and it seems likely degree of openness
is relevant to the political arena as well. Liberals
and conservatives, on average, differ in their openness
to atheism, homosexuality, communism, immigration,
and countercultural activities. These differences may
be entirely due to enculturation, but then again, they
may not be, and we will never know without testing for
the effects of genetics.
Based on behavioral geneticists’ study of religion,
it seems that group identiﬁcation is something that
is heavily inﬂuenced by the environment, especially
shared environment, and is mostly unconnected to genetics.
Children of Methodists are likely to be Methodists
not because there is a gene for Methodism or even
a personality particularly oriented toward Methodism,
but because of parental socialization. Thus, even as attitudes
connected to religiosity and religious beliefs and
activities (e.g., Sabbath observance, church authority,
belief in heaven, religious fundamentalism, frequency
of attendance) were found to be shaped more by genetic
inheritance than by parental views on those issues
(for details, see Bouchard et al. 1999, Eaves, Martin,
and Heath 1990, Maes et al. 1999, and Martin et al.
1999), identiﬁcation with a particular religious group
was shaped more by socialization and almost not at
all by genetics. We expect to ﬁnd a similar pattern
with political party identiﬁcation. Children are eager
to belong to the groups their parents belong to and
parents are frequently eager to encourage children in
this regard. Assuming these identiﬁcations have some
stickiness into early adulthood, our core expectation
is that party identiﬁcation will be inﬂuenced more by
parental socialization (shared environment) than by
genetic inheritance but that this pattern will be reversed
for political attitudes with inheritance playing
a role at least as large as the shared environment. By
predicting a large inﬂuence for genetic inheritance, we
depart from typical behavioralist expectations anticipating
that political attitudes will be predominantly
inﬂuenced by environmental factors, rendering genetic
inheritance largely, if not completely, inconsequential.
DATA AND METHODS
Since twin studies have not been conducted by political
scientists, political attitudes have been at best
a sidelight, and properly reﬁned measures of political
variables have not been constructed and employed (the
heritability of political behavior has not been analyzed
at all). Nonetheless, some previously employed variables
in twin studies have political relevance. For example,
the heritability of conservatism is frequently assessed
(see, e.g., Bouchard et al. 1990, Eaves, Eysenck,
and Martin, 1989, and Martin et al. 1986), and even
though conservatism is viewed by the scholars who
do twin studies more as a psychological trait than a
political ideology, measures of it include political items.
Of most relevance here is the Wilson–Patterson
(W–P) Attitude Inventory. This inventory is administered
by presenting subjects with a short stimulus
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Are Political Orientations Genetically Transmitted? May 2005
phrase such as death penalty or royalty and eliciting
a simple agree, disagree, or uncertain response. The
broadest version of the W–P inventory includes 50
items, 25 of which contribute positively to the conservatism
score and 25 of which contribute negatively
to the conservatism score. While some of the items relate
to a heavily social conception of conservatism—–for
example, pajama parties, nudist camps, computer music,
and horoscopes—–others have a much more direct
political content—–for example, disarmament, socialism,
patriotism, and death penalty. Studies typically
utilize reduced sets of W–P items or modify individual
items to better suit the country in which the items are
being administered. For political science this presents
two frustrations. The list of politically relevant items
is tantalizing but limited and unfocused, and the results
are often presented only for the entire combined
scale, making it difﬁcult to assess the contribution
of the directly political items to the overall index of
heritability.
We were granted access to the data for the W–P items
in the United States and were able to conduct comparable,
though more limited, twin correlation analyses
from published results of an Australian study.7
The U.S.
study included information on thousands of twin pairs
in Virginia, supplemented with twin pairs recruited
through the cooperation of AARP. A subset of these
twins and their relatives has been asked questions regarding
their social attitudes, including numerous items
from the W–P inventory.
A brief explication of twin methodology should help
readers make independent sense of the tables. The
standard techniques in behavioral genetics are based
on correlation analysis (in the case of limited response
items like the W–P inventory, the actual measure is the
polychoric correlation coefﬁcient, a technique that is
appropriate when individual subjects are using a limited
set of categories to express location on what is in
7 Our thanks go to Professor Lindon Eaves at Virginia Commonwealth
University for making the VA30K data available to us. The
data collection methods for both studies are summarized in Lake
et al. 2000 as follows: “The Australian sample was ascertained
through two cohorts of twins. The ﬁrst cohort was recruited in 1980–
1982 from a sampling frame which comprised 5967 twin pairs aged
18 years or older (born 1893 to 1964) then enrolled on the Australian
NHMRC Twin Registry (ATR). Responses were obtained
from 3808 complete pairs . . . and these were followed up with a second
mailed questionnaire in 1988–1990 with responses from 2708
complete pairs. . . . The second cohort of twins, born 1964–1971, was
recruited from the ATR in 1989 and was mailed similar questionnaires
in 1989–1991, with responses from 3,769 individuals of 4269
eligible pairs. . . . In total there were 21,222 respondents in the Australian
sample, of whom 20,945 had valid scores for EPQ Neuroticism.
The United States twins were ascertained from a populationbased
birth registry for the Commonwealth of Virginia and from
a volunteer sample through the American Association of Retired
Persons (AARP), described in detail by Truett et al. (1994). Their
ﬁrst-degree relatives and spouses were recruited in a similar fashion
to the Australian sample, and in total there were 24,905 respondents
(of 29,080) with valid scores for Neuroticism and for whom the zygosity
of the proband twins could be determined. The response rates
were 70% for twins and 45% for relatives” (224–25). The original
U.S. twin data collection was funded in part by NIH grants GM30250
and AG04954, by ADAMHA grants AA06781, AA07728, AA07535,
and MH40828, and by a gift from R. J. R. Nabisco.
fact a continuous trait). The correlations are computed
separately for male/male and female/female twin pairs
to provide an appropriate comparison, since all MZ
twins are same-sex pairs, while DZ twins are a mix
of same-sex and opposite-sex pairs (in other words,
female/male DZ twin pairs are excluded from the analysis).
Without this control, the presence of any male/
female differences would spuriously deﬂate the correlations
for DZ pairs relative to the same-sex MZ
pairs.
Heritability is typically estimated by subtracting the
correlation for DZ pairs from the correlation for MZ
pairs and then doubling the resulting difference. At one
extreme, if the correlations are the same for MZ and
DZ pairs, suggesting that genetic similarity plays no
role in similarity for that particular trait, then the result
will be an estimate of heritability of zero. At the other
extreme, a purely genetic additive trait should produce
a correlation of .5 for DZ pairs and 1.0 for MZ pairs, resulting
in an estimate of heritability of 1.0 (1.0 − .5 = .5,
and 2 ∗ .5 = 1.0). In a similar way, we can estimate the
inﬂuence of shared environment, as opposed to shared
genetic material, by doubling the correlation for DZ
pairs and then subtracting the correlation for MZ pairs.
Again, a purely genetic additive trait should produce
a correlation of .5 for DZ pairs and 1.0 for MZ pairs,
resulting in an estimate of the impact of shared environment
of zero (2 ∗ .5 = 1.0, and 1.0 − 1.0 = 0). At
the other extreme, if the correlations are the same for
MZ and DZ pairs, suggesting that genetic similarity
plays no role in similarity for that particular trait, then
the result will be an estimate of the impact of shared
environment that is equal to the MZ or DZ correlation
(e.g., if MZ = DZ = .4, then 2 ∗ .4 = .8, and .8 − .4 = .4).
Whatever is left over is taken to be attributable to the
unshared environment.
