two genes predict voter turnout -...

Two Genes Predict Voter Turnout

James H. Fowler University of California, San Diego

Christopher T. Dawes University of California, San Diego

Fowler, Baker, and Dawes (2008) recently showed in two independent studies of twins that voter turnout has veryhigh heritability. Here we investigate two specific genes that may contribute to variation in voting behavior. Usingdata from the National Longitudinal Study of Adolescent Health, we show that individuals with a polymorphismof the MAOA gene are significantly more likely to have voted in the 2004 presidential election. We also findevidence that an association between a polymorphism of the 5HTT gene and voter turnout is moderated byreligious attendance. These are the first results ever to link specific genes to political behavior.

Social scientists have shown that basic politicalattitudes like liberalism and conservatism arelikely to be heritable (Alford, Funk, and Hibbing

2005, Hatemi et al. 2007). While the choice of a particu-lar candidate or party does not appear to be heritable,a significant proportion of the variation in the decisionto participate in politics can be attributed to geneticfactors. Fowler, Baker, and Dawes (2008) recently stud-ied the voting behavior of two populations of twinsand showed that heritability accounted for 53% ofthe variation in validated turnout of those living inLos Angeles county and 72% of the self-reported turn-out in a nationally representative sample of youngadults. They also showed that heritability accountedfor 60% of the variation in a general index of politicalparticipation, including contributing to campaigns,running for office, volunteering for political organ-izations, and attending protests. These results werethe first to suggest that humans exhibit inherent vari-ability in their willingness to participate in politics.

However, these initial results based on twinstudies beg the question ‘‘which genes?’’ The naturalplace to start the search for such genes is among thosethat have already been shown to account for variationin social behavior. And among these, MAOA and5HTT are prime candidates. These two genes tran-scribe neurochemicals that exert a strong influenceon the serotonin system in parts of the brain thatregulate fear, trust, and social interaction (Bertolinoet al. 2005; Eisenberger et al. 2007; Hariri et al. 2002;Hariri et al. 2005; Heinz et al. 2005; Meyer-Lindenberget al. 2006). MAOA and 5HTT have been studied formore than 20 years, and much is known about the

way different versions of their genes regulate tran-scription, metabolism, and signal transfers betweenneurons, all of which have an effect on socialinteractions (Craig 2007). In particular, the lesstranscriptionally efficient alleles of these genes havebeen associated with a variety of antisocial behaviors(Rhee and Waldman 2002).

In this article, we hypothesize that people withmore transcriptionally efficient alleles of the MAOAand 5HTT genes are more likely to vote. An associ-ation between a gene and political behavior may alsobe moderated by environmental factors. This phe-nomenon is known as a gene-environment (GxE)interaction (Shanahan and Hofer 2005). We thereforealso hypothesize that an association between each ofthese genes and voting may be moderated by socialactivity. Using data from the National LongitudinalStudy of Adolescent Health, we conduct gene andgene-environment association tests on the relation-ship between turnout and MAOA and 5HTT. Theresults show that both genes are significantly asso-ciated with the decision to vote. Moreover, theassociation between 5HTT and turnout is moderatedby exposure to religious social activity. These findingshave important implications for how we both modeland measure political interactions.

Past Work on the Genetic Basis ofPolitical Participation

Although we are not the first to suggest a linkbetween genes and political participation, this study

The Journal of Politics, Vol. 70, No. 3, July 2008, Pp. 579–594 doi:10.1017/S0022381608080638

! 2008 Southern Political Science Association ISSN 0022-3816

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is the first to investigate an association between spe-cific genes and political behavior. Early work studiedthe importance of personality in political participa-tion, but this literature focused exclusively on envi-ronmental factors, asserting that people who arereared in similar ways will have similar personalities(Lane 1959; Levinson 1958) or that the role of per-sonality is to mediate social influences on participa-tion (Krause et al. 1970). Additional earlier studiesfocused on the importance of adolescent socializationin the development of political behaviors, but thesescholars never considered the role of genes in the linkbetween parent and child. Merelman (1971) explicitlyaddressed this shortcoming, arguing that both genesand environment are probably important. He la-mented the fact that genetic explanations had beenignored by social scientists:

‘‘[T]his natural tendency to examine one environmentalfactor after another ad infinitum does a genetic explan-ation something of an injustice. The problem is thatwhile we can examine environmental variables directly,we can usually only infer genetic effects, and so ournatural tendency is to slight the latter perspective. Inshort, our procedures, following the line of leastmethodological resistance, impinge heavily upon ourtheoretical perspectives.’’ (1044)

In spite of Merelman’s early call for attention,genetic studies of participation were not forthcom-ing. Scholars continued to focus on personalityfactors underlying participation like efficacy (Finkel1985) and self-esteem (Sears 1987) without consid-ering the fact that these factors may be heritable. Afew political scientists have argued on general prin-ciple that genes must play a role in political behaviorslike participation (Carmen 2004; Masters 1990; Somitand Peterson 1998) but they have left the work oftesting their hypotheses to others.

A wide range of studies have already shown thatvariation in prosocial personality and behavior can beattributed to genes (McGue, Bacon, and Lykken 1993;Rushton et al. 1986; Scourfield et al. 2004). Thisliterature suggests that innate dispositions play animpotant role in an individual’s willingness to par-ticipate in social activities or to engage in acts thatprimarily benefit others. Meanwhile, a growing num-ber of observational studies, theoretical models, andlaboratory experiments suggest that prosocial atti-tudes and behavior are important factors for explain-ing voter turnout and political participation. Forexample, Knack (1991) creates an index of ‘‘socialaltruism’’ from questions about charity, volunteerwork, and community involvement on the 1991 NESPilot Study and finds a positive relationship between

the index and turnout. Similarly, Jankowski (2007)finds a relationship between turnout and human-itarian norms from questions on the 1995 NES PilotStudy. Edlin, Gelman, and Kaplan (2007) show that avariety of aggregate features of turnout can be easilyexplained by incorporating prosocial preferencesinto the decision-theoretic calculus of voting, andJankowski (2002) shows that this reasoning extendsto a game-theoretic model. Finally, experimental stud-ies utilizing dictator games to measure revealed so-cial preferences show that individuals who are morewilling to engage in costly giving to others are alsomore likely to vote (Fowler 2006a) and participate inpolitics (Dawes and Fowler 2007, Fowler and Kam2007).

