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DOI: 10.1017/ehs.2024.3
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SEX RATIOS AND GENDER NORMS: WHY BOTH ARE NEEDED TO UNDERSTAND
SEXUAL CONFLICT IN HUMANS.
Renée V. Hagen and Brooke A. Scelza
Department of Anthropology, University of California, Los Angeles. United States of America.
ABSTRACT
Sexual conict theory has been successfully applied to predict how in non-human animal
populaons, sex raos can lead to conicng reproducve interests of females and males and
aect their bargaining posions in resolving such conicts of interests. Recently this theory has
been extended to understand the resoluon of sexual conict in humans, but with mixed success.
We argue that an underappreciaon of the complex relaonship between gender norms and sex
raos has hampered a successful understanding of sexual conict in humans. In this paper, we
review and expand upon exisng theory to increase its applicability to humans, where gender
norms regulate sex rao-eects on sexual conict. Gender norms constrain who is on the marriage
market, how they are valued, and may aect reproducve decision-making power. Gender norms
can also directly aect sex raos, and we hypothesize that they structure how individuals respond
to market value gained or lost through biased sex raos. Importantly, gender norms are in part a
product of women’s and men’s somemes conicng reproducve interests, but these norms are
also subject to other evoluonary processes. An integraon of sexual conict theory and cultural
evoluonary theory is required to allow for a full understanding of sexual conict in humans.
Keywords gendered conict, bargaining, sex rao, gender ideology, gender norms
1 Social Media summary
Hagen and Scelza argue for the integraon of sexual conict and cultural evoluonary theory to understand sexual
conict in humans.
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2 Introducon
When two individuals mate, they have both converging and diverging interests. They share an interest in the success
of any joint ospring, but may dier in the opmal trade-os between current and future reproducon, or in the
benets they may be able to gain through inclusive tness (Parker, 1979; Arnqvist and Rowe, 2005). Conict occurs
any me two partners cannot simultaneously reach their opmal tness outcome (Parker, 2006). Sexual conict
theory is a framework in evoluonary biology that seeks to explain how such conicts result in adaptaons over
evoluonary me, including behavioural exibility within the lifespan of individuals. One important determinant of
conict is the number of reproducve opons that each party has outside of their current partnership, which
regulates how much bargaining power each partner has.
Anthropologists are increasingly applying sexual conict theory to understand human reproducve strategies
(e.g. Käär et al., 1998; Bird, 1999; Borgerho Mulder and Rauch, 2009; Schacht and Borgerho Mulder, 2015; Schacht
and Smith, 2017; Lawson et al., 2021; Akurugu et al., 2022; Baraka et al., 2022). In recent years, interest has grown
in examining local sex raos and their eect on reproducve decision-making and bargaining power in romanc
relaonships (e.g. Abramitzky et al., 2011; Francis, 2011; Wei and Zhang, 2011; Lainiala and Mienen, 2013; Schacht
and Borgerho Mulder, 2015; Porter, 2016; Uggla and Mace, 2017; Schacht and Smith, 2017), and scholars have
extended the theory to test predicons on how sex raos may inuence gender norms (Guentag and Secord, 1983;
Grosjean and Khaar, 2019; Brooks et al., 2022).
In this arcle, we propose a revision of sexual conict theory when applied to humans, and discuss important
consideraons that are essenal to understanding the relaonship between human sex raos, bargaining power and
gender norms. Here we dene gender norms as social norms, rules or ideals that govern what counts as socially
acceptable and virtuous behaviour and that apporon resources, roles, power and entlements based on (perceived)
sex (Ridgeway and Correll, 2004; Cislaghi and Heise, 2020). The term social norm is used to describe convenons or
common behaviours in a parcular community, but many norms are also injuncve or prescripve; they refer to
moral values and societal standards (Bicchieri, 2005). Social norms are thought to have evolved because they render
the acons of others more predictable and thereby facilitate coordinaon between community members (O’Connor,
2019). Violaon of norms can be costly because it can lead to miscoordinaon with cooperave partners, or to
punishment in reacon to deviance from prescripve norms. Gender norms pertain to behaviour related to marriage,
sexual divisions of labour, respecul conduct, and other forms of behaviours that are dependent on one’s sex. They
are oen not merely descripve, but prescribe ‘morally right’ behaviour. While theorecally gender norms could
culminate into complete gender egalitarianism, more typically across cultures they describe a gender hierarchy in
which men have more power and higher status than women (Schneider and Gough, 1961; Rosaldo, 1974; Smuts,
1995).
