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Assortative mating and couple similarity: Patterns, mechanisms, and consequences

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Abstract

Assortative mating refers to the tendency of two partners' characteristics to be matched in a systematic manner, usually in the form of similarity. Mating with a similar partner has profound implications at the species, societal, and individual levels. This article provides a comprehensive review of research on couple similarity since 1980s. The review begins with the general patterns and trends observed in couple similarity on a range of domains including demographic variables, physical/physiological characteristics, abilities, mental well-being, habitual behaviors, attitudes, values, and personality. Next the bulk of the review focuses on analyses of 4 mechanisms leading to similarity: initial active choice, mating market operation, social homogamy, and convergence. Specific future research avenues are outlined to improve understanding of these mechanisms. Finally, the review discusses genetic, social, and psychological consequences of couple similarity.
ARTICLE
Assortative mating and couple similarity: Patterns,
mechanisms, and consequences
S. Luo
University of North Carolina at Wilmington
Correspondence
S. Luo, University of North Carolina at
Wilmington, Wilmington, NC, USA.
Email: luos@uncw.edu
Abstract
Assortative mating refers to the tendency of two partners' character-
istics to be matched in a systematic manner, usually in the form of
similarity. Mating with a similar partner has profound implications
at the species, societal, and individual levels. This article provides a
comprehensive review of research on couple similarity since
1980s. The review begins with the general patterns and trends
observed in couple similarity on a range of domains including demo-
graphic variables, physical/physiological characteristics, abilities,
mental wellbeing, habitual behaviors, attitudes, values, and person-
ality. Next the bulk of the review focuses on analyses of 4 mecha-
nisms leading to similarity: initial active choice,mating market
operation,social homogamy, and convergence. Specific future
research avenues are outlined to improve understanding of these
mechanisms. Finally, the review discusses genetic, social, and psy-
chological consequences of couple similarity.
1|INTRODUCTION
Why two romantic partners end up being with each other but not with many other potential partners is a fundamental
question to both academicians and laymen. While a definitive answer has yet to be found, one aspect we are certain
about is that the pairing of two partners is far from random (see Buss, 1985). Assortative mating (AM) refers to the
tendency for two partners to be matched systematically on one or more characteristics, most commonly examined
in the form of similarity or complementarity (e.g., Thiessen & Gregg, 1980; Vandenberg, 1972). Because the bulk of
the literature supports similarity (see next section for details), researchers often equate AM to couple similarity. This
can be problematic because similarity can be present at the onset of the relationship as part of the partner selection
decision or show up later in the relationship due to partner interactions. In fact, the majority of previous studies can-
not differentiate these two possibilities because their findings were based on couples who had been together for
some time. To improve conceptual clarity, I wish to distinguish these two terms in the current review: Whereas AM
specifically refers to initial partner similarity evident at the beginning of the relationship that reflects active or passive
assortment, (couple) similarity refers to partner similarity at any point of the relationship, which may result from initial
assortment and/or development within the relationship.
Research on this topic can be traced back to Pearson (1903) who reported positive correlations in spouses' height,
span of arms, and forearm length in 1,000 couples. Since then, this line of research has expanded from physical to
DOI: 10.1111/spc3.12337
Soc Personal Psychol Compass. 2017;11:e12337.
https://doi.org/10.1111/spc3.12337
© 2017 John Wiley & Sons Ltdwileyonlinelibrary.com/journal/spc3 1of14
many demographic, psychological, and even genetic characteristics. The purpose of the current review is threefold:
First, because two previous reviews surveyed this area through early 1980s (Epstein & Guttman, 1984; Vandenberg,
1972), this article aims to provide an update of the literature since then. Second, the bulk of the article is devoted to
an analysis of the underlying mechanisms of couple similarity. Finally, genetic, social, and psychological consequences
of similarity are discussed.
2|PATTERNS AND TRENDS OF COUPLE SIMILARITY
2.1 |Demographics
Age arguably shows the strongest couple similarity. In more than a century's research, age homogamy (i.e., marry-
ing someone with similar characteristics) remains consistently high (e.g., Schwartz & Graf, 2009; Van de Putte
et al., 2009). Spousal age correlation typically ranges from .70s to .90s (e.g., Feng & Baker, 1994; George et al.,
2015; Watson et al., 2004). For race/ethnicity and religion, although endogamy (i.e., marrying an ingroup member)
remains the norm in the United States (e.g., Schwartz & Graf, 2009), it has declined significantly over the 20th
century (see Kalmijn, 1991a; Schwartz, 2013). This decline in racial and religious endogamy is evident elsewhere
in the West (e.g., Frimmel, Halla, & WinterEbmer, 2013).
In contrast to the decline in racial and religious endogamy, most studies have indicated an increase in educational
homogamy in the United States, particularly between 1940s and 1970s (e.g., Blackwell, 1998; Kalmijn, 1991a, 1991b).
Similarity correlations in spouse education range from .40s to .60s (e.g., George et al., 2015; Hur, 2016; Watson et al.,
2004). Likewise, endogamy in socioeconomic status (SES), commonly measured in employment status and income
level, has been on the rise since 1960s in the United States as women's earnings and education continue to increase
(e.g., Sweeney & Cancian, 2004).
2.2 |Attitudes and values
Previous research has examined couple similarity on a variety of attitudes and values: political orientation, religiosity,
authoritarianism, family role attitudes, risk attitudes, personal values, etc. Strong similarity correlations with a typical
range of .40s to .70s are reported for attitudes and a weaker range of .10s to .40s for values (e.g., Bacon, Conte, &
Moffatt, 2014; Feng & Baker, 1994; Gaunt, 2006; Watson et al., 2004).
2.3 |Abilities and intelligence
Moderate partner similarity for abilities and intelligence has been documented, with correlations typically ranging
from .20s to .40s (e.g., MascieTaylor, 1989; Reynolds, Baker, & Pedersen, 2000; Van Leeuwen, van den Berg, &
Boomsma, 2008), although there appears to be a gradual tendency to decline (Johonson, Ahern, & Cole, 1980). Addi-
tionally, partner correlations on verbal ability are higher than those on perceptual speed or spatial ability (Epstein &
Guttman, 1984; Watson et al., 2004).