Thus, we hypothesize that genes may influencevoting and political participation because they influ-ence a generalized tendency to engage in prosocialbehavior via their functional role in nuerochemicalprocesses. Although Fowler, Baker, and Dawes (2008)have already shown that a large fraction of thevariation in voter turnout and political participationcan be attributed to genetic factors, to date nospecific genes have been identified in this process. Itis crucial to point out at the outset that we cannottest, given our data, the potential causal pathways wesuggest. Therefore, the goal of this study is to showassociation rather than causality.

Some Basic Genetics Concepts

Genes are distinct regions of human DNA that formthe blueprint for molecules that regulate the develop-ment and function of the human body. There are anestimated 25,000 genes (most of which exist in mul-tiple copies) in the 46 chains, or chromosomes, thatmake up all human DNA. Almost all human cellscontain the same inherited DNA chains that are fixedfrom the moment of conception. This is an impor-tant point for social scientists. Since genes are fixed,they represent the purest measure of biological in-heritance, virtually unaffected by environment andable to be collected at any point throughout aperson’s life.

At conception individuals inherit one-half oftheir DNA from each parent, with one copy of eachgene coming from the mother and one copy from thefather. Some genes come in different versions, knownas ‘‘alleles’’—for example, sickle cell disease resultsfrom a particular allele coding for abnormal ratherthan normal hemoglobin. Each parent has two

580 james h. fowler and christopher t. dawes

separate copies of an allele at each ‘‘locus,’’ orlocation, on the chromosome, but each sperm oregg cell contains only one of these alleles. Thus a childhas a 50% chance of receiving a particular allele froma particular parent. For example, suppose that at agiven locus there are two possible alleles, A and B. Ifboth parents are ‘‘heterozygous’’ at that locus, mean-ing they each have an A and a B allele (AB), then agiven offspring has a 25% chance of being ‘‘homo-zygous’’ for A (AA), a 25% chance of being homo-zygous for B (BB) and a 50% chance of beingheterozygous (AB or BA—order is irrelevant).

Genes transcribe proteins and following thisprocess, these proteins begin a cascade of interactionsthat regulate bodily structure and function. Many ofthe observable traits and behaviors of interest, re-ferred to as ‘‘phenotypes,’’ are far downstream fromthe original ‘‘genotypes’’ present in the DNA. Whilein some cases one allele can single-handedly lead to adisease (such as Sickle Cell Anemia, Huntingtonsdisease, and cystic fibrosis), the vast majority ofphenotypes are ‘‘polygenic,’’ meaning they are influ-enced by multiple genes (Mackay 2001; Plomin 2008)and are shaped by a multitude of environmentalforces. As a result, simple association models betweengenotype and phenotype are an important first stepto establish candidate genes, but they are not the endof the story. It is also important to investigate theextent to which genetic associations are moderated byenvironmental factors (‘‘environmental modifiers’’)and other genes (‘‘genetic modifiers’’).

Serotonin, Genes, and SocialBehavior

Twin studies have already established that geneticfactors account for a significant proportion of thevariation in antisocial behaviors (Rhee and Waldman2002), including substance abuse, impulsivity, crim-inality, precocious sexuality, and a combination ofthese behaviors called antisocial personality disorder(ASPD). However, twin studies cannot establishwhich genes are implicated. It is likely that dozens,if not hundreds of genes influence sociability (Mackay2001; Plomin 2008). As a result, scientists typicallystart with ‘‘candidate’’ genes that are known to in-fluence related behaviors or processes in the body.For social behavior, this means focusing on genes thataffect brain development, neurotransmitter synthesisand reception, hormone regulation, and transcriptionfactors (Damberg et al. 2001).

To determine whether genes affect voting behav-ior, we chose two candidate genes that have alreadyreceived a great deal of attention for their associationwith antisocial behavior. These genes, MAOA and5HTT,1 are critical to the metabolism of serotonin inthe brain. As shown in Figure 1, serotonin is achemical that is released when a neuron ‘‘fires’’ andsensed by a receptor on the receiving neuron, passingan electric potential across a gap called a nervesynapse (the nerve that fires is on the ‘‘presynaptic’’side of the gap). Signals are carried throughout thebody by the sequential firing of one neuron afteranother across these synapses. When an individualexperiences stress, it causes increased neuron activity,stimulating the release of excess serotonin into thegaps between the synapses (Chaouloff, Berton, andMormede 1999). If serotonin remains outside thecells, it can oxidize into a toxin that kills both thepresynaptic and postsynaptic neurons. The body’shomeostatic response to this excess serotonin is toreabsorb it into the presynaptic neuron via a trans-porter in the cell wall, called 5HTT. Once the‘‘reuptake’’ of serotonin is complete and it is backinside the neuron, an enzyme called monoamineoxidase A (MAOA) degrades the serotonin so thatits components can be reabsorbed in the cell. Thegenes responsible for transcribing 5HTT and MAOAare eponymous—the 5HTT gene produces 5HTT andthe MAOA gene produces MAOA.

Animal studies indicate that the serotonin systemhas an important effect on social behavior. Rhesusmacaque monkeys with impaired serotonin metabo-lisms are impulsive and aggressive in response tosocial stressors (Kraemer et al. 1989) and studies ofrodents show that acute emotional stress affects theway MAOA breaks down serotonin in several areas ofthe brain (Popova, Voitenko, and Maslova 1989;Virkkunen et al. 1995). In mice, social stress increasestranscription of both MAOA and 5HTT (Filipenkoet al. 2002) and knock-out studies that eliminate theMAOA gene in subjects cause enzymatic activity tocome to a complete halt (Cases et al. 1995). Inmonkeys, 5HTT is densely concentrated in the out-put regions of the amygdala, which affects fearrecognition (O’Rourke and Fudge 2006) and MAOAhas been shown to alter the structure of the brain inmice (Cases et al. 1996). This evidence suggests thatany deficiency in the genes that regulate serotoninmetabolism will have a direct effect on the brain thattends to reduce the ability to process and respond to

1The 5HTT gene has several other names, including HTT,SLC6A4, and SERT.

two genes predict voter turnout 581

social stress. These effects have been linked specifi-cally to a genetic polymorphism in monkeys that isclosely related to that observed in humans (Newman,et al. 2005, Suomi 2003). There is also strongevidence that the serotonin system affects complexsocial traits in humans (Balciuniene and Jazin 2001),and 5HTT and MAOA frequently serve as targets forantidepressants and illegal recreational drugs (Craig2007, Livingston and Livingston 1996).