THE HERITABILITY OF POLITICAL
ATTITUDES
Table 1 contains the results of a standard polychoric
correlation analysis for the 28 W–P items available in
the Virginia 30K data set and for a select set of additional
items to provide some sense of perspective for
the level of these correlations. Even the quickest glance
at the results in Table 1 is enough to set aside the traditional
view that genes do not play any role in explaining
political attitudes. All 28 of the MZ correlations are
larger than their corresponding DZ correlations, and
in every case the difference is statistically signiﬁcant at
the .01 level. Far from typically being at or near zero,
none of the 28 heritability estimates falls in the single
digit range, and more than half of the 28 items have
heritability estimates of .3 or more. Heritability ranges
from a high of .41 to a low of .18, all suggesting that the
inﬂuence of heredity on political attitudes is very real,
and given the diverse range of items included here,
this genetic inﬂuence is also pervasive. So the view
that heritability of social and political attitudes will be
nonzero but small relative to shared environment is
also called into question. We see from Table 1 that the
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American Political Science Review Vol. 99, No. 2
TABLE 1. Genetic and Environmental Inﬂuences on Political Attitudes: The 28 Individual
Wilson–Patterson Items
Polychoric Correlation
MZ DZ Shared Unshared
Heritability, Environment, Environment, z for (MZ–DZ)
Attitude Item Corr. n Corr. n 2 ∗ (MZ − DZ) (2 ∗ DZ) − MZ 1 − MZ Differencea
School Prayer 0.66 2,687 0.46 1,774 0.41 0.25 0.34 9.83
Property Tax 0.47 2,643 0.27 1,748 0.41 0.06 0.53 7.66
Moral Majority 0.42 2,614 0.22 1,717 0.40 0.03 0.58 7.16
Capitalism 0.53 2,609 0.34 1,720 0.39 0.14 0.47 7.85
Astrology 0.48 2,631 0.28 1,721 0.39 0.09 0.52 7.39
The Draft 0.41 2,641 0.21 1,753 0.38 0.02 0.59 6.94
Paciﬁsm 0.34 2,576 0.15 1,686 0.38 −0.04 0.66 6.43
Unions 0.44 2,661 0.26 1,752 0.37 0.07 0.56 6.89
Republicans 0.48 2,627 0.30 1,734 0.36 0.12 0.52 6.91
Socialism 0.43 2,616 0.25 1,726 0.36 0.07 0.57 6.53
Foreign Aid 0.41 2,669 0.23 1,771 0.35 0.06 0.59 6.42
X-Rated Movies 0.63 2,685 0.46 1,783 0.35 0.28 0.37 8.15
Immigration 0.45 2,658 0.29 1,748 0.33 0.12 0.55 6.20
Women’s Liberation 0.46 2,666 0.30 1,779 0.33 0.13 0.54 6.27
Death Penalty 0.56 2,684 0.40 1,775 0.32 0.24 0.44 6.83
Censorship 0.40 2,629 0.25 1,718 0.30 0.10 0.60 5.36
Living Together 0.67 2,679 0.52 1,771 0.30 0.37 0.33 7.54
Military Drill 0.38 2,625 0.24 1,733 0.29 0.09 0.62 5.24
Gay Rights 0.60 2,658 0.46 1,767 0.28 0.32 0.40 6.26
Segregation 0.38 2,653 0.24 1,743 0.27 0.11 0.62 4.83
Busing 0.43 2,670 0.30 1,766 0.26 0.16 0.57 4.92
Nuclear Power 0.42 2,646 0.29 1,744 0.26 0.16 0.58 4.84
Democrats 0.47 2,639 0.34 1,726 0.26 0.21 0.53 4.96
Divorce 0.47 2,659 0.34 1,765 0.26 0.21 0.53 4.99
Abortion 0.64 2,668 0.52 1,768 0.25 0.39 0.36 6.23
Modern Art 0.43 2,662 0.30 1,765 0.25 0.18 0.57 4.69
Federal Housing 0.36 2,665 0.26 1,766 0.20 0.16 0.64 3.61
Liberals 0.44 2,629 0.35 1,734 0.18 0.26 0.56 3.40
28-item mean 0.47 2,648 0.31 1,748 0.32 0.16 0.53
Source: Access to the data provided by Eaves et al., principal investigators, Virginia 30K twin study (see note 7).
a The MZ–DZ correlation difference is statistically signiﬁcant for all of the table items at the 0.01 level or above.
impact of shared environment exceeds that of heredity
for only four of the 28 items, and the mean estimate of
heritability for the 28 W–P items is .32, compared to
a mean estimate of shared environmental inﬂuence of
.16.
The second-to-last column in Table 1 reports the estimates
for the proportion of the variation in an attitude
that is attributable to the unshared environment. As
described above this is essentially a residual variance
category, reﬂecting such factors as random choice as
well as external inﬂuences such as the unique experience
of each individual, including those from childhood,
and later inﬂuences in life that have been termed
“adult socialization” in the political science literature.
Across the 28 W–P items the estimate of the impact
of unshared environment varies from about one-third
(for School Prayer) up to about two-thirds (for Paciﬁsm)
of the overall variation. The average impact of the
unshared environment for these items is .53, or roughly
half of the overall variation. The summary picture for
this set of political attitudes, then, is that shared inﬂuences
(genetic and environmental) account for about
half of the variation in these political reactions, with
unique individual and environmental factors accounting
for the remainder. Within the half that is accounted
for by shared inﬂuences, genetic inﬂuences, in contradiction
to behavioralist expectations, are roughly twice
as inﬂuential as environmental inﬂuences.
While the individual items provide interesting variation,
the purpose of the W–P inventory is to provide
an overall index of conservatism. We compute a simple
index by assigning a value of +1 to any “conservative”
response (i.e., a “yes” to an item like Death Penalty
or a “no” to an item like Women’s Liberation) and −1
to any “liberal” response (i.e., a “no” to an item like
Death Penalty or a “yes” to an item like Women’s
Liberation). Items where the respondent chose a noncommital
(?) response are coded as zero. When these
individual scores are summed across the 28 items they
yield an index that varies from a potential low of −28
(indicating a set of uniformly “liberal” responses) to
a high of +28 (indicating a set of uniformly “conservative”
responses). The actual index scores for the
twins in the study range from −26 to +26, with the
median response falling between +2 and +3. Given
the far more continuous nature of this overall index,
we can now utilize the more traditional Pearson’s correlation
coefﬁcient. The results for the overall index
159
Are Political Orientations Genetically Transmitted? May 2005
TABLE 2. Genetic and Environmental Inﬂuences on Political Attitudes: Summary Index and
Additional Non–Wilson–Patterson Items
Correlation
MZ DZ Shared Unshared
Heritability, Environment, Environment, z for (MZ–DZ)
Attitude Item Corr. n Corr. n 2 ∗ (MZ − DZ) (2 ∗ DZ) − MZ 1 − MZ Differencea
Pearson’s correlation coefﬁcient
28-item index score 0.65 2,107 0.43 1,384 0.43 0.22 0.35 8.93
Partial corr. for parent, 0.64 173 0.37 131 0.53 0.11 0.36 3.08
same index
Opinionation 0.39 2,107 0.20 1,384 0.36 0.02 0.61 5.77
Polychoric correlation coefﬁcient
28-item mean 0.47 2,648 0.31 1,748 0.32 0.16 0.53
Educational Attainment 0.86 2,683 0.66 1,771 0.40 0.46 0.14 16.40
Party Afﬁliation 0.55 2,417 0.48 1,554 0.14 0.41 0.45 2.99
Mean of affect toward Reps. 0.48 2,633 0.32 1,730 0.31 0.17 0.52 5.94
and Dems.