2.4 |Mental wellbeing and psychopathology
Moderate positive associations have consistently been observed on partners' subjective wellbeing (e.g., Hoppmann,
Gerstorf, Willis, & Schaie, 2011b) and a wide range of disorders including schizophrenia, affective disorders,
autism, antisocial behaviors, alcoholism, and other substance dependence disorders (e.g., BaronCohen, 2006;
Frisell, Pawitan, Langstrom, & Lichtenstein, 2012; Maes et al., 1998; Mathews & Reus, 2001; Sakai et al., 2004).
The similarity correlations typically range from .20s to .50s (Frisell et al., 2012; Grant et al., 2007; RhuleLouie
& McMahon, 2007). Additionally, there is evidence for crossassortative matingpatients' partners tend to have
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related disorders. For example, partners of schizophrenic patients are more likely to have nonaffective functional
psychosis such as schizotypal, schizoid, paranoid, borderline, and narcissistic disorders, but not neurotic disorders
(Parnas, 1985).
2.5 |Habitual behaviors and lifestyle
Moderate similarity in couples has been observed on a variety of habitual behaviors, hobbies, and lifestyle including
alcohol, coffee, and tea consumption; smoking; circadian rhythm; and physical exercise with a typical range of .20s
to .50s (e.g., Buss, 1984; George et al., 2015; Randler & Kretz, 2011). Diet similarity is lower than alcohol consumption
(e.g., Epstein & Guttman, 1984). Moreover, these correlations are not due to age assortment (e.g., Buss, 1984).
2.6 |Physical and physiological characteristics
Small, positive correlations ranging from .10s to .20s have been reported for couples' physical characteristics including
height, weight, various measures of body shape, and finger length as well as biomarkers of physical wellbeing such as
blood pressure, kidney function, and cholesterol level; importantly, these associations are independent of effects of
age, education, and race (e.g., Di Castelnuovo, Quacquaruccio, Donati, De Gaetano, & Iacoviello, 2009; Peek &
Markides, 2003; Voracek, Dressler, & Manning, 2007). Partner similarity on perceptions of physical attractiveness
and healthiness are higher with a range of .30s.40s (e.g., George et al., 2015).
2.7 |Personality
Research on couples' personality similarity is extensive, covering traits such as Big Five factors, attachment avoidance
and anxiety, positive and negative affectivity, selfesteem, and sensation seeking. Overall, similarity correlations on
personality variables tend to be positive though with weak magnitude rarely above .30 (e.g., Botwin, Buss, &
Shackelford, 1997; Luo, 2009; Watson et al., 2004) and occasionally negative (e.g., Markey & Markey, 2007).
3|MECHANISMS FOR COUPLE SIMILARITY
The above review suggests two general patterns: First, similarity is the rule and complementarity is an exception. Sec-
ond, the degree of similarity depends on the specific domain, showing a hierarchy with strong similarity in demo-
graphic variables and attitudes; moderate similarity in values, intelligence, interests, and mental wellbeing; but
weak similarity in physical and personality characteristics. These two patterns are evident in the findings before
1980s (Epstein & Guttman, 1984; Vandenberg, 1972). It is only natural to wonder why and how these patterns hap-
pen. Four possible mechanisms have been suggested to account for similarity: Perhaps the most intuitive mechanism
would be initial, active choice of a similar partner due to personal preference; alternatively, similarity may be a passive
product of initial assortment due to mating market operationcompetition for desirable partner characteristics leads to
matching, and social homogamyfinding a partner within one's immediate geographic and social surroundings entails
sharing similarity on other characteristics; finally, similarity may not only be due to initial assortment but also be due to
convergencepartners simply grow to be similar (e.g., Caspi & Herbener, 1993; Luo & Klohnen, 2005; Watson, Beer, &
McDadeMontez, 2014).
Although these four mechanisms can all help to explain couple similarity, to date there has been little discussion
as to why and how the similarity hierarchy should occur. I argue that understanding this hierarchy may bring about a
breakthrough in our understanding of the underlying process of AM and similarity in general. The following sections
review each of the four mechanisms and discuss possible pathways by which the mechanisms may lead to the simi-
larity hierarchy.
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3.1 |Mechanism 1: Is couple similarity due to initial, active choice?
If similarity is primarily out of an active choice, it must come with significant benefits. Thiessen, Young, and Delgado
(1997) suggested that AM may be an evolved strategy because of two major evolutionary benefits: First, AM
increases the benefit/cost ratio of altruism according to inclusive fitness theory (Hamilton, 1964). Second, when
two parents share similarity, each parent will be able to contribute more than 50% to their offspring's genetic material.
Epstein and Guttman (1984) even suggested that an optimal mating strategy would be to partner with someone who
maximizes similarity to the degree that it is not incestuous.
If AM is truly beneficial, individuals should evolve to consider a similar partner as psychologically appealing.
Indeed, people find sharing similarity with their partner strengthens selfverification, improves understanding, and
increases closeness (e.g., Aron, Aron, Tudor, & Nelson, 1991; Murray, Holmes, Bellavia, Griffin, & Dolderman,
2002). People strongly desire their ideal partner to be similar to themselves (e.g., Hitsch, Hortascu, & Ariely, 2010);
in fact, desired similarity exceeds actual similarity in strength (Watson et al., 2014). Attraction literature has long
established similarity as a major attraction principle (e.g., Byrne, 1971; Montoya, Horton, & Kirchner, 2008). Attrac-
tiveness of similarity extends to nonromantic friendships (e.g., McPherson, SmithLovin, & Cook, 2001). Finally, cou-
ples and friends display a similarity biasbelieving they have more in common with each other than the reality (e.g.,
Luo & Snider, 2009; Montoya et al., 2008). Thus, findings across different stages and types of relationships provide
converging evidence that similarity is attractive.
If AM is a selected product of evolution, it should generalize across different relationship types and cultures. So
far such comparisons are mostly constrained to demographic variables.