The 5HTT gene contains a 44 base-pair variable-number tandem repeat (VNTR) polymorphism2 inthe promoter region3 that is believed to be respon-sible for variation in transcriptional activity. Thetranscriptional efficiency of the ‘‘long’’ version ofthis allele is associated with a much higher basalactivity than the shorter allele (Lesch et al. 1996;Little et al. 1998). MAOA has a 30 base-pair VNTRpolymorphism located in the promoter region. The‘‘high’’ version of this polymorphism significantlyincreases the transcriptional efficiency of MAOA(Denney, Koch, and Craig 1999; Denney et al. 1994;Sabol, Hu, and Hamer 1998). The less transcription-ally efficient alleles of both 5HTT and MAOA havebeen linked to antisocial behavior (Vanukov et al.1995; Contini et al. 2006; Domsche et al. 2005; Hsuet al. 1996; Lawson et al. 2003; Saito et al. 2002;Samochowiec, Lesch et al. 1999; Schmidt et al. 2000)which appears to be mediated by certain parts of the

brain. For example, the development of the amygdalaand orbitofrontal cortex has been linked to a smallgenetic locus which contains the gene for MAOA(Good, Lawrence, and Thomas 2003). Furthermore, anumber of studies show that the amygdala becomes‘‘hypersensitive’’ during the presentation of aversiveor threatening social stimuli in individuals with eitherthe short 5HTT allele (Bertolino et al. 2005; Haririet al. 2002; Hariri et al. 2005; Heinz et al. 2005) or thelow MAOA allele (Meyer-Lindenberg et al. 2006).Eisenberger et al. (2007) report similar results, notingthat the link to antisocial behavior results from anincreased sensitivity to negative socioemotional ex-periences (though in their study they claim the effectis mediated via the dorsal anterior cingulate cortex).

Not all studies show a direct relationship betweenthese polymorphisms and behavior. Instead, devel-opmental or concurrent environments may moderatean association between genes and observed socialbehavior. A gene-environment (GxE) interaction hasbeen identified in many cases for antisocial behavior(Caspi et al. 2002; Foley et al. 2004; Haberstick et al.2005; Kim-Cohen et al. 2006; Nilsson et al. 2006), themost famous of which is the Caspi et al. (2002) paper.This work shows that exposure to stressors like childabuse at early developmental stages may interact withthe low MAOA polymorphism resulting in antisocialbehavior later in life. This is an important point—in these studies the gene itself was not associatedwith the behavior. Rather, it was the combination ofboth gene and environment that yielded a significantassociation.

Two Hypotheses for Genes andTurnout

A growing literature suggests that voter turnout isa prosocial behavior that is strongly influencedby other-regarding preferences (Edlin, Gelman, andKaplan 2007; Fowler 2006a; Fowler and Kam 2007;Jankowski 2002, 2007). Given that polymorphisms ofMAOA and 5HTT appear to influence antisocialbehavior, we therefore hypothesize that they will alsobe associated with voting behavior. One difficulty ofthe voting experience is that one’s preferred candi-dates sometimes lose. This loss has been theorized toreduce future motivations to vote (Bendor, Diermeier,and Ting 2003; Fowler 2006b) and Kanazawa (1998)has even shown empirically that turnout declinesamong those whose favorite candidates lost theprevious election. In addition, people may prospec-tively consider how they will feel about a loss before

FIGURE 1 Simple representation of the release,reception, and recycling of serotoninin neurons.

5HTT SerotonintransporterSerotonin

Serotonin receptors

Storagevesicle

MAOA

2A VNTR polymorphism is a repeated segment of DNA thatvaries among individuals in a population.

3A promoter region is the regulatory region of DNA that tellstranscription enzymes where to begin. These promoter regionstypically lie upstream from the genes they control.


deciding whether or not to vote, or whether they willeven pay attention to the election. Those who areoverly sensitive to social conflict may choose to stayhome and ignore politics, while less sensitive indi-viduals will not take the potential emotional stresscaused by the loss of their favorite candidates intoconsideration. Thus, we expect individuals with the‘‘high’’ MAOA polymorphism and ‘‘long’’ 5HTTpolymorphism will be more likely to turn out to vote.

However, an association between either MAOAand 5HTT and voting may not be direct. Instead, anassociation between a gene and turnout may bemoderated by environmental factors. A vast literatureon turnout suggests the importance of voter mobi-lization efforts (Wielhouwer and Lockerbie 1994),religious group activity (Cassel 1999), and otherkinds of social contacts that have an influence onpolitical participation (Huckfeldt 1979, Verba,Schlozman, and Brady 1995). Religious group activityin particular has been singled out as one of thestrongest predictors of voter turnout, even moreso than socioeconomic status (Olsen 1972; Sallach,Babchuk, and Booth 1972). However, scholars havehad difficulty interpreting this association. Religiousgroups might stimulate political activity directly or asbyproducts of their tendency to increase civic skills,political interest, feelings of efficacy, access to polit-ical information, and a sense of civic duty. Testing allof these possible explanations, Cassel (1999) suggeststhat the main reason for the association is thatreligious groups build a sense of belonging to a largercommunity. However, it may not be possible to buildsuch a sense in people who are too averse to socialconflict, since they will resist appeals to becomeinvolved. We therefore hypothesize that MAOA and5HTT, when interacted with religious group activity,may be significantly associated with turnout. Specif-ically, individuals who are actively involved in theirreligious organizations and who have the ‘‘high’’MAOA allele or the ‘‘long’’ 5HTT allele will be morelikely to vote than others.