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The evoluon of social norms is an important topic of study in research on cultural evoluon. Cultural evoluon
theory has been fruiul in showing the importance of biased transmission in the spread and maintenance of cultural
traits (Boyd and Richerson, 1985; Henrich and Boyd, 1998; Boyd and Richerson, 2002; McElreath et al., 2003; Eerso n
et al., 2008; Mesoudi, 2011b), but limited consideraon has been given to power dynamics and their role in the
spread of ideas (Singh et al., 2017; Cofnas, 2018) and this eld of inquiry has somewhat ignored gender (Lawson et
al., 2023). Instead, social inuence is mostly considered in the form of deference and voluntary copying of the
behaviour of high-status individuals rather than coercion by those in power. In order to gain a deeper understanding
of how gender norms spread, persist and change over me, and of how sex raos and gender norms interact to aect
gendered bargaining dynamics, sexual conict theory and cultural evoluon theory need to be beer integrated and
applied together.
In this review we summarize sexual conict theory and discuss how it has been used to understand the
inuence of sex raos on the reproducve behaviour of non-human animals. Next, we examine how in human
populaons, sex-rao eects on gendered bargaining power can interact with culturally-specic gender norms in
important ways. We propose that to understand this complex relaonship, we need to consider gender norms both
as products of sexual conict and variables aecng sexual conict by constraining women and men’s market values
and their freedom of choice in reproducve decision-making. Importantly, gender norms can directly aect sex raos
and at the same me aect how people respond to market dynamics that are a product of those sex raos. We argue
that an integraon of models of cultural evoluon with sexual conict theory can help to elucidate these interacons,
improving our understanding of sexual conict in humans.
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Adult sex rao, rao of adult men to women in a given populaon.
High sex raos refer to populaons where men
ASR outnumber women, low sex raos refer to populaons
where women outnumber men
(Queller, 1997; Jennions and Fromhage, 2017).
OSR
Operaonal sex rao, rao of reproducvely available males
to females, which excludes juveniles, pregnant or lactang
females and post-reproducve individuals
(Emlen and Oring, 1977; Jennions and Fromhage, 2017).
Cultural
evoluon
theory
Theorecal framework in evoluonary anthropology that
hypothesizes that cultural traits, like social norms, are subject
to processes similar – but not idencal - to Darwinian evoluon
as traits vary and some are more likely to spread than others.
At the micro-level, this theorecal framework considers how
cultural traits are passed on between individuals, such as
through biased social learning.
Gender norms
Social norms, rules or ideals that govern what counts as socially
acceptable and virtuous behaviour and that apporon resources,
roles, power and entlements based on (perceived) sex (Ridgeway
and Correll, 2004; Cislaghi and Heise, 2020). Gender norms
prescribe behaviour related to marriage, divisions of labour,
respecul conduct, and other forms of behaviours depending on
one’s (perceived) sex.
Gender
ideology
Ideologies are collecons of social norms or ‘cultural beliefs that jusfy
parcular social arrangements, including paerns of inequality’
(Macionis, 2010, p. 257). Gender ideologies then refer to collecons of
norms that jusfy a parcular hierarchy (or lack of hierarchy) between
people on the basis of their (perceived) sex.
Gendered
conict
Sexual conict as applied to human behaviour, where women and
men’s conicng interests can not be reduced to sex-based biological
dierences, but are shaped by cultural pracces and gender norms
(Lawson et al., 2023).