3.1.1 |Unmarried vs. married couples
Several studies used large, representative datasets to compare cohabiting couples to married couples in similarity on
age, race/ethnicity, education, and religion. The most consistent finding was that both cohabiting and married couples
are overwhelmingly similar. The discrepancies, if any, are small, mostly indicating that cohabiting couples are less sim-
ilar than married couples (Blackwell & Lichter, 2000; Blackwell & Lichter, 2004; Schoen & Weinick, 1993; Schwartz,
2010a; Verbakel & Kalmijn, 2014). Additional evidence suggests that less committed couples display a comparable
level of similarity to cohabiting and married couples (Blackwell & Lichter, 2004; Luo, 2009). Overall, relationship stage
appears to make small quantitative rather than qualitative differences in AM.
3.1.2 |Remarriage vs. first marriage
It is important to examine similarity in remarriages because remarriage market offers a smaller, more heterogeneous
pool of potential partners relative to first marriage market (Kalmijn & Flap, 2001). Three studies, using large samples
from UK, the Netherlands, and United States, respectively, examined similarity of demographic background in
remarried couples (Gelissen, 2004; Ni Bhrolchain, 1988; Shafer, 2012). Their findings suggested that the new marriage
somewhat differs from the first. For instance, age homogamy becomes weaker in remarriages as both genders are
more likely to marry someone younger than the former spouse (Gelissen, 2004; Ni Bhrolchain, 1988). Despite the
changes, all three studies indicated that similarity remains strong in remarriages.
3.1.3 |Arranged marriage vs. freechoice marriage
Almost all couple similarity studies are conducted in relationships based on partner choices of free will. Arranged mar-
riages, in contrast, primarily reflect parents' or other relatives' spouse considerations for the adult child rather than the
child's own wish. Studying the similarity pattern in arranged marriages provides an interesting comparison. Unfortu-
nately research on arranged marriages is extremely scarce. One exception was Ahmad, Gilbert, and Naqui's (1985)
study of the height of 1,500 Pakistan couples in arranged marriages, which yielded a similarity correlation (r= .36)
comparable to freechoice marriages.
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3.1.4 |Samesex vs. differentsex relationships
It is particularly informative to study similarity in samesex relationships because of a far smaller pool of partners and
the unique profile of homosexual individuals (e.g., more open and egalitarian). Several studies compared similarity pat-
terns in samesex and differentsex couples using large datasets from United States, Sweden, Norway, and the
Netherlands (N= 20,098 to 2 million couples), and two consistent findings were observed: First, regardless of sex
composition of the partnership, all couples demonstrate substantial similarity in demographics including age, educa-
tion, race/ethnicity, work hours, and earnings. Second, gay partners tend to show the least similarity, whereas married
differentsex partners show the highest similarity, except that for work hours and earnings, samesex couples are
more similar than differentsex couples (Andersson, Noack, Seierstad, & WeedonFekjaer, 2006; Jepsen & Jepsen,
2002; Schwartz & Graf, 2009; Verbakel & Kalmijn, 2014).
3.1.5 |Eastern vs. Western culture
The vast majority of similarity research is conducted in Western countries. One exception was Chen, Luo, Yue, Xu, and
Zhaoyang's (2009) study of two nationally representative samples of newlywed couples from mainland China. Their
results indicated that whereas the degree of similarity in demographics and attitudes largely mirrored the Western
findings, Chinese couples showed stronger similarity on personality traits (ranging from .33 to .66). This suggests that
certain culture differences promote personality similarity while maintaining demographic and attitudinal homogamy.
So far the limited research across relationship types and cultures shows that homogamy is highly robust, at least
for demographic variables, which is consistent with the hypothesis that AM may be an evolved, universal strategy.
More importantly, there has been direct and indirect evidence that people are genetically drawn to a similar partner.
Directly, Domingue, Fletcher, Conley, and Boardman (2014) used genetic information from a nationally representative
sample of nonHispanic White U.S. adults and found that married couples were indeed genetically more similar than
randomly paired couples. Indirectly, Rushton and colleagues' work shows that from physical features to intelligence,
attitudes, and hobbies, spousal similarity was consistently greater on more heritable items within sets of homogenous
attributes (see Rushton & Bons, 2005). Additionally, identical twins' spouses were more alike than fraternal twins'
spouses, particularly on more heritable characteristics (Rushton & Bons, 2005).
Although these studies suggest a genetic basis for similarity, heritability alone does not explain the similarity hier-
archy across different characteristics. For example, while personality traits are generally more heritable than education,
educational similarity is much stronger than personality similarity. Other factors must be at play in initial assortment.
For instance, one explanation for the similarity hierarchy in Western couples is characteristic visibility: Personality traits
are generally less visible compared to demographics and attitudes, and thus, it is not as easy for one to identify a partner
with similar personality initially (Luo & Klohnen, 2005). This may also help explain why Chinese couples displayed higher
personality similarity: As the Chinese culture strongly emphasizes interpersonal relatedness, individuals may be better
at gauging others' personality (see Chen et al., 2009). Given the limited research beyond the West, it is imperative to
study AM nonWestern cultures, particularly what cultural aspects bring about the similarities and differences in AM.
3.2 |Mechanism 2: Could mating market operation contribute to similarity?
Although previous evidence indicates that similarity is advantageous, desirable, hence likely a result of active choice, it
does not rule out other mechanisms. Xie, Cheng, and Zhou (2015) mathematically demonstrated that assortment can
arise from systematic structural forces other than personal will. Using actual couple data, Fisher et al. (2014) showed
that similarity on body mass index remained when partners' preferences for adiposity and body mass index were sta-
tistically controlled, suggesting that processes other than active choice contributed to similarity.
Mating market operation is such an alternative process. For example, John very much likes Mary who is highly
physically attractive, but due to the strong competition for such an attractive partner, John settles for Susan who is
more of his equal in looks, based on principles of social exchange (Kelley & Thibaut, 1978). This matching idea has
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received moderate support (e.g., Hunt, Eastwick, & Finke, 2015; Little, Burt, & Perrett, 2006). In reality, the way
mating market impacts similarity can be highly complex. Partners have many factors to weigh in their mate
selection decision. For example, John may trade his high SES or good humor for Mary's attractiveness. John may
also prioritize sharing a common faith over the partner's attractiveness. Decisions like these will all affect matching
in physical attractiveness. Currently it is unclear how individuals balance the tradeoff between competing desires
such as desires for similarity on different domains and desires for an absolute level of various characteristics (e.g., a
certain income level).
