Assortative Mating on Ideology Could Operate
Through Olfactory Cues
Rose McDermott Brown University
Dustin Tingley Harvard University
Peter K. Hatemi Pennsylvania State University
Mates appear to assort on political attitudes more than any other social, behavioral, or physical trait, besides religion. Yet
the process by which ideologically similar mates end up together remains ambiguous. Mates do not appear to consciously
select one another based on ideology, nor does similarity result from convergence. Recently, several lines of inquiry have
converged on the finding that olfactory processes have an important role in both political ideology and mate selection. Here
we integrate extant studies of attraction, ideology, and olfaction and explore the possibility that assortation on political
attitudes may result, in part, from greater attraction to the scent of those with shared ideology. We conduct a study in which
individuals evaluated the body odor of unknown others, observing that individuals are more attracted to their ideological
concomitants.
Similarity between spouses is common across do- into understanding some of the foundations of assorta-mains, but in humans, long-term mates correlate tive mating on the basis of political ideology. A growingmore highly (between 0.60 and 0.75) on social and body of evidence reveals that the mechanisms that ac-
political attitudes than almost any other trait, with the ex- count for differences in ideological attitudes are geneti-
ception of religion (Alford et al. 2011; Byrne 1961; Cavior, cally and biologically influenced and conscript olfactory
Miller, and Cohen 1975; Curry and Kenny 1974; Eaves et processes (Chapman et al. 2009; Chapuisat 2009; Fowler
al. 1999; Eaves and Hatemi 2011; Feng and Baker 1994; and Christakis 2013; Fowler and Schreiber 2008; Haidt
Hatemi et al. 2010;Martin et al. 1986; Stoker and Jennings 2012; Hatemi et al. 2011; Hatemi et al. 2014; Herz and
1995, 2006; Zietsch et al. 2011; Zuckerman, Fitzgerald, Inzlicht 2002; Inbar et al. 2012; Inbar, Pizarro, and Bloom
and Dasovic 2005). The processes that result in religious 2012; Navarrete and Fessler 2006). In this research note,
assortation are well understood, resulting largely from we integrate these lines of inquiry and reveal that people
social homogamy and proximity effects (e.g., Eaves, Mar- find the smell of ideologically similar others more attrac-
tin, and Heath 1990; Kalmijn 1998). However, we know tive, thereby providing preliminary evidence suggesting
much less about the mechanisms that produce high levels that one of the mechanisms by which political assortative
of spousal correlation on left-right political dimensions. mating occurs is through subconscious sexual attraction
Unlike religion, assortative mating on attitudes does not to variant body odors.
appear to result from partners becoming more similar
over time, social homogamy, or direct selection (Alford
et al. 2011; Klofstad,McDermott, andHatemi 2012, 2013;
Kofoed 2008; Rantala and Marcinkowska 2011). Olfaction and Mate Selection
The findings that attitudes are not only socially
driven, but are equally informed by genetic and neu- Olfactory mechanisms have proven important in mate
robiological mechanisms may provide valuable insight seeking and reproduction in both humans and animals
RoseMcDermott is Full Professor of Political Science, BrownUniversity, 36Prospect St., Providence, RI 02906 (rmcdermott21@gmail.com).
Dustin Tingley is Paul Sack Associate Professor of Political Economy, Government Department, Harvard University, 1737 Cambridge St.,
K208, Cambridge, MA 02138 (dtingley@gov.harvard.edu). Peter K. Hatemi is Associate Professor of Political Science, Microbiology, and
Biochemistry, Pennsylvania State University, 307 Pond Lab, University Park, PA 16802 (phatemi@gmail.com).
We thank Kelly Gildersleeve and the Martie Haselton Lab at UCLA for their scent protocol and guidance. Replication data for this article
are available in the AJPS Data Archive on Dataverse (http://thedata.harvard.edu/dvn/dv/ajps).