Sexual
conict
theory
Theorecal framework in evoluonary biology that seeks to
explain how conicts of interest between females and males
result in adaptaons over evoluonary me, including
behavioural exibility within the lifespan of individuals.
Adapve lag
A period of mismatch when adaptaons have not yet caught up
with current selecon pressures.
Behaviour aimed at maintaining exclusive sexual access to a
sexual partner, for example by physically prevenng a partner
from interacng with other potenal partners. In humans,
Mate guarding behaviours such as prevenng a partner from leaving the
house or liming a partner’s nancial independence may be
interpreted as mate guarding.
Table 1: Glossary.
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3 Theory and applicaons of sexual conict theory and sex raos in non-human animals
Research on non-human animals shows that female and male interests may diverge over shared physiological (Trivers,
1972; Kokko and Jennions, 2008), morphological (Bonduriansky and Chenoweth, 2009) or behavioural traits, such as
the number of mangs (Galliard et al., 2005), control over ferlizaon (Parker, 2006), and levels of parental
investment (Arnqvist and Rowe, 2005). Sexual conict theory posits that the degree of bargaining power individuals
have in conicts of interest depends on the relave value of all possible conict outcomes for each partner, which
includes the opportunies each has outside the current interacon. Sexual conicts of interest are expected to be
resolved in favour of the party with a beer bargaining posion.
A crucial factor for determining bargaining power in sexual conict is the quality and number of alternave
partners available to each party. Holding all else equal, the party with more alternave reproducve opportunies
has more leverage to achieve their interests. A classic example of sexual conict is parental investment: when two
individuals reproduce, both are interested in the survival of their ospring, but either party may prefer that the other
provide the parental investment required so they can spend their own me and energy pursuing other reproducve
opportunies (Trivers, 1972; McNamara et al., 2000; Kokko and Jennions, 2008). The individual with beer opons
outside the current partnership has a beer ‘fallback posion’ and therefore is in a stronger bargaining posion. They
will, on average, see the conict resolved closer to their opmal outcome (Cluon-Brock, 1991; Parker, 1979; Arnqvist
and Rowe, 2005; Kokko and Jennions, 2008).
At the level of the populaon, the rao of males to females is strongly associated with the reproducve opons
of each sex, and the signicance of sex rao as a determinant of bargaining power in sexual conict has been
exemplied in numerous experimental and observaonal studies (Carroll, 1991; Székely et al., 2006; Karlsson et al.,
2010; Liker et al., 2013; Carmona-Isunza et al., 2015; Eberhart-Phillips et al., 2018).
Sex raos are usually operaonalized as the rao of males to females in a populaon; where high sex raos
reect a bias towards more males, and low sex raos a bias towards more females. There are several sex raos that
are commonly used in these studies. One is the adult sex rao (ASR), the rao of reproducvely aged males to females.
Another is the operaonal sex rao (OSR), the rao of reproducvely available males to females, which excludes
pregnant, lactang and post-reproducve individuals (Emlen and Oring, 1977) and in humans somemes excludes
married people. The ASR and OSR do not always track each other because of the higher temporal variaon of the
laer, especially in smaller populaons (Carmona-Isunza et al., 2017; Jennions and Fromhage, 2017). In addion,
because the me and energy individuals spend on reproducon is itself a product of sexual conict dynamics (Kokko
and Jennions, 2008; Kappeler et al., 2022), which rao is most relevant depends on the reproducve biology of the
species, and on the phenotypic trait and evoluonary me-scale being studied (Jennions and Fromhage, 2017). For
example, when tesng for sex-rao eects on male mate guarding behaviour in primates, the rao of adult males to
estrous females may be most relevant, but when examining sexual conict over parental investment, the OSR is a
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dependent rather than an independent variable (as which sex spends me in and out of the mang market is the
subject of conict) and ASR may be more appropriate. ASR is usually a suitable measure for considering behavioural
plascity and short-term responses to environmental cues in within-species comparisons, and is the measure that is
used in most contemporary human studies (Kappeler et al., 2022).