In order to dissect the complexities of the market operating mechanism, it is important to distinguish two types of
characteristic desirability critical to the trade: the absolute level of desirability determined by consensus (i.e., attrac-
tive to everyone) and the relative level of desirability determined by personal idiosyncrasy (i.e., only attractive to
myself; see Watson et al., 2014; Wood & Brumbaugh, 2009). Assortment through market ruling is more likely to take
place on attributes high in consensual desirability, e.g., physical attractiveness, intelligence, education, and SES. How-
ever, assortment through active choice is more likely to occur on attributes where relative desirability is more rele-
vant, e.g., faith, attitudes, values, hobbies, and lifestyle. An important finding in ideal partner research is that people
desire differential levels of similarity on different domains; further, desired similarity across domains displays a tiered
pattern parallel to the hierarchy in actual similarity (Buss, 1984; Luo, Johnson, Chen, & Zhaoyang, 2010; Watson et al.,
2014). This suggests that actual similarity hierarchy primarily reflects active preferences, although market forces may
reinforce this pattern. Specifically, both active choice and market force are likely to contribute to matching on demo-
graphic variables. Active choice is likely the main push behind matching on values, interests, and lifestyle, while market
operation may be the primary driving force behind the (weak) matching on personality variables.
One fruitful avenue for future research is to quantify absolute and relative desirability, which enables us to
partition to what extent initial assortment is due to active choice and forced selection by market force. For
example, partners with psychological disorders tend to be paired together, an outcome primarily through market
operation due to their absolute (un)desirability. But the fact that these individuals tend to partner with someone
with a similar disorder rather than just any disorder indicates that their choice is active to some degree within
the limits (Parnas, 1985).
3.3 |Mechanism 3: Can social homogamy lead to couple similarity?
Some scholars suspect that couple similarity may be a result of social homogamy (see Epstein & Guttman, 1984).
The idea is that potential partners tend to meet each other within physical proximity, particularly in certain social
settings they frequent (e.g., school, workplace, and church). If people who share geographic and social back-
grounds also share other characteristics such as attitudes and values, then the latter similarity is not due to
an active selection but to a passive assortment due to the selection for shared background. Although this sounds
plausible, it has received little support because couple similarity on attitudes and personality remain robust when
common backgrounds are statistically controlled (e.g., Botwin et al., 1997; MascieTaylor & Vandenberg, 1988;
Watson et al., 2004).
Online dating has become a mainstream means of searching for romantic partners. Since couples who meet online
are not necessarily constrained by geographical location or social setting, they provide a neat test for the social
homogamy hypothesis: Specifically, online couples' similarity, if any, should reflect more of their active choice rather
than of a byproduct of living in a common area or belonging to the same social group. Gonzaga, Carter, and
Buckwalter (2010) examined similarity on personality, emotion experiences, and interests in married couples who
met on eHarmony. They found that couples held significant similarity in personality and interests even before they
met (online), although couples did not show aboverandom similarity in emotions.
Since there is little evidence for social homogamy in either traditional or online couples, this mechanism is unlikely
to play a central role in similarity. The implication is that there must be adequate variation in personal attributes such
as interests and attitudes even among people sharing geographical or social backgrounds. This variation allows them
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to actively pursue a partner whose personal attributes align with their own. However, it is noteworthy that effects of
social homogamy may not be negligible in assortment on a small number of characteristics such as IQ (MascieTaylor
& Vandenberg, 1988) and violent crimes (Frisell et al., 2012).
3.4 |Mechanism 4: Can convergence lead to couple similarity?
The convergence hypothesis suggests that partners may grow to be more alike over time as a function of increased
familiarity, interactions, and synchronized routines. Although it sounds plausible, previous studies consistently found
that acquaintance length and relationship length have little association with couple similarity in personal characteris-
tics including intelligence, attitudes, values, personality, and psychological wellbeing (e.g., Buss, 1984; Feng & Baker,
1994; Glicksohn & Golan, 2001; Watson et al., 2004). Further, couples at early dating stage (6 months) exhibit sim-
ilarity patterns parallel to longterm couples (Luo, 2009). Longitudinal studies of marital couples showed their similar-
ity level in attitudes and personality hardly changed over 20 years (Caspi & Herbener, 1993). Moreover, couple
similarity in personality and interests was evident even before partners met each other (Gonzaga et al., 2010). For-
merly married couples' similarity in life satisfaction declined after divorce (Wortman & Lucas, 2016). These findings
provide strong evidence for Caspi, Herbener, and Ozer's (1992) claim that shared life experiences and circumstances
play a significant role in maintaining rather than increasing couples' initial similarity.
However, research of similarity in physical and biological characteristics shows a different picture. In a famous
experiment by Zajonc, Adelmann, Murphy, and Niendenthal (1987), participants had better success matching the pho-
tographs of couples taken at their 25year anniversary than those taken during their honeymoon, suggesting that cou-
ples had become more alike in appearance. Longitudinal research shows that longterm couples not only have sizable
similarity in functional limitations (e.g., ability to shop for food, make a hot meal, and take medications), depression
symptoms, and happiness, but spouses' changes in these areas over the years are in tandem (Hoppmann, Gerstorf,
Hibbert, 2011a; Hoppmann, Gerstorf, Willis, et al., 2011b). Thus, it is possible that partners can become more similar
on certain characteristics as a result of shared living.
Additionally, similarity in longterm couples may reflect effects of attrition rather than convergence alone. That is,
couples who do not have much in common may break up more often, leaving a net effect for increased similarity over
time (see Bacon et al., 2014). Schwartz's (2010a, 2010b) longitudinal study found support for this idea: Although
cohabiting couples overall showed less educational homogamy than married couples, this gap was not apparent when
cohabiting and marital unions began; the gap seemed to be driven by differential dissolution rate between cohabiting
and married couples.