Source: Access to the data provided by Eaves et al., principal investigators, Virginia 30K twin study (see note 7).
a The MZ–DZ correlation difference is statistically signiﬁcant for all of the table items at the 0.01 level or above.
are presented in Table 2 and clearly support a powerful
role for heredity in inﬂuencing conservatism, at least
as measured by the W–P inventory. The estimate for
heritability is .43, higher than for any of the individual
items. The estimate for shared environment is .22,
falling within the upper range of the individual items,
while the estimate for unshared environment is only
.35, falling very near the bottom of the range for individual
items. The overall picture is again a very strong
role for heredity and a less powerful, but clear role for
shared environment. What is different for the overall
index is that the role of shared inﬂuences (genetic and
environmental) account for almost two-thirds of the
variation in the index (compared to about one-half
for the individual items), with unique individual and
environmental factors accounting for only about onethird
of the variation. This decline in the role of unique
individual and environmental factors seems sensible,
as we are moving from individual and highly speciﬁc
items that could involve a host of unique experiential,
associational, and informational perturbations to an
index where those idiosyncratic features of individual
items have the opportunity to cancel each other out.
The W–P items can also be used to construct a rough
index of political opinionation by taking advantage of
the frequency of ? responses. The number of times
that a respondent chose a yes or no response over a
neutral ? response was summed to produce an index
that varies from zero to 28, with a 28 indicating that the
respondent was willing to express a directional opinion
on all 28 items and a score of zero indicating that the
respondent was unwilling to offer a directional opinion
on any of the 28 items. The median for this index
is 21 yes or no response choices of 28 possible. The
results for the overall index clearly support a powerful
role for heredity in inﬂuencing political opinionation,
at least as it is captured by the admitted rough
gauge of the frequency of nonneutral responses to the
W–P inventory items. The estimate for heritability is
.36 and the estimate for shared environment is only
.02. The estimate for unshared environment is high,
at .61, falling near the top of the range for individual
items. To the extent that there is a family effect on
political opinionation, it would appear to be entirely a
genetic one, with the remaining roughly two-thirds of
the variation being due to nonshared factors.
Two items from the survey that are not a part of the
W–P inventory are included in Table 2. Party afﬁliation
is the most clearly political of the items in the broader
questionnaire, and it is useful here on its own, as well
as in contrast to the attitudinal items. Party identiﬁcation
is distinct among U.S. political attitudes both
in our conception of it as an identiﬁcation, and hence
as something at least potentially distinct from simple
item evaluation, and in its established tendency to correlate
well between parent and child (see Jennings and
Niemi 1968). This distinctiveness is apparent in Table 2.
As we expected, the pattern for party identiﬁcation is
nearly the exact reverse of that for the average attitude
item. Heritability for party afﬁliation is relatively low
(r = .14), while shared environment is much stronger
(r = .41). Note also that not one of the 28 W–P items
has an average heritability that is as low as that for party
afﬁliation, and likewise, not one of the 28 items has an
average coefﬁcient for the impact of shared environment
that is as high as that for party afﬁliation. Clearly,
party identiﬁcation is, at least for the United States, a
different sort of beast than reactions to issue items.
In this regard it is particularly interesting that the
two major parties also appear in the W–P battery, but
here they are objects of affect rather than labels of possible
identiﬁcation, and the “pro” or “con” reactions to
the parties that these items pick up do not exhibit the
same patterns of genetic and environmental inﬂuence
that we see for party afﬁliation. In fact, if we average
the polychoric correlation for the “Democrats” item
with the correlations for the “Republicans” item and
compute the resulting estimates we get a heritability
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estimate of .31 and a shared environment estimate of
.17, almost exactly the same as the mean results for all
28 attitude items. It would appear that affect toward
the major parties is largely a matter of genetic predisposition
but that, just as the political socialization literature
has concluded all along, party identiﬁcation itself
is primarily the result of parental socialization. This
pattern is intriguing in and of itself but it also should
give pause to those who would dismiss the ﬁndings
on attitude items as the product of some methodological
quirk of twin studies. If estimates of heritability
are somehow artiﬁcially inﬂated, why does this alleged
contamination not occur for party identiﬁcation?
Table 2 also reports the results for a summary
indicator of educational attainment from the survey.
We include it here partly because it reﬂects an actual
behavior, if only a self-reported one, and partly because
it carries the role of genetics more directly into
the world of actual and meaningful social variation.
Educational attainment is also useful as an example
of a behavior that is traditionally thought to be heavily
inﬂuenced by shared environment, particularly by
parental example, expectations, and resources. This traditional
view is supported by the shared environment
estimate of .46, a ﬁgure higher than any of the estimates
for the 28 attitude items and even somewhat higher
than the estimate for party ID. What may surprise
readers is that as important as shared environment is
to educational attainment, heredity, at .40, is almost as
important. Taken together, family effects are almost
the entire story for variation in education attainment.
The estimate for the impact of the unshared environment
is only .14, a value markedly lower than any other
in the table.
ASSORTATIVE MATING
Assortative mating is a particular concern here. As detailed
above, the assumption that DZ twins, like any
other pair of biological siblings, share on average 50%
of the variable genetic code is crucial to the estimation
of heritability. This contrasts with MZ twins, where the
shared proportion is 100%, and the DZ level forms
the baseline for separating genetics from the shared
environment. What may not be immediately apparent
is that the assumption that purely genetic traits in DZ
twins will on average correlate at .50 is itself built on the
assumption that their biological parents will on average
correlate at .00 for the same traits. In other words, the
assumption is that the parents are not related to each
other in any close degree, and this is typically true, as
close relatives generally do not mate, and the amount
of average shared genetic code drops geometrically as
we move away in relatedness and quickly approaches
zero. This assumption that mates are genetically uncorrelated
on the trait of interest is, however, violated if
mate choice is itself based on the trait of interest. If,
for example, parents have identical genetic codes for a
trait of interest, then the shufﬂing of that genetic code
produced by sexual reproduction will not result in any
variation among DZ twins, or any other siblings, with
regard to their genotype for that trait. In other words,
DZ twins of these parents will be as genetically alike
on this one trait as MZ twins are on this trait. Across a
study population, the higher the proportion of spouses
that share identical genetics for a trait, the closer the
DZ correlation will be to the MZ correlation. Since
heritability of a trait is estimated as 2 ∗ (MZ − DZ), the
increased similarity of DZ and MZ pairs will lead to an
underestimation of heritability for this genetic trait.
This is important for our assessment of the heritability
of political attitudes. If there is a tendency for people
to choose mates with similar positions on political issues,
then the estimates of heritability in Tables 1 and 2
are biased. Fortunately for us, the direction of the bias
is uniform and conservative. Any measurable tendency
toward assortative mating on political orientation will
push up the DZ twin correlation while leaving the
MZ correlation unaffected, and this reduction in the
MZ–DZ gap will have the related effect of lowering
estimates of heritability. Note also that any increase in
similarity of DZ twins will inﬂate the estimate of the
importance of shared environment, as the estimation
formula of (2 ∗ DZ) − MZ makes clear.
The immediate empirical question is how much of
a role assortative mating plays in political issue positions.