American Journal of Political Science, Vol. 00, No. 00, May 2014, Pp. 1–9
©C 2014, Midwest Political Science Association DOI: 10.1111/ajps.12133
1
2 ROSE MCDERMOTT, DUSTIN TINGLEY, AND PETER K. HATEMI
because smell may signal mate immunocompetence, so- uing through early development (Bereczkei, Gyuris, and
cial compatibility, or other characteristics associated with Weisfeld 2004; Rikowski and Grammer 1999; Thornhill
mate quality and optimal reproduction (Blaustein 1981; and Gangestad 1999), and pathogen avoidance (Faulkner
Brennan 2004; Folstad and Karter 1992; Milinski and et al. 2004; Navarrete and Fessler 2006). We discuss these
Wedekind 2001; Miller, Tybur, and Jordan 2007; Penn processes in more detail below.
and Potts 1998; Thornhill and Gangestad 1999). While
most senses operate through the brain stem and thala-
mus, the olfactory bulb has direct connections with the
seat of emotion in the amygdala and the center of mem- Olfaction and Political Ideology
ory in the hippocampus. This indicates the importance
that natural selection placed on odorants, in some ways Why and how might smell signals be linked to politi-
privileging speed of learning by smell over that offered by cal ideology? Smell signals help maximize prospects for
vision or hearing. For example, Herz and Inzlicht (2002) disease avoidance, cheater detection, defense against out-
found that women ranked body odor as more important groups, and social cohesion (Chapuisat 2009; Fowler and
for attraction than any other physical factor, including Christakis 2013). These topics are embedded within at-
looks; only the social factor of “pleasantness” emerged titudinal measures of ideology. For example, greater dis-
more dominant in evaluations. Moreover, liking some- gust sensitivity, which is intimately interconnected with
one’s natural body odorwas themost influential olfactory the neural substrates of smell, predicts more conservative
variable driving sexual interest for bothmen and women. positions, particularly around issues involving morality
The most prominent explanation for how attraction and sexual reproduction. These underlying, physically
operates through olfactory and chemosensory channels experienced predilections can come to be expressed as
has relied on the finding that individuals tend to be more opinions on such topics as abortion, homosexuality, gay
attracted to the odor of those whose major histocompati- marriage, and a host of other ideological topics (Heining
bility complex (MHC) offers optimal complementarity to et al. 2003; Inbar et al. 2012; Inbar, Pizarro, and Bloom
one’s own(Brennan2004;Thornhill et al. 2003;Wedekind 2012; Phillips et al. 1997). In a related vein, Navarrete
and Furi 1997). MHC variants have a significant role in and Fessler (2006) find that ethnocentrism represents an
immune, odor, and kin recognition; susceptibility to dis- evolved function designed to avoid disease and to pro-
eases; mate selection; and pregnancy outcomes. Indeed, tect individuals from dissimilar others. These attitudes
many scholars propose that the loss of MHC variation manifest in left-right political orientations (for earlier de-
negatively affects population survival (Radwan, Biedrzy- velopments, see Faulkner et al. 2004; Folstad and Karter
cka, and Babik 2010). Milinski et al. (2013) find that 1992).
humans have the ability to detect and evaluate MHC
peptides in body odor, and this ability is mediated by
genotype, whichmay provide a basis for the sensory eval-
uation of potential partners during human mate choice Olfaction, Mating, and Political
(Davis 2013; Woelfing et al. 2009).1 In this way, optimal Ideology
complementarity in MHC may include some combina-
tion of different and sharedMHC alleles (Nowak, Tarczy- Olfaction correlates in specific ways with differing
Hornoch, and Austyn 1992). political preferences through genetic and biological
MHC, however, constitutes only one mechanism mechanisms similar to those employed in choice of
by which olfactory processes influence sexual selection sexual partners. As noted above, spouses and long-term
(Havlicek and Roberts 2009). Indeed, numerous other partners appear to be more similar in their political pref-
mechanisms have been proposed to explain why indi- erences than almost any other trait. This affinity exists
viduals find those with particular scents more attractive, prior to marriage, and the length of marriage appears to
including processes of imprinting, defined as acquisition have little effect on spousal similarity in ideology (Alford
of sexual preferences that emerge through unrewarded et al. 2011; Caspi and Herbener 1993; Feng and Baker
experiences with parents beginning in utero, and contin- 1994; Kofoed 2008; Watson et al. 2004). Suggestively,
Hatemi et al. (2011) identified several genomic regions
1There remains some debate as to whether MHC retains as sub- that account for variation in ideological orientation, one
stantial a role in social behavior (reviewed in Penn and Potts 1999). of which contained a large number of olfactory receptors.