Data

All of our analysis is based on individual-level geneticand survey data collected as part of the The NationalLongitudinal Study of Adolescent Health (AddHealth). Add Health is a study that explores the causesof health-related behavior of adolescents in grades 7through 12 and their outcomes in young adulthood.The first wave of the Add Health study (1994–95)selected 80 high schools from a sampling frame of

26,666. The schools were selected based on their size,school type, census region, level of urbanization, andpercent of the population that was white. Participatinghigh schools were asked to identify junior high ormiddle schools that served as feeder schools to theirschool. This resulted in the participation of 145middle, junior high, and high schools. From thoseschools, 90,118 students completed a 45-minute ques-tionnaire, and each school was asked to complete atleast one School Administrator questionnaire. Thisprocess generated descriptive information about eachstudent, the educational setting, and the environmentof the school. From these respondents, a core randomsample of 12,105 adolescents in grades 7–12 weredrawn plus several over-samples, totaling more than27,000 adolescents. These students and their parentswere administered in-home surveys in the first wave.Wave II (1996) was comprised of another set of in-home interviews of more than 15,000 students fromtheWave I sample and a follow-up telephone survey ofthe school administrators. Finally, Wave III (2001–2002) consisted of an in-home interview, six yearslater, of 15,170 Wave I participants. The result of thissampling design is that Add Health is a nationallyrepresentative study. Women make up 49% of thestudy’s participants, Hispanics 12.2%, blacks 16.0%,Asians 3.3%, and Native Americans 2.2%.4 Partici-pants in Add Health also represent all regions of thecountry: the Northeast makes up 17% of the sample,the South 27%, the Midwest 19%, and the West 17%.

In Wave I of the Add Health study, researcherscreated a genetically informative sample of siblingpairs based on a screening of the in-school sample of90,114 adolescents. These pairs include all adolescentsthat were identified as twin pairs, half siblings, orunrelated siblings raised together. Twins and halfbiological siblings were sampled with certainty. TheWave I sibling-pairs sample has been found to besimilar in demographic composition to the full AddHealth sample (Jacobson and Rower 1998). Allelicinformation for six genetic markers are available for2,574 individuals as part of Wave III,5 includingmarkers that identify alleles of MAOA and 5HTT.Details of the DNA collection and genotyping process

4A breakdown for those providing DNA samples is presented inthe appendix.

5We do not use the AddHealth samplingweights becausemore thana third of subjects in the genetic sample had a co-sibling that wasinterviewed as part ofWave III but not as part of the originalWave Isampling frame (Lessem et al. 2006). Therefore, sampling weightscould not be constructed for these subjects. Limiting our analysisto only individuals in the genetic sample for which weights couldbe determined would greatly reduce statistical power.


are available at the Add Health website (Add HealthBiomarker Team 2007).

MAOA alleles consist of 2 repeats, 3 repeats, 3.5repeats, 4 repeats, and 5 repeats with 291, 321, 336,351, and 381 base-pair fragment sizes, respectively.The 291 and 321 base-pair alleles are believed to havelower transcriptional efficiency than the 336, 351, and381 base-pair alleles (Denney et al. 1994, Sabol, Hu,and Hamer 1998). Following Haberstick et al. (2005),we group the 291 and 321 base-pair allele to form a‘‘low’’ transcription group and the 336, 351, and 381base-pair alleles to form a ‘‘high’’ transcriptiongroup. Allele frequency for the low grouping is 41%and high grouping is 59% in our sample. 5HTTalleles similarly vary in their transcriptional activity,with a ‘‘long’’ 528 base-pair allele associated with amuch higher basal activity than the shorter 484 base-pair allele. Allele frequency for the short allele is 42%and long allele is 58%.

Nearly 80% of the sibling-pairs sample partici-pants in Wave I also participated in Wave III andprovided information about their recent religious andpolitical activity and attitudes as well as DNA samples.In particular, subjects reported how often they at-tended church, synagouge, temple, mosque, or otherreligious services in the past 12 months. The categoriesfor response were ‘‘never,’’ ‘‘a few times,’’ ‘‘severaltimes,’’ ‘‘once amonth,’’ ‘‘2 or 3 times amonth,’’ ‘‘oncea week,’’ and ‘‘more than once a week’’. We simplifiedthese responses by grouping them into three categoriesof attendance: never, at least a few times but no morethan once a month, and more than once a month.

Subjects also answered ‘‘Did you vote in the mostrecent [2000] presidential election?’’ While this ques-tion gives us a valuable opportunity to explore thegenetic basis of political behavior, we want to makeclear two limitations of the data. First, it would bepreferable to have information about validated turn-out because of the well-known problem of over-reporting—many people who say they voted actuallydid not (Karp and Brockington 2005). However,Fowler, Baker, and Dawes (2008) show that a sub-stantial genetic component exists for both validatedand self-reported turnout, and they do not find astatistically meaningful difference in the size of thecomponent for the different measures. Second, itwould also be preferable to have information aboutthe voting behavior of older adults. The Add Healthsample is restricted to individuals who are 18–26years old during Wave III, so it is possible that ourresults apply only to the initiation of turnout behav-ior in young adults and not to its subsequentdevelopment as people age (Plutzer 2002).

Genetic Association

Genetic association studies test whether an allele orgenotype occurs more frequently within a groupexhibiting a particular trait than those without thetrait. For example, is the frequency of a particularallele or genotype higher among voters than non-voters? However, a significant association can meanone of three things: (1) The allele itself influencesvoting behavior; (2) the allele is in ‘‘linkage disequi-librium’’ with an allele at another locus that influen-ces voting; or (3) the observed association is a falsepositive signal due to population stratification.

Population stratification occurs because groupsmay have different allele frequencies due to theirgenetic ancestry. Turnout in these groups may be theproduct of their environments, alleles other than theone of interest, or some unobserved reason.6 Forexample, two groups may not have mixed in the pastfor cultural reasons. Through the process of naturalselection or genetic drift these groups may developdifferent frequencies of a particular allele. At the sametime, the two groups may also develop divergent be-haviors that are not influenced by the allele but com-pletely by the environment in which they live. Oncethese two groups mix in a larger population, simplycomparing the frequency of the allele to the observedbehavior would lead to a spurious association.

There are two main research designs employed inassociation studies, case-control designs, and family-based designs. Case-control designs compare thefrequency of alleles or genotypes among subjects thatexhibit a trait of interest to subjects who do not.7 As aresult, case-control designs are vulnerable to popula-tion stratification if either group is especially prone toselection effects. A typical way to control for thisproblem is to include controls for the race/ethnicityof the subject or to limit the analysis to a specificracial or ethnic group. Family-based designs eliminatethe problem of population stratification by usingfamily members, such as parents or siblings, ascontrols. Tests using family data compare whetheroffspring exhibiting the trait receive a risk allele fromtheir parents more often than would be expected bychance. This design is very powerful in minimizing

6Given our data, we cannot differentiate between 1 and 2. Inorder to do so we would need additional genetic informationabout loci in close proximity to the locus of interest. Thus, asignificant association means that either a particular allele, or onelikely near it on the same gene, significantly influences votingbehavior.