A quick answer can be found by looking at the
interspouse polychoric correlations for the individuals
included in the Virginia 30K study. The average interspouse
polychoric correlation for the 28 items is .41 and
the individual correlations range from a low of .26 for
Censorship to a high of .64 for School Prayer. While
some of this interspouse similarity could plausibly be
attributed to persuasion effects taking place after mate
choice rather than to assortative mating, the levels of
similarity are probably too high to dismiss assortative
mating entirely. This is conﬁrmed by a preliminary look
at the impact of controlling for assortative mating on
these 28 attitude items. The Virginia 30K study includes
data for parents of twins in the study, including
parents’ individual responses to the same W–P items
that the twins responded to. The usable sample size
does drop substantially when we restrict our analysis
to only twin pairs with completed W–P results for both
parents (there are a total of 304 pairs of male/male
or female/female twins with complete twin and parent
W–P data, compared to approximately 4,400 pairs in
the twin only analysis in Table 1). This effectively limits
us to an assortative mating analysis that focuses on the
overall index score, rather than looking at each item in
the inventory individually.
The approach we used is to compute the partial correlation
for twin similarity in the overall index for the 28
W–P items, controlling for (partialing out) the inﬂuence
of the degree of parental similarity on the overall index.
The implication for relative twin agreement is simple;
if parental agreement results from assortative mating,
then the resulting increase in genetic similarity will
increase DZ twin correlations (the more alike genetically
the parents are on a trait, the more alike siblings
will be on a trait). Controlling for parental similarity
will therefore reduce the size of the DZ twin correlations.
However, parental agreement resulting from
assortative mating and the resulting increase in genetic
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Are Political Orientations Genetically Transmitted? May 2005
TABLE 3. Comparison of Australian and U.S. Estimates of Genetic and Environmental Inﬂuences
on Political Attitudes
Virginia 30K Data Australian Data
Shared Shared
Heritability, Environment, Heritability Environment
2 ∗ (MZ – DZ) (2 ∗ DZ) – MZ 2 ∗ (MZ – DZ) (2 ∗ DZ) – MZ
Attitude Item Male Female Mean Male Female Mean Attitude Item Male Female Mean Male Female Mean
Astrology 0.49 0.33 0.41 −0.01 0.14 0.06 Horoscopes 0.30 0.32 0.31 0.07 0.18 0.13
Paciﬁsm 0.46 0.34 0.40 −0.09 −0.03 −0.06 Disarmament 0.26 0.48 0.37 0.09 −0.08 0.01
Censorship 0.55 0.20 0.37 −0.12 0.17 0.02 Censorship 0.30 0.48 0.39 0.11 −0.03 0.04
Socialism 0.33 0.36 0.35 0.05 0.08 0.07 Socialism 0.04 0.24 0.14 0.38 0.24 0.31
Military Drill 0.42 0.24 0.33 −0.02 0.12 0.05 Military Drill 0.32 0.42 0.37 0.20 0.08 0.14
Immigration 0.29 0.35 0.32 0.16 0.10 0.13 Nonwhite −0.06 0.20 0.07 0.50 0.23 0.37
Immigration
Death Penalty 0.27 0.35 0.31 0.27 0.19 0.23 Death Penalty 0.42 0.54 0.48 0.10 −0.03 0.04
Women’s 0.23 0.35 0.29 0.08 0.17 0.13 Working 0.22 0.32 0.27 0.15 0.16 0.16
Liberation Mothers
Segregation 0.34 0.23 0.29 0.04 0.14 0.09 Apartheid 0.44 0.38 0.41 0.09 0.08 0.09
Modern Art 0.31 0.21 0.26 0.11 0.22 0.16 Modern Art 0.40 0.22 0.31 −0.02 0.26 0.12
Abortion 0.26 0.24 0.25 0.29 0.44 0.36 Legalised 0.10 0.42 0.26 0.50 0.25 0.38
Abortion
Divorce 0.20 0.28 0.24 0.22 0.21 0.21 Divorce 0.16 0.44 0.30 0.31 0.10 0.21
Mean 0.35 0.29 0.32 0.08 0.16 0.12 Mean 0.24 0.37 0.31 0.21 0.12 0.16
Source: Access to the original U.S. data provided by Eaves et al., principal investigators, Virginia 30K twin study. Australian data
computed from Martin et al. 1986, Table 1, p. 4365.
similarity will not increase MZ twin correlations (MZ
twins are already genetically identical, regardless of
parental similarity or dissimilarity). Therefore, controlling
for parental similarity should have no effect on the
size of the MZ twin correlations. In contrast, if parental
agreement results from persuasion or from a shared environment
for the couple, then the impact of parental
agreement has no genetic implications and operates
on their offspring solely through its inﬂuence on the
offsprings’ shared environment. This should produce
relatively higher correlations of equal magnitude for
both MZ and DZ twins and, therefore, lead to roughly
comparable reductions in both the MZ and the DZ
correlations when we partial out the effect of parental
agreement.
The results for a partial correlation analysis controlling
for parental agreement are reported in Table 2,
on the row just below the results for the overall index.
For MZ twins the issue of whether their parents agree
or disagree on a particular item makes little difference
(.65 without control versus .64 after partialing out the
effect of parental agreement). In contrast, the correlation
between DZ twins decreases modestly when the
impact of parental agreement is removed (.43 without
control versus .37 after partialing out the effect of
parental agreement). Further, the tendency of assortative
mating to deﬂate estimates of heritability while
inﬂating estimates of the impact of shared environment
is clear. Without controls, the estimate of heritability
for the overall index is .43 and the average estimate
of the impact of shared environment is .22. When the
impact of parental agreement is partialed out, the average
estimate of heritability rises to .53, and the average
estimate of the impact of shared environment drops to
.11. Note that the traditional socialization account of
attitude formation is not at odds with this last ﬁnding.
If the issue positions of parents are in conﬂict, then we
would hardly expect this shared conﬂicted setting to
yield sibling agreement.8
COMPARATIVE POLITICAL GENETICS:
EVIDENCE FROM AUSTRALIA
Even with a data set as large as the Virginia 30K, questions
may arise over the extent to which conclusions
are bound by time and geography. As a result, it is
helpful to note results from a quite different context
and a slightly different time period. Table 3 presents
a comparison of the key summary results in Table 1
from the Virginia 30K study to comparable results in
the Australian data described before (Truett et al. 1994;
see also Lake et al. 2000). While the Australian study
8 The same sort of control for parental agreement that was applied
to the W–P inventory was applied to the party afﬁliation analysis. Because
this is only a single item, the results are much less reliable than
those averaged across the 28 items. However, despite the fact that
assortative mating clearly takes place with regard to party ID (only
24 of the 543 parent pairs had opposite party afﬁliations), the general
pattern of party ID being due more to shared environment than to
heredity holds up. Using a very broad deﬁnition of disagreement
(i.e., anything short of exact agreement on a ﬁve-point scale), the
shared environment estimate weakens modestly but remains high, at
almost twice the heritability estimate in the subset of twin pairs with
parents in some degree of disagreement on party afﬁliation.
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American Political Science Review Vol. 99, No. 2
utilized a larger set of W–P items (50 in all, compared
to 28 in the U.S. study), the items were a mix of political
and social items, and only six items appeared in
exactly the same form in both studies. An additional six
items were similar enough, in our judgment, to merit
comparison, and they are included in Table 2 with the
Australian wording italicized.
The broad picture from Table 3, and its comparison
to Tables 1 and 2, is one of remarkable similarity.
The mean heritability for the 12 item subset of the
Virginia 30K data is .32 for the full 28 items in Table 1
and .31 for the 12-item subset of the Australian data.