For example, much of the evidence that supports MHC’s influence
on sexual selection operating through heterozygous attraction is They proposed that “if olfactory receptors account for
based on inbred groups of mice. some variation on political preferences, they may do so
ASSORTATIVE MATING, IDEOLOGY, AND OLFACTION 3
through intended optimal breeding and rearing strategies system that is designed to optimize prospects for success-
such as spousal selection” (Hatemi et al. 2011, 280). Davis ful reproduction by selecting partners who closely align
(2013), in a thorough examination of genetics and mate on parental investment strategies and other critical values
selection, provides evidence that genotype influences regarding sex and reproduction. Just as adult individuals
how attractive a person is to a potential partner because prefer smells that populate their mother’s world in utero
suitable mates display differential smell preference for and their world in childhood (Browne 2008; Mennella,
partners with an optimal genetic code for them. In this Jagnow, and Beauchamp 2001), we suggest that mates
way, people may be subconsciously choosing an optimal find those who are homogamous on social and political
mating partner who would increase the probability of ideologies more attractive, and propose that this pref-
having children with more social and genetic advantages. erence helps maximize prospects for successful and en-
These dynamics may represent an extension of im- during mating, interpersonal bonding, and compatible
printing whereby offspring “fashion a mental model parenting strategies.
of their opposite–sex parent’s phenotype that is used
as a template for acquiring mates” (Bereczkei, Gyuris,
and Weisfeld 2004, 1129). For example, fetuses come to
prefer scents that most populate their mother’s world Materials and Methods
(Browne 2008), and humans, including mothers, spend
most of their time around ideologically similar others We conducted a study where participants rated the at-
(e.g., Fowler, Settle, and Christakis 2011; Posner et al. tractiveness of the body odor of unknown strong liberals
1996). If social attitudes are linked to odor, as the litera- and strong conservatives, hereafter referred to as “target”
ture suggests, then one mechanism that odor preferences subjects; all “evaluator” subjects remained blind to the
transfer from parents to children may operate through ideologyof the target samples.Onehundred forty-six par-
their mother’s choice of mate. In this way, social pro- ticipants between 18–40 years oldwere drawn froma large
cesses may drive some of the pathways by which indi- city in the northeast United States; half were drawn from
viduals come to prefer those whose ideological “smell” a volunteer subject pool associated with a large university,
matches their own (e.g., Schaal, Marlier, and Soussignan and the other half were taken from the general population
2000). solicited through media advertisements, electronic mail-
Political compatibility may also serve as a modern ing lists, and other forms of social networks. Informed
representation of a host of mechanisms and values that consent was obtained from all participants. Twenty-one
directly affect physiological and sexual compatibility as target participants were selected for their high scores on
well as child-rearing strategies. This is because parental opposite ends of the political spectrum (10 liberals and
similarity in values increases the likelihood that such 11 conservatives) and provided body odor samples, in
individuals may be able to stay together long enough to accordance with established protocols for sample col-
raise their children successfully into adulthood in order lection and storage (Haselton and Gildersleeve 2011).