7Controls may be randomly selected from the population orthose known not to exhibit the trait.


type I error but also suffers from much lower power indetecting a true association. Xu and Shete (2006)show, based on extensive simulation work, that a case-control association study using a mixed-effects logisticregression outperforms family-based designs in detect-ing an association while at the same time effectivelylimiting type I error.

Model

To test for genetic association we employ a mixed-effects logistic regression model (Guo and Zhao 2000;Xu and Shete 2006):

P!Yij5 1jZkij;Uj"5 logit b0 # bGGij

!

#bEEij # bGxE$Gij % Eij& # bkZkij # Uj

"

where i and j index subject and family respectively.For the MAOA gene, G 5 1 if the subject’s genotypeis HH, and G 5 0 for genotypes Hl or ll (where Hrepresents having a copy of a 336, 351, or 381 base-pair ‘‘high’’ allele, and l represents having a copy of a291 or 321 base-pair ‘‘low’’ allele). For the 5HTTgene, G5 1 if the subject’s genotype is LL or Ls and G5 0 if the subject’s genotype is ss (where L representshaving a copy of the 528 base-pair ‘‘long’’ allele and srepresents having a copy of the 484 base-pair ‘‘short’’allele).8 The variable E is an environmental variablewe believe moderates the influence of the genotype onvoting behavior. We test one such variable, regularattendance of religious services. Z is a matrix ofvariables to control for underlying population struc-ture of the Add Health samples as well as potentiallymediating factors like age, gender, income, andeducation that have been found to significantlyinfluence turnout. Finally, the variable U is a familyrandom effect that controls for potential genetic andenvironmental correlation among family members.

To control the effects of the underlying popula-tion structure, we include indicator variables for

whether a subject self-reported as black, Hispanic,Asian, or Native American (base category is white).Following the policy of the United States Census, AddHealth allows respondents to mark more than onerace. Since this complicates the ability to control forstratification, we exclude these individuals (N 5 117),but supplementary analysis including them yieldssubstantively identical results. We also exclude fromthe data analysis noncitizens and people less than18 years of age on Election Day since they are notlegally eligible to vote. This leaves us with a samplesize of 2,329 individuals.9

The odds ratio of bG is an individual’s odds ofvoting if he or she is HH genotype for the MAOAgene compared to an individual with an Hl or llgenotype. A significant odds ratio means that lacking a‘‘short’’ allele is associated with higher turnout whencompared to having at least one ‘‘short’’ allele. For the5HTT gene, the odds ratio of bG is an individual’s oddsof voting given he or she has at least one ‘‘long’’ allele(LL or Ls) compared to having no long alleles (ss).Therefore, a significant odds ratio implies that having a‘‘long’’ 5HTT allele is associated with the decision tovote. Finally, the odds ratio of bGxE is whether having aspecific genotype combined with being exposed to anenvironmental effect influences turnout behavior, evenafter controling for both main effects.

Results

Table 1 shows the results of several specifications ofthe models to test the hypothesis that genes areassociated with voter turnout and whether the asso-ciation is moderated by religious service attendance.Each of these specifications includes variables for age,gender, and race to control for population stratifica-tion. Model 1 shows that the ‘‘high’’ allele of MAOAis significantly associated with increased voter turn-out (p 5 0.03). This model suggests that the odds ofa person with the ‘‘high’’ version of the MAOA genevoting are 1.26 times greater than that of a personwith the ‘‘low’’ version.10 Model 2 rejects the hy-pothesized relationship between 5HTT and voting(p 5 0.99). Thus, only MAOA appears to be directlyassociated with turnout.

8We classified genotypes based on transcriptional efficiency. Thelong allele of the 5HTT gene and the high allele of the MAOAgene has been associated with higher transcriptional efficiency(Denney, Koch, and Craig 1999, Denney et al. 1994, Lesch et al.1996, Little et al. 1998, Sabol, Hu, and Hamer 1998). Males arehomozygous for the MAOA gene (HH or ll), however femalesmay be heterozygous (Hl). Following previous studies, we classifyheterozygous females as the Low genotype and all homozygousindividuals as the High genotype (Fan et al. 2003, Frazzetto et al.2007). Previous research has shown that being homozygous forthe short allele (ss) makes one more vulnerable to negativeenvironmental stimuli compared to being heterozygous for theshort allele or homozygous for the long allele (Ls or LL; Caspiet al. 2003). Therefore, we combined Ls and LL into the Longgenotype and ss into the Short genotype.

9Our sample contains 451 single-person families, 884 familieswith two siblings, 34 families with three siblings, and two familieswith four siblings.

10We also checked whether the odds ratio was significantlyinfluenced by race or gender and it was not. We added an inter-action with High MAOA and gender (male51) which was notsignificant (p 5 0.98) and also an interaction with High MAOAand race (white51) which was not significant (p 5 0.79).


The next two models test the possibility thatattendance of religious services moderates an associ-ation between each of the genes and turnout. Model 3suggests that no moderation relationship exists forMAOA (p5 0.93), but it also shows the robustness ofthe direct association with turnout since includingattendance and an interaction in the model does notalter the significance of the main effect (p 5 0.02). Incontrast, Model 4 indicates that an association be-tween 5HTT and voting is in fact moderated byattendance (p 5 0.01). The odds of voting for thosewith the ‘‘long’’ version of the 5HTT gene and whofrequently attend religious services are 1.58 greaterthan people with the ‘‘short’’ version.11 To test therobustness of the direct and moderated associations,we model both of them simultaneously in Model 5.The results show that both odds ratios remainsignificant at p , 0.04.12

In Figure 2 we summarize our results for MAOAand the interaction between 5HTT and attendance bysimulating first differences from the coefficient co-variance matrix of Model 1 and Model 4. Holding allelse constant and changing the MAOA gene of allsubjects from ‘‘low’’ to ‘‘high’’ would increase aver-age turnout in this hypothetical population by about5 percentage points. Changing the 5HTT gene ofall religious attendees from ‘‘short’’ to‘‘long’’ wouldincrease average turnout in that group by about 10percentage points.

Model 6 includes a number of factors previousstudies have found to influence turnout. These vari-ables may in fact mediate the relationship between thegenes we have identified and turnout.13 For example,MAOA and 5HTT may be associated with a disposi-tion towards partisanship, which is known to signifi-cantly influence political participation (Bartels 2000).We might also expect genes to contribute to variationin socioeconomic factors like income (Bowles andGintis 2002), which in turn would yield greaterparticipation. Also, several twin studies have suggestedthat variation in cognitive ability can be attributed to

TABLE 1 Models of Association Between MAOA, 5HTT, and Voter Turnout.