The mean estimate for the effect of shared environment
for the 12 item subset of the Virginia 30K data is .12
compared to .16 for the full 28 items in Table 1 and .16
for the 12-item subset of the Australian data. Thus the
general pattern of a relatively greater role for heredity
compared to shared environment detailed above in
the discussion of the U.S. data in Tables 1 and 2 also
applies to the Australian data in Table 3. While most
of the individual items also have broadly comparable
results in the two countries, a few, speciﬁcally “socialism”
and “immigration” (“nonwhite immigration” in
the Australian study), are noticeably different. In both
cases the U.S. pattern of substantially higher relative
heritability is reversed in the Australian data, where we
see evidence of relatively higher shared environmental
effects. Whether these are meaningful reﬂections of
differences in how these items relate to deeper political
orientations is not clear, but they are in any case the
exceptions rather than the rule.9
THE GENETICS OF POLITICAL IDEOLOGY
The possibility that attitudes and behaviors are inﬂuenced
by genetic variables is an emotionally charged
topic so it is important that readers understand the
claims being made. Partitioning the origins of human
traits, whether they be physiological or behavioral, into
the discrete, quantiﬁable components of genetic inheritance,
shared environment, and unshared environment
should not be taken to imply that these components
work separately. Rather these numbers only provide a
rough indication of the inﬂuence of three categories of
independent variables that are intimately intertwined.
(Moreover, they are estimates of the ability of independent
variables to account for variance in the dependent
variables not for the variables themselves.)
As mentioned earlier, gene–culture interaction is the
key to understanding the source of political attitudes
and behaviors, just as it is the key to understanding
most physical and behavioral aspects of the human
condition. Genes do not work in isolation and instead
generally inﬂuence the extent to which organisms are
9 In this case, the different results with regard to socialism could
reﬂect different meanings of the phrase in the two countries. In
Australia, the term socialism is closer to a party identiﬁcation label,
whereas in the United States it has more loaded ideological
connotations. Likewise, the addition of the qualiﬁer “nonwhite” to
immigration raises questions of what the key stimulus is.
responsive to particular environmental conditions (see
Boyd and Richerson 1985 and Masters 1993).
And this conditioning inﬂuence of genetics on complex
social behaviors is not the product of a single gene
but rather numerous genes that, to make matters more
complicated, appear to combine in conﬁgural as opposed
to additive ways. The same set of multiple genes
may inﬂuence behavior in different ways depending
on the order in which they express themselves and
the manner in which they interact with other genes.
Recent discoveries also suggest that biological markers
of phenotypic manifestations include the manner
in which DNA is packed in the nucleus, particularly
the physical location of genes relative to other genes
and to the histones that help to give DNA its structure.
An accurate understanding of gene expression appears
to require knowledge not just of the sequence of nucleotides
(e.g., ATCAGG) that constitutes the gene
itself but also of the context in which each gene resides,
thus forming an interesting parallel to the way
we must try to understand the organisms (e.g., human
beings) genes help to construct (for a good summary,
see Kosack and Groudine 2004; also see Lykken 1999).
Individual genes for behaviors do not exist and no
one denies that humans have the capacity to act against
genetic predispositions. But predictably dissimilar correlations
of social and political attitudes among people
with greater and lesser shared genotypes suggest
that behaviors are often shaped by forces of which
the actors themselves are not consciously aware, a
point that is made with some force by Bargh and
Chartrand (1999), Marcus (2002), Marcus, Neuman,
and MacKuen (2000), McDermott (2004), and Wegner
(2002). It is not biological determinism to posit the existence
of complex collections of genes that increase the
probability that certain people will display heightened
or deadened response patterns to given environmental
cues. And it is not antibehavioralism to suggest that
true explanations of the source of political attitudes
and behaviors will be found when we combine our currently
detailed understanding of environmental forces
with a recognition that genetic variables subtly but importantly
condition human responses to environmental
stimuli.
IMPLICATIONS FOR POLITICS
It is important to note that none of the data or arguments
presented in this paper indicates that extant empirical
knowledge about political socialization is useless.
In fact, it strongly reinforces many of the most
salient ﬁndings in that research stream. We know from
that research, for example, that if both parents share a
political identiﬁcation, there is a high degree of likelihood
that their offspring will have that same political
identiﬁcation (Jennings and Niemi 1968; Tedin 1974).
Our “twin study” results conﬁrm this ﬁnding. One
of the peculiar ﬁndings in the political socialization
literature even makes more sense when a role for genetic
inheritance in conceded. Scholars have occasionally
puzzled over the fact that family arrangements and
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Are Political Orientations Genetically Transmitted? May 2005
styles of operation have little if any impact on the extent
to which there is a match between parental and
offspring political attitudes on a wide variety of items
(see Jennings and Niemi 1968, 180–83). Fathers do not
have more inﬂuence over sons, and mothers do not
have more inﬂuence over daughters; fathers are not
generally more inﬂuential; the distribution of power
within the family is irrelevant to parent–child correlations
(i.e., neither highly autocratic, highly permissive,
nor middling arrangements affect the extent to which
attitudes are correlated); the degree to which children
and parents feel close to each other does not matter; the
frequency with which the family discusses politics does
not much affect correspondence between offspring and
parent views (though, as we would have predicted since
it is based on active socialization, party identiﬁcation is
more sensitive to family arrangements); and the extent
to which politics is important to the parents is also irrelevant.
Scholars grounded in traditional behavioralism
have difﬁculty accounting for these “perplexing
conﬁgurations” (Jennings and Niemi 1968, 183), but
recognizing that the correlations between the views of
parents and children derive more from genetics than
familial socialization makes it much less surprising that
the strength of these correlations is not reliant on family
arrangements (for an example of political science
work that does posit a role for genetics, see Peterson
1983).
Still, the substantive ﬁndings we present here offer
a direct challenge to common assumptions and
interpretations that political attitudes and behavioral
tendencies are shaped primarily or even exclusively
by environmental, especially familial, factors. Setting
aside the important special case of party identiﬁcation,
we ﬁnd that political attitudes are inﬂuenced much
more heavily by genetics than by parental socialization.
For the overall index of political conservatism, genetics
accounts for approximately half of the variance in
ideology, while shared environment including parental
inﬂuence accounts for only 11%. And in the case of
the variance in people’s tendencies to possess political
opinions at all, regardless of their ideological direction,
genetics explains one-third of the variance, and shared
environment is completely inconsequential.
What are the implications of these ﬁndings for political
science? Acknowledging a role for heritability
in politics affects our understanding of, ﬁrst, political
issues, second, political learning, and, third, political
cleavages. Inherited attitudes seem to be demonstrably
different than acquired attitudes. Tesser (1993) provides
evidence that attitudes higher in heritability are
manifested more quickly, are more resistant to change,
and increase the likelihood that people will be attracted
to those who share those particular attitudes. It has long
been known that certain political issues seem “hard”
to people, and others seem “easy,” presumably because
some issues trigger “gut responses” while others do not
(Carmines and Stimson 1980, 79), but no explanation
has yet been offered for why given issues do or do
not elicit gut responses. Why do social, more than economic,
issues tend to hit people in the gut, even though
both constitute ongoing and equally complex societal
concerns? In light of the new ﬁndings, one distinct possibility
is that easy “gut” issues tend to be those that
are more heritable.
To the extent that political ideologies are inherited
and not learned, they become more difﬁcult to manipulate.