to potentiate their own reproductive success. Uniform Eleven of these target participants were female and 10
parental rearing practices also provide consistently better were male (see the online supporting information for
outcomes for child learning and development than cross-tabulations and additional descriptive information,
conflicting value structures (Block, Block, and Morrison including instructions given to participants). Ideology
1981). In addition, parents who share particular political was measured using the standard 7-point (strongly lib-
values, and thus get along better,may find it easier to elicit eral to strongly conservative) American National Election
social support and social capital than those constantly Studies (ANES) self-report measure: “Here is a 7-point
prone to infighting. Such social support increases health scale on which the political views that people might hold
and longevity (Cacioppo and Hawkley 2003; Hawkley et are arranged from extremely liberal to extremely conser-
al. 2003; Hawkley and Cacioppo 2003). vative. Where would you place yourself on this scale?”
In summary, olfactory signals communicate impor- (ANES 2012). Target participants washed in fragrance-
tant characteristics of mate quality, providing an efficient free shampoo and soap and then taped one 2×2 Johnson
evolutionary mechanism by which to enhance reproduc- & Johnson gauze pad to each underarm using Johnson
tion. In this way, attraction to the smell of those with & Johnson paper tape, all of which we provided. Par-
similar attitudesmay reflect the ontological remnants of a ticipants wore these pads for 24 hours following a strict
more primitive behavioral adaptation designed to ensure protocol that prohibited smoking, drinking, deodorants,
reproductive success. We propose that modern assorta- perfumes, being around strong odors or candles, animals,
tion on political attitudes may rest, in part, on an evolved eating strong-smelling foods, having sex, or sleeping in a
4 ROSE MCDERMOTT, DUSTIN TINGLEY, AND PETER K. HATEMI
FIGURE 1 Sample Population Ideology and Sex
Target Ideology Evaluator Ideology
1 2 3 4 5 6 7 1 2 3 4 5 6 7
Liberal : Conservative Liberal : Conservative
Target Sex Evaluator Sex
Female Male Female Male
Note: The sexual categories are binary, and the ideological ones exist on a 7-point scale ranging from
very liberal (1) to very conservative (7).
bed with any other people or pets. Smokers, those who to recognize any target body odor. Figure 1 provides the
reported more than four drinks a day, and menstruating descriptives of our evaluator subject sample. The sample
or pregnant women were excluded from the study. Com- is slightly more female and liberal, although we made ev-
pliance was ascertained when participants returned their ery effort to recruit equal numbers by sex and across the
samples to the lab24hours later.One samplewas excluded political spectrum, including contacting every Republi-
because the participant wore the pads too long, return- can club at five universities in the greater metropolitan
ing them the day after they were requested, making this area and asking the conservative subjects we ascertained
sample not commensurate with the others. Samples were to contact any of their ideological compatriots and en-
transferred into sterile containers and frozen at –32 de- courage them to participate in the study.
grees Celsius in a secure freezer in the laboratory. A week
later, 125 participants evaluated the body odor of each of
the target participants by smelling each vial containing
a target participant’s gauze pads. Samples were thawed Analyses and Results
one hour prior to use and used for two hours subsequent
to thaw. Each subject smelled each vial individually in We estimated interpersonal attraction for odor as a func-
randomized order; vials were identified by number. Sub- tion of both targets’ and evaluators’ ideologies and sex.
ject fatigue was minimized by having subjects smell small We use ordinary least squares with robust standard errors
pieces of paper dipped in peppermint essential oil be- clustered at the evaluator level. The dependent variable
tween each sample to refresh the nasal canal, in keeping is the reported level of attraction. Subtracting the target’s
with previous protocol in this area (Haselton and Gilder- average attractiveness and using this transformed vari-
sleeve 2011). Each subject rated the attractiveness of each able as a dependent variable produces similar results. We
vial on a 5-point Likert scale ranging from least to most estimate several different models (see Table 1).
attractive. In addition, each subject was asked to guess the Model 1 uses binary versions of our ideology vari-
political ideology of each sample on theANES left-right 7- ables, classifying anyone with an ideology score of 4 and
point continuum. Upon inquiry, no participant claimed up as conservative. As explanatory variables, we use an
Frequency Frequency
0 5 10 0 2 4 6 8
Frequency Frequency
0 20 40 60 80 0 10 20 30 40 50
ASSORTATIVE MATING, IDEOLOGY, AND OLFACTION 5
TABLE 1 Odor Attraction as a Function of Models 2 and 3 use a different estimation strategy.