Model 1 Model 2 Model 3 Model 4 Model 5 Model 6

OR SE OR SE OR SE OR SE OR SE OR SE

High MAOA 1.26 0.10 1.29 0.11 1.28 0.11 1.28 0.12High*Attend 0.99 0.14Long 5HTT 1.00 0.13 0.97 0.13 0.96 0.13 1.04 0.15Long*Attend 1.58 0.17 1.46 0.17 1.51 0.20Attend 2.17 0.10 1.46 0.15 1.59 0.16 1.28 0.18Black 1.59 0.14 1.45 0.13 1.33 0.14 1.19 0.14 1.28 0.14 1.54 0.18Hispanic 0.76 0.17 0.76 0.17 0.75 0.17 0.73 0.17 0.73 0.17 0.93 0.20Asian 0.83 0.22 0.79 0.22 0.75 0.22 0.72 0.22 0.74 0.22 0.91 0.25Nat Am 0.81 0.37 0.82 0.36 0.81 0.38 0.82 0.36 0.80 0.37 0.96 0.43Age 1.13 0.03 1.12 0.03 1.13 0.03 1.12 0.03 1.13 0.03 1.06 0.04Male 0.96 0.10 1.04 0.10 1.06 0.10 1.15 0.10 1.07 0.10 1.09 0.12Partisan 3.66 0.12Income 1.02 0.02Cognitive 1.01 0.00College 2.34 0.13Intercept 0.05 0.66 0.07 0.66 0.05 0.67 0.06 0.67 0.05 0.68 0.02 0.80

N 2273 2290 2283 2265 2254 1800Deviance 3059 3092 2966 2936 2918 2125NullDeviance 3091 3117 3109 3081 3067 2458

Note: Variable definitions are in the appendix. All results are expressed in odds ratios (OR). Standard errors (SE) are also presented.

11We also checked whether the odds ratio was significantlyinfluenced by race or gender and it was not. We added aninteraction with Long 5HTT*Attend and gender (male51) whichwas not significant (p 5 0.25) and also an interaction withLong 5HTT*Attend and race (white51) which was not signifi-cant (p 5 0.32).

12We also tested whether there was a significant gene-gene (GXG)interaction between 5HTT and MAOA by regressing Long 5HTT,High MAOA, Long*High, race controls, gender, and age onturnout. The estimated parameter on the interaction was insig-nificant (p 5 0.89).

13A variable M mediates the relationship between an independentvariable X, in our case a genotype, and a dependent variable Y, inour case voting, if (1) X significantly predicts Y, (2) X signifi-cantly predicts M, and (3) M significantly predicts Y controllingfor X (Baron and Kenny 1986).


genetic factors (McGue and Bouchard 1998). If so,then variation in the ability to process politicalinformation, which has an impact on turnout (Verba,Schlozman, and Brady 1995), may also be linked togenes. We can measure at least part of this ability usingthe Picture Vocabulary Test (PVT) administered byAdd Health, which is thought to be a good measure ofverbal IQ (Rowe, Jacobson, and Van den Oord 1999).Variation in educational attainment is another factorthat has been found to be heritable (Baker et al. 1996;Heath et al. 1985) and is frequently shown to influenceturnout (Leighley and Nagler 1992b). In order to testwhether these variables are potentially mediators, weregress each of them separately on High MAOA andLong 5HTT along with race, age, and gender controls.Since High MAOA and Long 5HTT are not signifi-cantly associated with any of these variables, we canrule them out as mediators.14

Even after including all of these variables in themodel, both High MAOA and the interaction be-tween Long 5HTT and attendance remain significant.We also observe something rather unexpected in

Model 6—the main effect of religious service attend-ance ceases to be significant (p 5 0.17). In otherwords, it appears that the direct effect of churchattendance on voter turnout that has been reportedin so many other studies (see Cassel 1999 for areview) may be driven by two factors: (1) a spuriousassociation caused by the relationship between othercorrelates of turnout and religious service attendance,and (2) the previously unmodeled interaction be-tween religious service attendance and the function-ing of the serotonin transporter, 5HTT. To test thisassertion we simply remove 5HTT from the Model 6specification (not shown). Religious attendance re-turns to exerting a significant direct effect on turnout(p 5 0.00001). Our results show that individuals withthe ‘‘short’’ 5HTT allele (the base category of the5HTT variable) who are active in religious organ-izations are not more likely to vote. Similarly,individuals with the ‘‘long’’ allele who are not asactive in religious organizations are not more likelyto vote. In fact, voting is only higher among thosewho are both strongly exposed to the sense ofcommunity offered by religious groups (Cassel1999) and potentially better equipped to handle thepotential pain associated with social risks due to afully-functioning serotonin metabolism conferred bya ‘‘long’’ 5HTT allele.

It is worthwhile pointing out how these resultsare reminiscent of the Caspi et al. (2002) findingson child abuse. Prior to that publication, scholarshad reported a weak but significant relationshipbetween receiving abuse as a child and abusingone’s own children as an adult. What Caspi and hiscolleagues showed was that this weak effect wasmoderated by the MAOA gene. People with the‘‘high’’ MAOA allele who suffered abuse as a childwere not more likely to abuse their own children.Similarly, people with the ‘‘low’’ allele who had notbeen abused were not more likely to abuse theirown children. It was only those who experiencedthe trauma of child abuse and who lacked theprotective effect of a fully-functioning serotoninmetabolism conferred by a ‘‘high’’ MAOA allelethat continued the cycle of violence. In the case ofboth voting and child abuse, bringing genes intothe study of social behavior not only highlights therole of biology—it clarifies and sharpens the effectof the environment.

Discussion

The results of this analysis are clear: we have foundthat two extensively studied genes are significantly

FIGURE 2 Changing MAOA and 5HTT to HighActivity Allele Yields SignificantlyHigher Turnout.

MAOAAll Subjects

5HTTReligious Attendees

Diff

eren

ce in

Tur

nout

(%)

If Su

bjec

ts C

hang

e Fr

om L

ow to

Hig

h Ac

tivity

Alle

le0

510

1520

Note: First differences in turnout simulated from the coefficientcovariance matrix of Model 1 and Model 4, assuming all othervalues are held at their means. Horizontal bar indicates 95%confidence interval.