Given these seemingly inconsistent findings, it is time to reconceptualize convergence. Rather than defining con-
vergence as increased similarity as a function of the relationship, it might be more productive to expand convergence
to the totality of the effects that sharing a relationship has on similarity over time. According to this expanded con-
ceptualization, convergence is not a mutually exclusive alternative to initial, active choice of similarity but rather a
mechanism of how initial similarity develops over the course of the relationship.
It appears that initial similarity can either be upheld or increase due to active interactions or passive attrition. A
useful way to disentangle these competing processes would be to examine the characteristics' malleability/controlla-
bility. The odds to find evidence that initial similarity changes over time should be better on characteristics that are
more malleable and people have better control over. For example, weight and lifestyle are likely to be more plastic
and responsive to personal will than height and IQ. Needless to say, longitudinal research, particularly research follow-
ing couples up from the beginning of the relationship, is needed because the vast majority of similarity research has
relied on partners who have been together for a while.
The above review suggests that all mechanisms except social homogamy are likely to contribute significantly to
couple similarity. Further, different dimensions vary in their sensitivity to different mechanisms. Thus, each character-
istic may display a distinct similarity pattern depending on its specific combination of these dimensions. Delineating
these mechanisms' influence on different dimensions provides a useful explanation for the similarity hierarchy
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observed across dimensionsthis hierarchy is likely a combined outcome of multiple mechanisms. Specifically, demo-
graphic similarity is usually the strongest because active selection (due to high relative desirability and partly heredity),
market operation (due to absolute desirability), and best visibility all contribute to it. Next on the ladder are values,
interests, and lifestyles, where active choice and better visibility contribute to couple similarity; however, market influ-
ence is minimal. Personality similarity is the weakest because it is not particularly desirable nor is easily visible, making
market operation the main force behind this similarity. Convergence is likely to exert a subtle influence on initial sim-
ilarity, particularly on more malleable characteristics. Ultimately, to completely comprehend the underlying process of
similarity, it will be necessary to examine possible overlapping and interactive effects between the dimensions. For
instance, a characteristic's malleability, visibility, and desirability may be related to its heritability. Accessibility is likely
to moderate effects of all other dimensions.
4|CONSEQUENCES OF SIMILARITY
4.1 |Genetic consequences
At the microlevel, similar parents produce more genetically homogenous offspring. Built upon life history (LH) theory,
Figueredo and Wolf (2009) further argued that the genetic advantages of homogamy depend on the predictability of
the environment. According to LH theory, an organism's LH strategy is contingent on their environment. Fast strate-
gies, featuring fast sexual maturity and early reproduction onset, are adaptive in a harsh and unpredictable environ-
ment. Hence heterogamy is a desirable mating strategy to a fast strategist because heterogamy produces more
genetically diversified offspring who survive better in unpredictable environments. In contrast, slow strategies charac-
terized by slow sexual maturity and delayed reproduction are more successful in stable environments. Therefore,
homogamy is beneficial for slow strategists as more homogenous offspring resulted from homogamy will have a better
ability to preserve the integrity of their locally adapted genomes.
At the macrolevel, homogamy on a particular attribute increases the proportion of individuals with extreme
levels of the attribute and diversifies the population. For example, models simulating AM effects on obesity suggest
that switching from random mating to complete homogamy could more than double the percentage of obese individ-
uals in the population within just two generations (Speakman, Djafarian, Stewart, & Jackson, 2007). Psychopathology
literature shows a strong relationship between the number of mentally ill parents and rates of major mental disorders
in children (e.g., Gottesman, Laursen, Bertelsen, & Mortensen, 2010; Merikangas, Weissman, Prusoff, & John, 1988).
These genetic consequences have significant implications for public health. It is important to note that the genetic
effects of AM depend on fertility (Epstein & Guttman, 1984). For example, infertility risk is considerably higher in cou-
ples if both partners are obese (Pasquali, Patton, & Gambineri, 2007). Educational homogamy reduces the odds of
reproductive failure (Huber & Fieder, 2011). Unraveling the complex genetic consequences of AM will require a break-
through in understanding of the role fertility plays in this process.
4.2 |Social consequences
At the societal level, AM has profound implications for social and economical inequality (e.g., Schwartz, 2013). The dis-
cussion here primarily focuses on variables related to SES such as income and education. Homogamy on these vari-
ables produces an increasing number of couples of equal status, which leads to a larger gap between classes and
reinforces their social status (Blossfeld, 2009). Harpending and Cochran (2015) used simulations to illustrate the pow-
erful effect AM has on class stratification: When a population free of castes starts to practice 100% homogamy on a
given variable (e.g., income), two castes begin to emerge after just one generation; after three generations, the two
castes have grown to be 1.75 standard deviations apart.
In United States, income inequality has grown substantially since 1960s (Burtless, 1999; Reed, Cancian, & Rising
family income inequality: The importance of sorting. Unpublished paper, 2009; Schwartz, 2010b). It is estimated that
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17% to 51% of the increase in income inequality is due to the growth in earning homogamy (Schwartz, 2010b). Higher
income inequality, in turn, further corroborates the boundaries between existing social groups, therefore reducing
likelihood of intermarriage (Schwartz, 2013).
As education is regarded an important determinant of income and earnings, one might expect educational AM to
be responsible for income inequality. Surprisingly, educational AM only accounts for a quarter of earning AM (Breen &
Salazar, 2011). Although education homogamy has increased in the United States, this change has little effect on
income inequality (Breen & Salazar, 2011; Kremer, 1997; Western, Bloome, & Percheski, 2008). Possible explanations
are (1) AM in lowest levels of education has declined, which decreases polarization in household income, and (2)
income heterogeneity within education categories has increased (Breen & Salazar, 2011). The social effects of AM
on subsequent generations are profound and complex, considering other factors may play a significant role, such as
differential fertility rate (Fernandez & Rogerson, 2001) and differential parental investment in children's education
(e.g., Beck & GonzálezSancho, 2009).