Ideological Similarity Here we retain the continuous nature of our ideology
variable and calculate the absolute value of the difference
Model 1 Model 2 Model 3 between the target’s and evaluator’s ideology, multiplied
Same Ideology 0.0853 by negative 1 (–Abs. Ideology Diff.). Larger values indi-
(0.0504) cate greater ideological similarity between the target and
−Abs. Ideology Diff. 0.0206 0.0226 evaluator. In Model 2, we control for evaluator (Ideol-
(0.0139) (0.0156) ogy of Eval.) and target ideology (Ideology of Target; both
Same Sex −0.144 −0.143 −0.143 continuous measures), sex, and average attraction levels.
(0.0430) (0.0431) (0.0443) In Model 3, we include evaluator and target fixed effects,
Conservative Eval. −0.00557 and any variables that are constant at the evaluator and
(0.00428) target levels are dropped. In both models, we observe the
Conservative Target 0.0196 hypothesized positive coefficient on the negative abso-
(0.0513) lute difference in ideology scores (–Abs. Ideology Diff.),
Ideology of Eval. −0.000919 though in both cases the coefficient is less precisely esti-
(0.00101) mated (t= 1.48 in Model 1 and t= 1.45 in Model 2), but
Ideology of Target 0.00561 still with one-sided p-values less than 0.1. In all cases, the
(0.0120) substantive effect of ideological similarity is small, which
Male Evaluator −0.0000277 0.000401 is to be expected.
(0.00412) (0.00417) The analyses above support our hypotheses and de-
Male Target −0.0174 −0.0141 sign based on our assumptions. However, different as-
(0.0439) (0.0464) sumptions and constructions of the relationships between
Avg. Target Attract 0.999 1.001 target and evaluator may necessitate using alternative
(0.0655) (0.0654) methods. As a result, we conduct additional analyses that
Avg. Eval. Attract 0.999 0.999 othersmight find beneficial, allowing readers to place our
(0.00339) (0.00367) results in different contexts, including the use of jackknife
Constant −3.576 −3.506 3.290 standard errors,mixedmodels to account for randomand
(0.231) (0.233) (0.142) fixed effects, and two-way clustering. We discuss the re-
Observations 2195 2195 2195 sults of these models in the supporting information due
to space constraints. Generally, the results are similar to
Note: Standard errors are clustered at evaluator level in parentheses. what is presented above.
indicator for whether the evaluator is conservative (Con-
servative Eval.), whether the target is conservative (Con-
servative Target), and whether the target and evaluator Discussion
have the same ideology (Same Ideology). We also control
for sex, using indicators for whether the evaluator is a Several important and unique results emerge from this
male (Male Evaluator), whether the target is a male (Male study. First, individuals find the smell of those who are
Target), and whether the target and evaluator both are more ideologically similar to themselves more attractive
the same sex (Same Sex). Finally, we include the average than those endorsing opposing ideologies; recall that par-
attractiveness of each target (Avg. Target Attract), as well ticipants never saw the individualswhose smells theywere
as each evaluator’s average reported attraction (Avg. Eval. evaluating, and the order of target subjects was random-
Attract).2 The results in Model 1 show a positive coeffi- ized for each evaluator. Thus, the recognition of political
cient for targets and evaluators having the same ideology alignment occurred through the medium of attraction,
(t = 1.69) and a negative coefficient on being the same not recognition.
sex (t = –3.34), which is consistent with our theoretical The amount of variation explainedby odor attraction
expectations. is small, but this should not be surprising and remains
consistent with studies on the biological properties
of other critical dimensions of social compatibility,
2We are aware that some of these results might be distorted by including sharing (Nettle et al. 2011), cheater detection
individual subjects’ sexual orientation. However, the institutional
review board would not allow us to inquire into subjects’ sexual (Cosmides et al. 2005), disease avoidance (Neuberg,
orientation, so we were not able to obtain this information. Kenrick, and Schaller 2011), and sexuality (Miller 2011).