Conservative parents who try to make their children
conservative by carefully controlling their children’s
environments are probably overestimating the
importance of those environments. Offspring of such
parents are likely to end up being conservative but
less because of the environment created by the parents
than the genes passed along by the parents. A political
match between parents and children should not be
taken to be the result of a socialization process—–that
is, the active postnatal transmission of views—–just as
political mismatches between parent and child should
not be taken as evidence against a role for genetics.
Parent–child mismatches are distinctly possible given
the uncertainties of meiosis (the random selection of
just 50% of each parent’s DNA) and the possibility for
occasional errors in the transcription and translation
of genes (mutations). These mismatches are likely to
be the primary cause of the fact that some children
rebel against the views of their parents but most do
not—–a pattern that environmental factors have never
explained satisfactorily.
Finally, we go into somewhat greater detail to illustrate
the manner in which results such as ours can
be of use in understanding the divisions characterizing
virtually all polities and, certainly, the United States
in the early twenty-ﬁrst century. Remember, genes inﬂuence
people’s outlooks and personalities, and it is
these broad features that then predispose individuals
toward suites of speciﬁc attitudes. This interpretation
likely explains the otherwise puzzling consistency in
ideological divisions that is present across space and
time. The package of attitudes held, for example, by
conservatives in the modern United States is remarkably
similar to that held by conservatives in other cultures
and at earlier times in American history (on the
durability of the liberal–conservative spectrum in the
United States, see Poole and Rosenthal 1997). Environmental
determinists have no convincing explanation
for the pervasiveness of this division but genetics
does.
If, as our results suggest, there is a genetic basis for
the varying political views people hold, and if, as seems
probable, genetic transmission frequently affects clusters
of political attitudes, we are likely to observe broad
but distinct political phenotypes. The number of these
phenotypes may vary, but for purposes of illustration
we discuss two probable orientations. One is characterized
by a relatively strong suspicion of out-groups
(e.g., immigrants), a yearning for in-group unity and
strong leadership, especially if there is an out-group
threat (“Do not question the President while we are
at war with terrorists”), a desire for clear, unbending
moral and behavioral codes (strict constructionists), a
fondness for swift and severe punishment for violations
of this code (the death penalty), a fondness for systematization
(procedural due process), a willingness to tolerate
inequality (opposition to redistributive policies),
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American Political Science Review Vol. 99, No. 2
and an inherently pessimistic view of human nature
(life is “nasty, brutish, and short”).
The other phenotype is characterized by relatively
tolerant attitudes toward out-groups, a desire to take
a more context-dependent rather than rule-based approach
to proper behavior (substantive due process),
an inherently optimistic view of human nature (people
should be given the beneﬁt of the doubt), a distaste
for preset punishments (mitigating circumstances), a
preference for group togetherness but not necessarily
unity (“We can all get along even though we are quite
different”), suspicion of hierarchy, certainty, and strong
leadership (ﬂip-ﬂopping is not a character ﬂaw), an
aversion to inequality (e.g., support for a graduated
income tax), and greater general empathic tendencies
(rehabilitate, don’t punish).
Common political usage would call the ﬁrst phenotype
conservative and the second liberal, but we seek
phrases that are less connected to political ideologies
and that indicate that these two phenotypes run to the
very orientation of people to society, leadership, knowledge,
group life, and the human condition. Thus, we
label the ﬁrst “absolutist” and the second “contextualist.”
This fundamental dimension offers a credible precursor
to basic cleavages manifested in a broad range of
human social activity: politics (conservatives/liberals),
religion (fundamentalists/secular humanists), law (procedural/substantive
due process), education (phonics/
whole language), art (traditional form-based realism/modern
free-form impressionism), sports (football/frisbee),
medicine (traditional AMA/wholistic),
morality (enduring standards/situational ethics), and
scientiﬁc inquiry (formal/empirical). In our view, all
of these vexing perennial dichotomies are related cultural
expressions of a deep-seated genetic divide in
human behavioral predispositions and capabilities. We
certainly are not asserting that everyone holds one of
these two orientations. Even if the individual genes involved
with absolutism or contextualism tend to move
together, this does not mean they always do. Some
individuals may carry, say, an absolutist’s aversion to
out-groups but a contextualist’s rejection of a universalistic
behavioral code. Moreover, genes not included
in these central packages, perhaps those related to extroversion,
ambition, and intelligence, often muddy the
waters.
More importantly, let us not forget that a heritable
component of 50% for political ideology and probably
somewhat higher for the absolutist-contextualist
dimension still leaves plenty of opportunity for the environment
to alter attitudes and behaviors—–and even
orientation. An individual with a contextualist genotype
who has been repeatedly victimized by out-group
members, or who has simply spent a great deal of time
listening to persuasive absolutists, may adopt attitudes
that run against type. Thus, even if a political system
started with two pure genotypes, it would soon display
a fascinating array of expressed orientations and
beliefs, intensity levels, and degrees of involvement
even as the system would continue to revolve around
the central division between absolutists and context-
ualists.
Such an account is speculative at this point but is
fully consistent with the ﬁndings presented here, with
previous research on the durability of political ideologies,
and with recent events in the United States.
Accounts of the 2004 election, for example, that do not
invoke this fundamental difference in orientation have
fallen ﬂat. Issues did not determine vote choice for the
many citizens who expressed disagreement with existing
economic policies and/or the war in Iraq yet still
voted for the incumbent president, George W. Bush.
Indeed, if the focus remains on issues, the resultant
description of the American public is grossly at odds
with reality. Morris Fiorina’s (2005) creative analysis
of survey responses indicates that Americans can be
placed in the middle on many important issues, but if
this is true, then what explains the vitriol and intensity
of feeling displayed by so many ordinary Americans in
2004?
Issues do not explain Americans’ politics. Many
Americans admit that they do not follow or understand
the issues (Hibbing and Theiss-Morse 2002), and
to the extent they do, they support whatever their
preferred politician and party seems to support (Page
and Jones 1979). In the 1990s, a Democratic president
(Bill Clinton) transformed welfare to workfare; then in
the 2000s, his Republican successor (George W. Bush)
greatly expanded federal involvement in both education
and the provision of prescription drugs for senior
citizens. If the enactors of these policies were reversed,
the groups of citizens displaying support for the policies
also would have reversed. Similarly, if a Republican
president had committed adultery with a young intern
or if a Democratic president had dramatically worsened
the deﬁcit and taken the country to war in a far-off
land on the basis of undeniably incorrect beliefs about
the opponents’ nuclear and chemical weapons capabilities,
the positions of most voters on the acceptability of
these conditions would be completely reversed. Issue
positions generally reﬂect divisions; they do not create
them.
Instead, the most accurate account of voting behavior
in 2004 moves beyond issues to the basic, partially
genotypic orientations described above. This sort of
broad orientation is not far removed from what most
commentators are trying to capture by reference to a
“moral” division in the electorate, but without tying it
to speciﬁc moral issues such as gay rights. The chasm inspiring
so much hostility between citizens of the United
States in the early twenty-ﬁrst century did not divide
supporters and opponents of privatizing Social Security;
it did not even divide supporters and opponents
of gun control. Rather, as has typically been the case,
it divided absolutists and contextualists.
And the prospects for eliminating this divide are
not promising. Since mate choice appears to be heavily
tilted toward those with similar social and political
attitudes, no genetic melting pot exists for these
traits. Thus, the evidence presented here on assortative
mating should be quite sobering to those in search of
unity and togetherness. If anything, the heritability of
orientation in combination with assortative mating
may exacerbate the current divide.