4.3 |Psychological consequences
At the individual level, couple similarity has been linked to two important psychological outcomesrelationship stabil-
ity and satisfaction. In general, in terms of relationship stability, heterogamous couples in race/ethnicity, religion, or
education are more likely to experience relationship dissolution; moreover, this pattern is more pronounced in dating
and cohabiting relationships than in marriages (e.g., Blackwell & Lichter, 2004; Frimmel et al., 2013; Wang, Kao, &
Joyner, 2006). However, similarity in demographic variables tends to be a poor predictor of relationship satisfaction;
that is, heterogamous couples overall are as happy as homogamous couples (e.g., Keizer & Komter, 2015; Schoen,
Rogers, & Amato, 2006).
Couple similarity on personal attributes such as personality and attitudes is theorized to be more pertinent
to relationship satisfaction (e.g., Luo & Klohnen, 2005; Murstein, 1980). There are two approaches in capturing
couple similarity: Whereas the variablecentered approach (VCA) concerns the similarity between two partners'
scores on individual characteristics (e.g., intelligence, extraversion), the person/couplecentered approach (PCA)
focuses on the similarity in two partners' entire profiles across a number of characteristics (e.g., all 44 items
of Big Five Inventory). To test how similarity is associated with satisfaction, researchers following VCA typically
use difference scores or the interaction approach, while those who follow PCA simply correlate the couple pro-
file similarity with satisfaction (for discussions surrounding the statistical issues of using difference scores, inter-
actions, and profile similarity, see Edwards, 2001; Griffin, Murray, & Gonzalez, 1999; Kenny, Kashy, & Cook,
2006). The overall conclusion from both VCA and PCA research is that couple similarity on personal attributes
usually has positive, weaktomoderate raw correlations with relationship satisfaction (e.g., Caspi & Herbener,
1990; Luo & Klohnen, 2005; Wilson & Cousins, 2003; see Shiota & Levinson, 2007 for an exception of a neg-
ative association between personality similarity and marital satisfaction in an older sample); however, these asso-
ciations diminish once other effects are statistically removed, e.g., each partner's scores (i.e., the main effects),
normative or stereotypic responses, and desirability of the profiles (Luo, 2009; Watson et al., 2004; Wood &
Furr, 2016). Overall, relationship satisfaction is better predicted by each partner's individual characteristics than
by couple similarity (e.g., Becker, 2013; Dyrenforth, Kashy, Brent, & Lucas, 2010).
5|CONCLUDING REMARKS
This article aims to provide a comprehensive review of couple similarity research in the last three decades. The review
of similarity patterns and trends indicates that overall, there is overwhelming evidence for similarity and little for com-
plementarity. This pattern is largely independent of couple's relationship status, sex composition, and sociocultural
environment. However, the degree of similarity depends on the specific domain, with strongest similarity shown in
LUO 9of14
demographic variables and attitudes; moderate similarity in values, intelligence, interests, and mental wellbeing; and
weak similarity in physical and personality characteristics. The review of the four mechanisms of similarity suggests
that while active preference is likely the leading cause, other processes such as mating market operation and conver-
gence may also contribute to similarity. Finally, AM can potentially change the genetic composition of the population,
amplify social inequality, and play a role in relationship stability although a much lesser role in relationship satisfaction.
In addition to providing an update for similarity research, this review attempts to identify directions for future
research. As the field has accumulated abundant evidence of the similarity patterns on a wide range of characteristics,
I believe it is time for researchers to shift energy to studying the underlying mechanisms of similarity. While this
review may give the (accurate) impression that these mechanisms are highly complex, an important mission of this
article is to articulate specific avenues to help disentangle these interwoven mechanisms that lead to observed phe-
notypic similarity hierarchy. The common theme of these avenues is focusing on the nature of differences in charac-
teristic dimensions such as desirability and visibility, and linking these differences to corresponding mechanisms such
as active choice and market force.
Ultimately, to fully understand the process, trends, and effects of similarity, integrative models are needed in
order to incorporate information from multiple disciplines (e.g., sociology, psychology, and biology) and address the
complexities and nuances in similarity. Inevitably, the methodologies to study these effects will become increasingly
sophisticated. These developments bring about both excitements and challenges to future research. I hope that this
review will stimulate a new wave of innovative research to further our understanding of the nature, mechanism,
and effects of AM and couple similarity in general.
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Shanhong Luo is currently a professor of psychology at University of North Carolina at Wilmington. She received
her MED and BS at Beijing University in China and her PhD at the University of Iowa. She is a socialpersonality
psychologist with a primary research interest in close relationships, particularly in ideal partner preference, initial
attraction, and partner choice.
How to cite this article: Luo S. Assortative mating and couple similarity: Patterns, mechanisms, and conse-
quences. Soc Personal Psychol Compass. 2017;11:e12337. https://doi.org/10.1111/spc3.12337
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... individuals with similar levels of that trait are more likely than chance to share other characteristics, such as a working environment, which may lead to an increased likelihood of a relationship starting (social homogamy); in addition, individuals may begin relationships with others who are more similar to themselves than expected by chance (initial assortment) or become more similar to their partner over the course of their relationship (convergence) 24,29 . Assortment also occurs in non-romantic relationships, with friends being more alike than chance for many variables 33,34 , including autistic traits 35 . ...
... ese observations may provide a basis for explaining why assortment occurs for autistic traits and systemizing, i.e., because those with high levels of these traits tend to communicate with each other more e ectively, increasing the chances of a relationship developing. Equally, as the current study detected within-couple correlations across a range of distinct albeit theoretically related measures, the ndings might simply re ect the generalised tendency for couples to be more alike than di erent 24,30 . It is surprising, however, that we did not observe signi cant results for empathizing 26 (although see also 78 ). ...
... It is noted that the within-couple correlations observed for autism-related variables in the current study were only statistically signi cant in unmarried couples. is was unexpected because couples that have been together for only a relatively short time (< six months) 82 tend to exhibit couple-similarity e ects that are broadly comparable to those observed in more established relationships 24 . It has also been suggested that partner resemblance in married and unmarried couples is 'overwhelmingly similar' , and, if anything, that married couples tend to be more alike than cohabiting (but unmarried) couples 24 . ...