6 ROSE MCDERMOTT, DUSTIN TINGLEY, AND PETER K. HATEMI
It is important to recognize that olfactory processes Howmight this process operate in the real world and
operate within complex social dynamics and environ- in real time, where our environments and backgrounds
mental contexts. This is particularly true of humans influence the people we are exposed to and meet, and
who can override or alter the importance of chemical affect the environments in which we communicate with
signals for conscious reasons. In humans, attraction those we see, hear, and smell? We expect that while hu-
remains idiosyncratic and culturally informed, with mans are generally aware ofwhat they are doing, andoften
greater emphasis placed on physical and sociocultural make conscious and cognitive choices to override more
features (Buss 1987; Jones et al. 1995). Various aspects of basic physical desires, it is most likely that odor operates
attraction indicative of fecundity, such as youth, beauty, subtly and may affect the regulation of hormonal states
hip-to-waist ratio in women (Singh 1993), and the ability and instigate changes in emotional mood. Such mecha-
to provide resources in men (Townsend and Levy 1990), nisms do not vitiate the reality or influence of odor on
have historically represented cross-cultural norms, to natural or initial attraction. Olfactory cues may provide
some degree. Nevertheless, such behaviors do not neces- automatic but indirect indicators that enhance immedi-
sarily obviate the basic “chemistry” that influences how ate attraction and increase the likelihood that politically
individuals feel toward specific others. We suggest olfac- similar partners find each other socially and sexually ap-
tion provides a similar signal of compatibility to potential pealing over time (Martins et al. 2005), and thus remain
mates. together long enough to improve their mutual prospects
Some insight on the potency of odormight be gained for reproductive success (Davis 2013). Conversely, poten-
from the participants’ comments and physical reactions tial mates who might otherwise appear appropriate may
during the study, not adequately captured in the quan- not spark any sense of natural “chemistry” with one an-
titative analyses, but consistent with Herz and Inzlicht other because they do not align politically or otherwise.
(2002). In one particularly illustrative case, a participant While people can choose to dismiss or ignore these sig-
asked the experimenter if she could take one of the vials nals in favor of more conscious considerations, it appears
home with her because she thought it was “the best per- nature stacks the deck to make politically similar part-
fume I ever smelled”; the vial was from amale who shared ners more attractive to each other in unconscious ways,
an ideology similar to the evaluator. She was preceded by at least over the long haul. Such information may not
another respondent with an ideology opposite to the per- be useful in all mating contexts, particularly among those
son who provided the exact same sample; this participant seeking short-termpartnerships. Nevertheless, long-term
reported that that vial had “gone rancid” and suggested it mates do align on political ideology more than would be
needed to be replaced. In this way, different participants expected by chance, and modern human mating strate-
experienced the exact same stimulus in radically different gies, which clearly rely on overt cues and communication,
ways only moments apart. likely remain informed by evolved mechanisms, such as
We do not claim that olfactory mechanisms establish olfactory cues, to trigger and signal sexual, social, and
an immediate or proximal cause of mate attraction, the political compatibility. Individuals are not run by instinct
strongest predictor of attraction, or represent the only alone, but complex anatomical processes continue to in-
influence on attraction, or that ideology is the only mod- formcognitive, emotive, and evaluative choices inmating,
erator of odor attraction. Indeed, the influence of smell and ideology has some role in this complex process.
constitutes only one of thousands of potential factors that
operate as part of the complex interaction between lo-
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