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But admitting that genetics inﬂuences political attitudes
could actually help to mute societal divisions.
Currently, absolutists and contextualists simply do not
connect, and the result is frustration. To contextualists,
absolutists appear simplistic and selﬁsh; to absolutists,
contextualists appear naive and indecisive. Each side
talks past, and is authentically miffed by, the other. Recognizing
that our political antagonists probably have a
different genetic predisposition to people, life, human
nature, and politics may serve to ease frustrations
and, eventually, to improve communications across the
chasm. If absolutists spent more time trying to think
like contextualists and contextualists trying to think
like absolutists, understanding would be increased and
debates could become more constructive. As frustrating
as it may be to debate with someone who holds such
different orientations, value exists in recognizing that
intransigence is not the result of willful bullheadedness
but, rather, genetically driven differences in orienta-
tion.
The exciting next step is to understand the reason
such distinct orientations have evolved and lasted.
Evolutionary psychologists tend to assume that all
enduring traits are adaptive (for a dissenting view,
see Gould 2000) since natural selection drives out
variation and makes adaptive traits ever more common.
In this organism-based interpretation, whichever
orientation—–absolutism or contextualism—–is evolutionarily
superior should soon come to numerically
dominate the other. This is possible but unlikely. An
alternative group-based interpretation sees variation
itself as adaptive (see Alford and Hibbing 2004 and
Sober and Wilson 1998). The beneﬁts of genetic variation
are most easily observed in the ability of differential
immune systems to prevent a group of organisms
from being completely wiped out by a single
pathogen, but it is easy to imagine how sociopolitical
variation could also create more viable groups. In fact,
computer simulations give support to the hypothesis
that divergent individual-level social behaviors, such as
cooperation and defection, are beneﬁcial at the group
level (Hammond 2000). As loathe as contextualists and
absolutists are to admit it, the presence of the other
orientation may make a society stronger.
REFERENCES
Adorno, Theodore W., Else Frenkel-Brunswik, Daniel Levinson, and
R. N. Sanford. 1950. The Authoritarian Personality. New York:
Harper.
Alford, John R., and John R. Hibbing. 2004. “The Origin of Politics:
An Evolutionary Theory of Political Behavior.” Perspectives on
Politics 2 (December): 707–23.
Bargh, J. A., and T. L. Chartrand. 1999. “The Unbearable Automaticity
of Being.” American Psychologist 54 (July): 462–79.
Bouchard, T. J., Jr. 1998. “Genetic and Environmental Inﬂuences on
Intelligence and Special Mental Abilities.” Human Biology 70 (2):
257–59.
Bouchard, T. J., Jr., and J. C. Loehlin. 2001. “Genes, Personality, and
Evolution.” Behavior Genetics 31 (January): 23–73.
Bouchard, T. J., Jr., and Matt McGue. 2003. “Genetic and Environmental
Inﬂuences on Human Psychological Differences.” Journal
of Neurobiology 54 (1): 4–45.
Bouchard, T. J., Jr., D. T. Lykken, M. McGue, N. L. Segal, and
A. Tellegen. 1990. “Sources of Human Psychological Differences:
The Minnesota Study of Twins Reared Apart.” Science 12 (October):
223–28.
Bouchard, T. J., Jr., Matt McGue, D. T. Lykken, and A. Tellegen. 1999.
“Intrinsic and Extrinsic Religiousness.” Twin Research 2 (June):
88–98.
Boyd, Robert, and Peter J. Richerson. 1985. Culture and the Evolutionary
Process. Chicago: University of Chicago Press.
Breland, Keller, and Marian Breland. 1961. “The Misbehavior of
Organisms.” American Psychologist 16 (9): 681–84.
Brewer, Marilyn B. 2000. “Superordinate Goals Vs. Superordinate
Identity as Bases of Cooperation.” In Social Identity Processes, ed.
Dora Capozza and Rupert Brown. London: Sage.
Campbell, Angus, Philip E. Converse, Warren E. Miller, and Donald
E. Stokes. 1960. The American Voter. New York: John Wiley.
Carmines, Edward G., and James A. Stimson. 1980. “The Two Faces
of Issue Voting.” American Political Science Review 74 (March):
78–91.
Caspi, Avshalom, Karen Sugden, Terrie E. Mofﬁtt, Alan Taylor, Ian
W. Craig, Honalee Harrington, Joseph McClay, Jonathan Mill,
Judy Martin, Antony Braithwaite, and Richie Poulton. 2003. “Inﬂuence
of Life Stress on Depression: Moderation by a Polymorphism
in the 5-HTT Gene.” Science 18 (July): 386–89.
Converse, Philip E. 1964. “The Nature of Belief Systems in Mass
Publics.” In Ideology and Discontent, ed. David Apter. New York:
Free Press.
Cook, Timothy E. 1985. “The Bear Market in Political Socialization
and the Costs of Misunderstood Psychological Theories.” American
Political Science Review 79 (December): 1079–93.
Crelia, R. A., and A. Tesser. 1996. “Attitude Heritability and Attitude
Reinforcement.” Personality and Individual Differences 21
(November): 803–8.
Easton, David, and Jack Dennis. 1969. Children in the Political System.
New York: McGraw–Hill.
Eaves, L. J., H. J. Eysenck, and N. G. Martin. 1989. Genes, Culture,
and Personality: An Empirical Approach. San Diego: Academic
Press.
Eaves, L. J., N. G. Martin, and A. C. Heath. 1990. “Religious Afﬁliation
in Twins and Their Parents.” Behavior Genetics 20 (January):
1–22.
Eysenck, H. J. 1954. The Psychology of Politics. London: Routledge.
Fiorina, Morris P. 1981. Retrospective Voting in American National
Elections. New Haven, CT: Yale University Press.
Fiorina, Morris P. 2005. Culture War? The Myth of a Polarized
America. New York: Pearson Longman.
Gould, Stephen Jay. 2000. “More Things in Heaven and Earth.” In
Alas, Poor Darwin, ed. Hillary Rose and Steven Rose. New York:
Harmony Books.
Greenstein, Fred I. 1960. “The Benevolent Leader: Children’s Images
of Political Authority.” American Political Science Review 54
(December): 934–43.
Hammond, Ross. 2000. “Endogenous Transition Dynamics in Corruption:
An Agent-Based Computer Model.” Brookings Institution.
Typescript.
Hibbing, John R., and John R. Alford. 2004. “Accepting Authoritative
Decisions: Humans as Wary Cooperators.” American Journal
of Political Science 48 (January): 62–76.
Hibbing, John R., and Elizabeth Theiss-Morse. 2002. Stealth Democracy:
Americans’ Beliefs about How Government Should Work.
New York: Cambridge University Press.
Jennings, M. Kent., and Richard G. Niemi. 1968. “The Transmission
of Political Values from Parent to Child.” American Political
Science Review 62 (March): 169–83.
Jennings, M. Kent, and Richard G. Niemi. 1991. “Issues and Inheritance
in the Formation of Party Identiﬁcation.” American Journal
of Political Science 35 (November): 970–88.
Kosack, Steven T., and Mark Groudine. 2004. “Gene Order and
Dynamic Domains.” Science 22 (October): 644–47.
Lake, Robert I. E., Lindon J. Eaves, Hermine H. M. Maes, Andrew C.
Heath, and Nicholas G. Martin. 2000. “Further Evidence Against
the Environmental Transmission of Individual Differences in Neuroticism
from a Collaborative Study of 45,850 Twins and Relatives
on Two Continents.” Behavior Genetics 30 (May): 223–33.