14The p values for High MAOA and Long 5HTT in eachregression are presented in the appendix.


associated with voter turnout. Further, these are thefirst two genes ever directly associated with politicalbehavior. The empirical approach we employ in thispaper improves on the twin study designs of Fowler,Baker, and Dawes (2008) in a profoundly importantway. Twin studies are valuable for determining theinfluence of genes over observed behavior, but theyare agnostic about causality. By focusing on specificgenes, our analysis is able to suggest potential causalpathways through which genes influence turnout. Asignificant body of research has found that the twogenes we study, 5HTT and MAOA, influence socialbehavior via their impact on the serotonin metabo-lism and research within political science has identi-fied prosociality as a significant determinant ofturnout, thereby establishing a potential causal chainleading from these genes to observed political behav-ior. Again, we cannot test any causal pathway givenour data so we are merely speculating based onprevious work done in behavior genetics and politicalscience.

More broadly, these results represent an importantstep for political science as a discipline. Specifically,they show that incorporating genetic information intoour theories and analysis may contribute to a greaterunderstanding of political behavior. The environment-only approach used for so long in political sciencehas frequently conceptualized human behavior asa ‘‘blank slate’’ on which any tendencies couldbe drawn, regardless of the unique biology of eachindividual (Pinker 2003). However, the results pre-sented here refute the blank slate theory of politicalbehavior. Although the environment is extremelyimportant for turnout and other political acts,perhaps even more so than genes, we can no longeract as if genes do not matter at all. Genetic differencesare likely to have important consequences for a wholerange of political behaviors.

We believe the significant interaction we findbetween 5HTT and church attendance adds animportant element missing from existing theoriesexplaining the relationship between religious activityand turnout. Cassel (1999) suggests religious groupsbuild a sense of belonging to a larger community, butany sense of belonging is likely mitigated by socialanxiety or an aversion to potential social conflict.Brady, Verba, and Schlozman (1995) argue that civicskills, acquired through church activities, increase thelikelihood of voting. Specifically, they find thatregular attendance prompts individuals to performacts on behalf of their religious organization thatenhance their civic skills, in turn better equippingthem for involvement in the political process. These

acts include writing a letter, taking part in decisionmaking, giving presentations or speeches, organizingor chairing a meeting, and/or contacting governmentofficials on behalf of the church. However, the leapfrom church attendance to completing these acts isnot a surety for every individual. These acts oftenrequire extensive interaction with others in a socialsetting, something that is likely to be uncomfortablefor those with social anxiety and fear of socialrejection. Even the simple act of writing a lettermay be difficult when the person writing it knowsothers will evaluate it. Rosenstone and Hansen (1993)suggest that religious organizations provide a venueamenable to the discussion of politics, thus loweringthe cost of gathering information. Since voting is lesscostly for regular attendees compared to those whodo not attend, we should observe a higher turnoutrate among this group. As in the case with thedevelopment of civic skills, engaging in politicaldiscussions with fellow members of one’s church islikely difficult for those with antisocial tendencies.Therefore, if our hypothesis is correct, we wouldexpect individuals with the ‘‘long’’ 5HTT allele to bemost likely to benefit from political discussions in achurch setting.

Even if one concedes genes do influence politicalbehavior, it is tempting to assume that since they arenot causally proximate to observed behaviors theycan be safely ignored for practical purposes. How-ever, this thinking is mistaken. Genes are the in-stitutions of the human body—they constrainindividual behavior just as political institutions con-strain the behavior of groups of people. In this articlewe demonstrate that possessing a particular gene isassociated with voting activity. Even after controllingfor factors known to influence turnout, having a highMAOA allele raises the likelihood of voting by about5%. Among people active in their religious organ-izations, having a long 5HTT allele raises the like-lihood of voting by about 10%. We theorize thatsince low efficiency MAOA and 5HTT alleles limit thedegree to which individuals are socially oriented,these alleles inhibit their desire or ability to partic-ipate in the political process.

Our theory that genetic differences within apopulation, in part, explain variation in politicalbehavior is in stark contrast to game-theoretic mod-els of voter turnout that typically predict very littlevariation in participatory behavior (Aldrich 1993). Inthese models, when one person votes, everyone withthe same preferences benefits from the increasedlikelihood that their preferred outcome will result.Yet those who do vote must bear the cost of time and


effort required to learn about election alternativesand go to the polls. In large populations, the prob-ability that a single vote will change the outcome ofan election is miniscule (Gelman, King, and Boscardin1998, Riker and Ordeshook 1968), meaning that evenvery small costs to the individual typically outweighthe expected benefits he or she would receive fromvoting. As a result, classic models that assume indi-viduals are self-interested and fully optimizing in theirbehavior show that the equilibrium amount of voterturnout approaches zero as the population becomeslarge (Palfrey and Rosenthal 1985). Yet in spite of thissocial dilemma, millions of people do vote, sug-gesting that something other than self-interest oroptimizing behavior drives their decision (Bendor,Diermeier, and Ting 2003; Edlin, Gelman, andKaplan 2007; Fedderson and Sandroni 2006; Fowler2006a, 2006b; Fowler and Kam 2007; Jankowski2002, 2007). In addition, the fact that millions ofpeople abstain concords with our finding here thatthere is inherent variation in the human tendencyto participate. Future models of prospective voterbehavior should account for this variation inpredispositions.

One popular extension of these models is toassume that some individuals experience an extrabenefit from voting that has nothing to do with theoutcome (the ‘‘D’’ term as Riker and Ordeshook(1968) called it). Instead, this benefit comes from thesatisfaction of fulfilling a civic duty or of contributingto the democratic process (Blais and Young 1999).Alternatively, one might vote to fulfill a desire forexpression (Schuessler 2000), which might be mod-elled as an ‘‘E’’ term. In other words, these modelsposit that there is an additional factor that yieldsinherent heterogeneity in the desire to vote. Whilemany scholars believe this line of argument isplausible (notably Aldrich (1993, 266): most of theaction is probably in the intrinsic values of voting perse’’), these models have failed to consider the possi-bility that this heterogeneity can be attributed togenes. Thus, our results suggest that a fruitful avenuefor future research is to study whether or notexpressiveness or feelings of civic duty intermediatethe association between genes and political partic-ipation. In fact, at least one study suggests this maybe the case for MAOA. Rosenberg et al. (2006) showthat the high activity allele of MAOA is associatedwith ‘‘straightforwardness’’ (frankness in expression)and ‘‘conscientiousness,’’ suggesting two additionalpathways between genes and voting.