Article
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It has been hypothesised that romantic partners are more similar than chance in relation to autistic traits. To test this theory, we recruited n = 105 heterosexual couples and examined within-couple correlations for autistic traits [measured using the Autism Spectrum Quotient (AQ)], empathizing [measured using the Empathy Quotient (EQ)], and systemizing [measured using the Systemizing Quotient-Revised (SQ-R)]. For a subsample that attended the lab (n = 58 couples), we also investigated theory of mind via facial expressions using the Reading the Mind in the Eyes Test (RMET) and attention to detail, a component within systemizing, using the Embedded Figures Task (EFT). Variable-centred analyses revealed positive within-couple correlations for all measures except EQ, although these effects were only statistically significant for unmarried couples and not for married/engaged couples. Follow-up analyses indicated that the observed couple similarity effects are likely consistent with people pairing with those more similar than chance (initial assortment) rather than becoming alike over time (convergence), and to seeking out self-resembling partners (active assortment) rather than pairing in this manner via social stratification processes (social homogamy). Additionally, a significant within-couple correlation for autistic traits was observed at the meta-analytic level. However, it should be noted that the meta-analytic effect size estimate was small (r = 0.153) and indicates that only ~2% of variance in a person’s score on a phenotypic measure of autistic traits can be predicted by that of their partner.
... Common wisdom (e.g., "birds of a feather flock together") aligns with the similarity attraction hypothesis, which assumes that people are attracted to others who are similar to themselves in a variety of variables, including personality traits (e.g., Buss, 1994;Byrne, 1971Byrne, , 1997. Empirical research has provided robust support for the notion that similarity attracts and that partners are on average similar at the on-set as well as later phases of relationships (for an overview, see Luo, 2017;Montoya et al., 2008). While many studies have examined partner similarity for broad personality traits such as the big five and HEXACO traits, research on character strengths in close relationships has received comparatively little attention thus far, which is surprising given that several character strengths are among the traits most sought after in potential partners (e.g., Buss & Barnes, 1986). ...
... The role of partner similarity has received major interest in research on romantic relationships. Studies testing the degree of partner similarity showed systematic similarity across a wide range of variables, and similarity has been argued to play a role for initial romantic attraction, satisfaction with and the longevity of relationships, and the heritability of traits (e.g., Buss, 1994Buss, , 2016Byrne, 1971Byrne, , 1997Luo, 2017;Watson et al., 2004Watson et al., , 2014Weidmann et al., 2016). Hence, the study of similarity concerns two distinct questions; namely, the description (i.e., "how similar are partners?") ...
... The description of similarity concerns quantifying the degree of similarity among partners. Luo (2017) suggests distinguishing between assortative mating, partner similarity, and potential mechanisms for why partners match non-randomly: "Whereas A[ssortative]M[ating] specifically refers to initial partner similarity evident at the beginning of the relationship that reflects active or passive assortment, (couple) similarity refers to partner similarity at any point of the relationship, which may result from initial assortment and/or development within the relationship" (p. 1). For both assortative mating and partner similarity, there is robust evidence for the systematic similarity of partners concerning various characteristics, including psychologically relevant variables such as attitudes, values, and personality traits (see e.g., Buss, 1994Buss, , 2016Luo, 2017;Montoya et al., 2008;Weidmann et al., 2016). ...
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We studied the similarity among partners' character strengths (i.e., positively valued traits) across two studies. In Study 1, N = 68 couples completed the 240-item VIA Inventory of Strengths and in Study 2, N = 143 couples completed a 24-item brief-form and measures of life-and relationship satisfaction. We computed raw, normative, and distinctive profile similarities for the 24 strengths and found support for partners' similarity in both studies (normative: rs ≥ .84; raw: rs ≥.23; distinctive: rs ≥ .06). Actor-Partner Interdependence Model analyses (Study 2) provided no evidence for the notion that similarity relates to couples' satisfaction. We discuss our findings regarding prior research, assortative mating preferences, and extensions to the study of partner-and ideal partner perceptions.
... Despite enduring popularity for the notion that "opposites attract", evidence from empirical research more readily supports the view that "birds of a feather flock together" (Luo, 2017;McPherson et al., 2001;Montoya et al., 2008). In the context of romantic relationships, this reflects the presence of assortative mating, that is, the non-random pattern in which partners show greater similarity to one another than expected by chance. ...
... In the context of romantic relationships, this reflects the presence of assortative mating, that is, the non-random pattern in which partners show greater similarity to one another than expected by chance. The highest within-couple correlations are typically observed for demographic variables, such as age and ethnicity, and are followed by educational attainment and attitudes; the weakest correlations are usually reported for personality traits and physical characteristics (see Luo, 2017). ...
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Assortative mating is a phenomenon in which romantic partners typically resemble each other at a level greater than chance. There is converging evidence that social behaviours are subject to assortative mating, though less is known regarding social cognition. Social functioning requires the ability to identify and understand the mental states of others, i.e., theory of mind. The present study recruited a sample of 102 heterosexual couples via an online survey to test if theory of mind as measured using facial expressions (Reading the Mind in the Eyes Test) or language (Stiller-Dunbar Stories Task) is associated with as-sortative mating. Results provide evidence of assortative mating for theory of mind via facial expressions, though there was no such effect for theory of mind via language. Assortative mating for theory of mind via facial expressions was not moderated by length of relationship nor by partner similarity in age, educational attainment, or religiosity, all variables relevant to social stratification. This suggests assortative mating for theory of mind via facial expressions is better explained by partners being alike at the start of their relationship (initial assortment) rather than becoming similar through sustained social interaction (convergence), and by people seeking out partners that are like themselves (active assortment) rather than simply pairing with those from similar demographic backgrounds (social homogamy).
... Positive assortative mating or homogamy (i.e., tendency to form intimate relationships with self-similar individuals) can be observed for socio-demographic (e.g., education and socioeconomic status), psychological (e.g., depression), and physical characteristics [11][12][13], even though the evidence is mixed [14]. Importantly, couples, where both are depressed, may be more likely to divorce [15]. ...