Laswell, Harold. 1930. Psychopathology and Politics. Chicago: University
of Chicago Press.
166
American Political Science Review Vol. 99, No. 2
Lodge, Milton, Kathleen McGraw, and Pat Stroh. 1989. “An
Impression-Driven Model of Candidate Formation.” American
Political Science Review 83 (June): 399–420.
Lykken, D. T. 1998. “The Genetics of Genius.” In Genius and the
Mind: Studies of Creativity and Temperament in the Historical
Record, ed. A. Steptoe. Oxford: Oxford University Press.
Lykken, David T. 1999. Happiness: What Studies on Twins Show Us
about Nature, Nurture, and the Happiness Set Point. New York:
Golden Books.
Maes, H. H., M. C. Neale, N. G. Martin, A. C. Heath, and L. J. Eaves.
1999. “Religious Attendance and Frequency of Alcohol Use.” Twin
Research 2 (June): 169–79.
Marcus, Gary. 2004. The Birth of the Mind. New York: Basic Books.
Marcus, George E. 2002. The Sentimental Citizen: Emotion in Democratic
Politics. University Park, PA: Penn State University Press.
Marcus, George E., John L. Sullivan, Elizabeth Theiss-Morse, and
Sandra L. Wood. 1995. With Malice Toward Some. New York:
Cambridge University Press.
Marcus, George E., W. Russell Neuman, and Michael MacKuen.
2000. Affective Intelligence and Political Judgment. Chicago: University
of Chicago Press.
Martin, N. G., L. J. Eaves, A. C. Heath, R. Jardine, L. M. Feingold,
and H. J. Eysenck. 1986. “Transmission of Social Attitudes.” Proceedings
of the National Academy of Sciences 15 (June): 4364–68.
Masters, Roger D. 1993. Beyond Relativism: Science and Human
Values. Hanover, NH: University Press of New England.
McCloskey, Herbert. 1958. “Conservatism and Personality.”
American Political Science Review 52 (March): 27–45.
McDermott, Rose. 2004. “The Feeling of Rationality: The Meaning
of Neuroscientiﬁc Advances for Political Science.” Perspectives on
Politics 2 (December): 691–707.
Merelman, Richard M. 1986. “Revitalizing Political Socialization.”
In Political Psychology, San Francisco: Jossey–Bass. ed. Richard
Herrmann.
Monroe, Kristen Renwick. 2004. The Hand of Compassion.
Princeton, NJ: Princeton University Press.
Orbell, John, Tomonori Morikawa, Jason Hartwig, James Hanley,
and Nicholas Allen. 2004. “Machiavellian Intelligence as a Basis
for the Evolution of Cooperative Dispositions.” American Political
Science Review 98 (March): 1–17.
Page, Benjamin L., and Calvin Jones. 1979. “Reciprocal Effects of
Policy Preferences, Party Loyalties, and the Vote.” American Political
Science Review 73 (December): 1071–89.
Peterson, Steven A. 1983. “Biology and Political Socialization: A
Cognitive Developmental Link?” Political Psychology 4 (2): 265–
88.
Plomin, R. 1990. “The Role of Inheritance in Behavior.” Science 13
(April): 183–248.
Plomin, R., D. W. Fulker, R. Corley, and J. C. DeFries. 1997. “Nature,
Nurture, and Cognitive Development from 1 to 16 years: A ParentOffspring
Adoption Study.” Psychological Science 8 (November):
442–47.
Plomin, R., R. Corley, A. Caspi, D. W. Fulker, and J. DeFries. 1998.
“Adoption Results for Self-Reported Personality: Evidence for
Nonadditive Genetic Effects?” Journal of Personality and Social
Psychology 75 (July): 211–18.
Plomin, R., J. C. DeFries, G. E. McClearn, and P. McGufﬁn. 2001.
Behavioral Genetics. 4th ed. New York: Worth.
Poole, Keith, and Howard Rosenthal. 1997. Congress: A Political–
Economic History of Roll Call Voting. New York: Oxford University
Press.
Prescott, Carol A., Ronald C. Johnson, and John J. McArdle. 1999.
“Chorion Type as a Possible Inﬂuence on the Results and Interpretation
of Twin Study Data.” Twin Research 2 (December): 244–
49.
Rhee, S. H., and I. D. Waldman. 2002. “Genetic and Environmental
Inﬂuences on Antisocial Behavior: A Meta-analysis of Twin and
Adoption Studies.” Psychological Bulletin 128 (May): 490–529.
Ridley, Matt. 2003. Nature Via Nurture. New York: Harper Collins.
Rushton, J. Philipe, Christine H. Littleﬁeld, and Charles J. Lumsden.
1986. “Gene-Culture Coevolution of Complex Social Behavior:
Human Altruism and Mate Choice.” Proceedings of the National
Academy of Sciences 1 (October): 7340–43.
Sacerdote, Bruce. 2004. “What Happens When We Randomly Assign
Children to Families?” National Bureau of Economic Research
Working Paper. Typescript.
Scarr S., and R. Weinberg. 1981. “The Transmission of Authoritarianism
in Families: Genetic Resemblance in Social-political
Attitudes.” In Race, Social Class, and Individual Differences, ed.
S. Scarr. Hillsdale, NJ: Erlbaum.
Searing, Donald, D. J. J. Schwartz, and A. E. Lind. 1973. “The
Structuring Principle: Political Socialization and Belief Systems.”
American Political Science Review 67 (June): 415–32.
Sears, David O. 1989. “Whither Political Socialization Research?
The Question of Persistence.” In Political Socialization, Citizenship
Education, and Democracy, ed. O. Ichilov. New York: Teachers
College Press.
Sober, Elliott, and David Sloan Wilson. 1998. Unto Others: The Evolution
and Psychology of Unselﬁsh Behavior. Cambridge, MA:
Harvard University Press.
Stouffer, S. 1955. Communism, Conformity, and Civil Liberties. New
York: Doubleday.
Tedin, Kent L. 1974. “The Inﬂuence of Parents on the Political
Attitudes of Adolescents.” American Political Science Review 68
(November): 1579–92.
Tesser, A. 1993. “The Importance of Heritability in Psychological
Research: The Case of Attitudes.” Psychological Review 100
(January): 129–42.
Truett, K. R., L. J. Eaves, E. E. Walters, A. C. Heath,
J. K. Hewitt, J. M. Meyer, J. Silberg, M. C. Neale, N. G. Martin, and
K. S. Kendler. 1994. “A Model for Analysis of Family Resemblance
in Extended Kinships of Twins. Behavior Genetics 24 (January):
35–49.
Wegner, Daniel M. 2002. The Illusion of Conscious Will. Cambridge,
MA: MIT Press.
Wiggins, J., and P. Trapnell. 1997. “Personality Structure: The Return
of the Big Five.” In R. Hogan, J. Johnson, and S. Biggs, ed.
Handbook of Personality Psychology. San Diego: Academic Press.
Zaller, John R. 1992. The Nature and Origins of Mass Opinion. New
York: Cambridge University Press.
Zhang, Xiaodong, Paul R. Gainetdinov, Jean-Martin Beaulieu,
Tatyana D. Sotnikova, Lauranell H. Burch, Redford B. Williams,
David A. Schwartz, K. Ranga, R. Krishnan, and Marc G. Caron.
2005. “Loss of Function Mutation in Tryptophan Hydroxlylase-2
Identiﬁed in Unipolar Major Depression.” Neuron 6 (January):
11–16.
167