[Paleopsych] Alford et alia: Are Political Orientations Genetically Transmitted?

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Are Political Orientations Genetically Transmitted?
American Political Science Review (2005), 99:2:153-167
JOHN R. ALFORD a1c1, CAROLYN L. FUNK a2c2 and JOHN R. HIBBING a3c3

a1 Rice University
a2 Virginia Commonwealth University
a3 University of Nebraska

Abstract

We 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.


Correspondence:
c1 John R. Alford is Associate Professor, Department of Political Science, Rice 
University, Houston, TX 77251 (jra at rice.edu).
c2 Carolyn L. Funk is Associate Professor, L. Douglas Wilder School of 
Government and Public Affairs, Virginia Commonwealth University, Richmond, VA 
23298 (clfunk at vcu.edu).
c3 John R. Hibbing is Foundation Regents Professor, Department of Political 
Science, University of Nebraska—Lincoln, Lincoln, NE 68588 (jhibbing at unl.edu).


List of Figures and Tables

Table 1 - Genetic and Environmental Influences on Political Attitudes: The 28 
Individual Wilson–Patterson Items

Table 2 - Genetic and Environmental Influences on Political Attitudes: Summary 
Index and Additional Non–Wilson–Patterson Items

Table 3 - Comparison of Australian and U.S. Estimates of Genetic and 
Environmental Influences on Political Attitudes

Why 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 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 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 genetically-induced 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 high-stress 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 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 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 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 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 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 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 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 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.

- Genetic and Environmental Influences on Political Attitudes: The 28 
Individual Wilson–Patterson Items
<http://journals.cambridge.org/fulltext_content/PSR/PSR99_02/S0003055405051579tbl001.gif>

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 [minus sign]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 non-commital (?) 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 [minus sign]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 [minus sign]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 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 one-third 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.

- Genetic and Environmental Influences on Political Attitudes: Summary Index 
and Additional Non–Wilson–Patterson Items
<http://journals.cambridge.org/fulltext_content/PSR/PSR99_02/S0003055405051579tbl002.gif>

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 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) [minus sign] 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 inter-spouse 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 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 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.

- Comparison of Australian and U.S. Estimates of Genetic and Environmental 
Influences on Political Attitudes 
<http://journals.cambridge.org/fulltext_content/PSR/PSR99_02/S0003055405051579tbl003.gif>

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 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 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), 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 contextualists.

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.

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 orientation.

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.

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.

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Footnotes

1Evidence consistent with an evolutionary theory of political behavior is found 
in Brewer 2000, Hibbing and Alford 2004, and Orbell et al. 2004.

2To his credit, Zaller (1992) goes on to acknowledge a possible role for 
“inherited” traits in shaping predispositions (23).

3To 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.

4However, 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.

5Conservatism 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%.

6Adoption 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).

7Our 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 
population-based 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.

8The 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.

9In 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.