... The couples with decreasing depressive symptoms (Class 2) also had the same direction of change (i.e., both decreasing), but they had different starting points converging to similar ending points. Using the traditional nomenclature [11], we may call the second group "convergent", but the first group could be both labeled "divergent" and "convergent". Even though the direction of change was the same in both partners in both classes, their levels of depression at the end of data collection were different. ...
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The occurrence of depression is influenced by social relationships, however, most studies focus on individuals, not couples. We aimed to study how depressive symptoms of couples evolve over time and determine, which characteristics are associated with their distinct trajectories. A multi-centric cohort sample of 11,136 heterosexual couples (mean age = 60.76) from 16 European countries was followed for up to 12 years (SHARE study). Information on depressive symptoms measured by EURO-D scale was collected every 2 years. Dyadic growth mixture modeling extracted four distinct classes of couples: both non-depressed (76.91%); only women having consistently high depressive symptoms while men having consistently low depressive symptoms (8.08%); both having increasing depressive symptoms (7.83%); and both having decreasing depressive symptoms (7.18%). Couples with increasing depressive symptoms had the highest prevalence of relationship dissolution and bereavement. In comparison to the nondepressed class, individuals with any depressive symptoms were less psychologically and physically well. Our results suggest that distinct mechanisms are responsible for couples’ various longitudinal trajectories of depressive symptoms.
... More specifically, in terms of evolutionary success, it would not pay for people to engage in intimate relationships, especially long-term ones, with individuals of a mate quality lower to their own, because by doing so they forgo the opportunity to engage in relationships with individuals of higher mate quality. Accordingly, people have evolved a preference for similarity; that is, they prefer as mates individuals who score similar to them in desirable traits (Buss & Barnes, 1986;Luo, 2017). Such preference would enable people to avoid mates of lower to their own mate quality as well as to avoid diverting their limited resources in attracting higher to their own mate quality individuals, whom they have limited chances to get. ...
... Along with the observation that earlier studies found only a little and inconsistent evidence of similarity effects (though the applied statistical approaches were even biased toward false-positive results), we consider it increasingly unlikely that the similarity-attracts hypothesis is empirically supported in the personality domain. Our findings could also help to explain why-with regard to existing relationships-romantic partners do not seem to be more similar in their personalities, whereas they notably resemble each other on a number of other characteristics, such as attitudes, religiosity, and physical attractiveness (Luo, 2017). Moreover, our findings are consistent with recent evidence that, although the largest share of variance in romantic attraction is unique to the specific dyad in question, it is difficult to predict this unique attraction from distal person variables, such as personality traits (Joel, Eastwick, & Finkel, 2017). ...
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A central assumption in lay and psychological theories is that people are attracted to potential mates who are similar to themselves in personality traits. However, the empirical findings on this idea have been inconclusive. Only a few studies have considered real-life dating contexts, and the statistical approaches they applied have sometimes spuriously identified similarity effects. In our study, 397 heterosexual singles (aged 18-28) participated in real speed-dates (Ndates = 940). Using dyadic response surface analysis, we investigated effects of actual similarity (similarity between self-reported personality trait levels) and perceptual similarity (similarity between an actor’s personality and his/her perception of the partner’s personality) concerning the Big Five traits. Neither type of similarity was related to initial romantic attraction. That is, the empirical evidence contradicted the idea that attraction occurs when people’s personalities match. We conclude that understanding initial attraction requires a deeper understanding of interpersonal dynamics in first encounters.
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The empirically related psychopathologies of stress and depression exact an enormous economic toll and have many physical and behavioral health effects. Most studies of the effects of stress and depression focus on their causes and consequences for a single, focal individual. We examine the extent to which depression, as indicated by filling antidepressant prescriptions (SSRI and Benzodiazepines), co-occurs across spouses, constituting a negative spillover effect. To better understand the conditions that affect within-household contagion of depression, we examine whether the stress and uncertainty occasioned by job change and financial stress (net worth) increases spillover effects among spouses. We use panel data from various Danish administrative registers from the year 2001–2015 with more than 4.5 million observations on more than 900,000 unique individuals and their spouses from Danish health registers. Spouses in a household with their partner using antidepressants have a 62.1% higher chance of using antidepressants themselves, with the one year lagged effect being 29.3% and a two-year lagged effect of 15.1%. The effects become larger by 14.8% contemporaneously and 20% in the two-year lagged model if the focal individual changed employers. There was also a substantively unimportant effect of lower financial wealth to increase inter-spousal contagion.
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In the present review, the author draws upon Bell's (1987) critical race theory – especially as reflected in Crenshaw's (1989) construct of intersectionality – en route to examining the results of four studies of international relationship processes that have been published since 2002 (i.e., Holzapfel et al., 2018; Kaya et al., 2019; Kuramoto, 2018 and van Mol & de Valk, 2016). One common theme that emerged from the four studies was the importance of satisfaction‐related processes in international relationships – a theme that Thibaut and Kelley's (1959) interdependence theory would anticipate within intranational and international relationships alike. Although persons from African, Central American, and South American nations are conspicuously missing from the studies in question, the author does not attribute such omissions to structural racism. Implications for future research on inclusivity and the dynamics of international relationships are discussed.
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Clear empirical demonstrations of the theoretical principles underlying assortative mating remain elusive. This article examines a moderator of assortative mating-how well couple members knew each other before dating-suggested by recent findings related to market-based (i.e., competition) theories. Specifically, competition is pervasive to the extent that people achieve consensus about who possesses desirable qualities (e.g., attractiveness) and who does not. Because consensus is stronger earlier in the acquaintance process, assortative mating based on attractiveness should be stronger among couples who formed a relationship after a short period rather than a long period of acquaintance. A study of 167 couples included measures of how long partners had known each other before dating and whether they had been friends before dating, as well as coders' ratings of physical attractiveness. As predicted, couples revealed stronger evidence of assortative mating to the extent that they knew each other for a short time and were not friends before initiating a romantic relationship. © The Author(s) 2015.
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