In contrast to the theoretical literature, theempirical literature has embraced variation in turn-

out behavior with models that test dozens of explan-atory variables (Plutzer 2002; Timpone 1998; Verba,Schlozman, and Brady 1995). These models include:demographic factors like age (Strate et al. 1989), gen-der (Schlozman et al. 1995), race (Verba, Schlozman,and Brady 1993), marital status (Stoker and Jennings1995), education (Leighley and Nagler 1992b), in-come (Leighley and Nagler 1992a), occupationalprestige (Nie, Powell, and Prewitt 1969a, 1969b),and home ownership (Highton and Wolfinger 2001);attitudinal and behavioral factors like interest in thecampaign (Verba, Schlozman, and Brady 1995),access to political information (DiMaggio, Hargittai,and Neuman 2001), general political knowledge(Galston 2001), strength of partisanship (Huckfeldtand Sprague 1992), feelings of civic duty (Blais andYoung 1999), internal and external efficacy (Finkel1985), political trust (Hetherington 1999), churchattendance (Cassel 1999), personal skill acquisition(Brady, Verba, and Schlozman 1995), humanitarian-ism (Jankowski 2007), altruism (Fowler 2006a), andpatience (Fowler and Kam 2006); social factors likeinterpersonal communication (McLeod, Scheufele,and Moy 1999), social identification (Fowler andKam 2007), group consciousness (Miller, Gurin, andGurin 1981), socialization (Cho 1999), the status ofneighbors (Huckfeldt 1979), political disagreement(Mutz 2002), and social capital (Lake and Huckfeldt1998); and institutional factors (Jackman and Miller1995) like closeness of the election (Shachar andNalebuff 1999), contact from political organizations(Wielhouwer and Lockerbie 1994), campaigns (An-solabehere and Gerber 1994), civic education (Somitet al. 1958), polling locations (Gimpel and Schuknecht2003), and barriers to registration (Rosenstone andWolfinger 1978). However, not one of these articleshas considered the possibility that genes may accountfor this variation.

Genes may also help us to explain two well-known features of voting. First, parental turnoutbehavior has been shown to be one of the strongestpredictors of turnout behavior in young adults(Plutzer 2002). Although this has previously beeninterpreted as the result of social influence, thefindings here suggest it may also be due to theinheritance of particular alleles of genes like MAOAand 5HTT. Second, turnout behavior has been shownto be habitual—the majority of people either alwaysvote or always abstain (Fowler 2006b; Gerber, Green,and Shachar 2003; Green and Shachar 2000;Miller and Shanks 1996; Plutzer 2002; Verba andNie 1972). Scholars previously interpreted this asthe result of reinforcement learning, but given that


genes like MAOA and 5HTT are fixed, it mightalso be largely due to inherent genetic variabilitywithin the population. Future longitudinal and fam-ily studies of voter turnout should investigate whatrole MAOA and 5HTT plays in the transmissionof political behavior over time within individualsand between parents and children. In particular, itwill be interesting to understand better why thesetwo genes that affect the serotonin system behavedifferently—why MAOA is associated with behaviordirectly, while 5HTT interacts with exposure to socialactivity.

Future work should use genetic association stud-ies to identify specific genes that are implicated inpolitical behaviors and attitudes. Finding out whichgenes they are and what physical function they havewill improve our understanding of the biologicalprocesses that underlie these complex social behav-iors and may also shed light on their evolutionaryorigin (Fitzpatrick et al. 2005). It is important to em-phasize that there is likely no single ‘‘voting gene’’—the results presented here suggest that at least twogenes do matter and there is some (likely large) set ofgenes whose expression, in combination with envi-ronmental factors, influences political participation.Finally, we offer a word of caution. Associationstudies like ours require further replication beforetheir findings can be truly considered anything morethan suggestive, therefore more work needs to bedone in order to verify and better understand thespecific associations we have identified.

Acknowledgments

This research was funded by National Science Foun-dation grant number SES-0719404. The most recentversion of this paper can be found at http://jhfowler.ucsd.edu. The contact author can be reached [email protected].

Appendix

Variable Definitions

High is an indicator variable for having two of the336, 351, or 381bp alleles of the MAOA gene. Longrefers to having at least one 528bp allele of the 5HTTgene. Partisan is the answer to the question ‘‘Do youidentify with a specific political party?’’ Attendance isconstructed from the response to the question, ‘‘How

often have you attended [church/synagouge/temple/mosque/religious] services in the past 12 months?’’The categories of attendance are never, at least a fewtimes but no more than once a month (baseline), andmore than once a month. We center this variable onthe category at least a few times a month (never5 -1,at least once a month 5 0, more than once a month5 1). Other race/ethnicity indicator variables basedon the questions ‘‘Are you of Hispanic or Latinoorigin?’’ and ‘‘What is your race? [white/black orAfrican American/American Indian or Native Amer-ican/Asian or Pacific Islander].’’ Age is self-reportedage, and Male is an indicator taking the value of 1 ifthe respondent is a male and 0 for a female. Income isthe log of the response to the question, ‘‘Including allthe income sources you reported above, what wasyour total personal income before taxes in [2000/2001]?’’ Cognitive Ability is the score on the PictureVocabulary Test, which measures verbal intelligence.College is an indicator variable taking the value 1 ifthe respondent completed at least one year of collegeand 0 for no college. It is based on the question,‘‘What is the highest grade or year of regular schoolyou completed?’’

TABLE 2 Percentage of subjects exhibiting thesecharacteristics.

Percent

Vote 45.7White 72.3Black 19.2Native American 2.2Hispanic 12.3Male 47.8High MAOA 48.1Long 5HTT 81.4Partisan 37.3College 55.5

TABLE 3 Sample means.

Mean Std Dev

Age 21.9 1.7Income 12912 13926Cognitive Ability 99.12 13.94Attend 2.1 0.8


Manuscript submitted 3 December 2007Manuscript accepted for publication 5 February 2008

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James H. Fowler is associate professor of politicalscience, University of California, San Diego, La Jolla,CA 92093. Christopher T. Dawes is a Ph.D. candidatein political science, University of California, SanDiego, La Jolla, CA 92093.


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