One important domain of sexual conict is parental investment. Following the principle of allocaon, organisms
have limited me and energy available to spend on reproducon. In sexually reproducing species a trade-o exists
between allocang me and energy towards behaviours aimed at accessing sexual partners and behaviours
associated with caring for exisng ospring, as the same resource unit cannot be spent on both (Trivers, 1972). This
trade-o can result in a conict of interest between sexual partners over parental investment, where either partner
is beer o if the other provides the investment needed to produce surviving ospring so that their own resources
are free to be spent on mang eort (Cluon-Brock, 1991; Székely and Cuthill, 2000).
The extent to which individuals can improve their tness through invesng me and energy in mang or
parenng behaviour depends on species-level, populaon-level and individual factors. For example, species vary in
the amount of parental care required to raise ospring to adulthood, populaon-level dierences in environments
may lead to variaon in the costs of parental investment and mang eort, and individuals’ reproducv e
opportunies are partly determined by their ability to compete for mates with others of the same sex. At the
populaon level, the density and rao of adult males to females shapes the relave payos of mang and parenng
behaviours, as it determines the number of potenal sexual partners and the severity of compeon between
members of the same sex. For example, when the sex rao is skewed, mate scarcity can make it more protable for
the abundant sex to devote energy into maintaining reproducve access to a current mate rather than into compeng
for addional mates (Kokko and Jennions, 2008). To increase their chances of mang, members of the abundant sex
can respond to an unfavourable sex rao by appealing to the preferences of the rare sex, such as by providing more
parental care. In addion, the opportunity costs of parental investment go down as the relave benets of searching
for mates decrease due to strong mang compeon. Members of the rare sex can drive a harder bargain in
situaons of sexual conict, for example forcing their mate to provide more care while they reenter into the mang
pool. We should expect ASR-responsive exibility in reproducve strategies to occur in species where at least some
paternal care is present, and where there is limited reproducve skew (when few males account for most or all
ospring in a populaon, the eect of sex raos on bargaining power will be minimal). Furthermore, the relaonship
between reproducve strategies and ASR is dynamic. For example, the mortality risks of parenng and mang
strategies can aect the ASR. If increased mang eort leads to higher mortality, the sex rao becomes more extreme
because of a self-reinforcing process where the rarer sex (which would benet the most from mang eort) becomes
even rarer (Kokko and Jennions, 2008).
Shorebirds (Scolopaci and Charadrii spp., sandpipers, plovers and allies) have provided an informave model
for invesgang the relaonship between sex raos and mang and parenng behaviour (Székely et al., 2023).
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Shorebirds exhibit broad inter-specic variaon in mang systems and in the ability of both sexes to provide full
parental care on their own, but are generally similar in ecology and variaon in sex raos as result of sex-biases in
juvenile survival (Benne and Owens, 2002; Székely et al., 2006; Liker et al., 2013; Carmona-Isunza et al., 2015;
Eberhart-Phillips et al., 2018). In line with predicons, research on these species has found that although sex
dierences in parental investment cannot be fully explained by considering the ASR alone, female-biased adult sex
raos are linked with more female care and higher levels of polygyny (Liker et al., 2013; Eberhart-Phillips et al., 2018).
In species with male-biased adult sex raos, males are more likely to provide the bulk of parental care, female
mulple mang is more common and females have more showy plumage, a trait that is generally associated with
investment in mang compeon (Liker et al., 2013). A compeve mang market can thus movate individuals of
the abundant sex to provide more parental care to exisng ospring (Székely et al., 2023). Similar evidence for
parenng behaviour as a exible response to ASR has been observed in other taxa, such as rails (Maynard Smith and
Ridpath, 1972) cichlids (Grüter and Taborsky, 2005), and dung beetles (Rosa et al., 2017).
Evidence in non-human primates for a sex rao-eect on reproducve strategies is limited, as direct paternal
care is absent in most species (Rosenbaum and Silk, 2022), but some supporve evidence comes from callitrichids.
Callitrichids are known to have exible mang systems. While monogamy is most common, polyandrous bonds are
more prevalent when the sex rao is high; under these circumstances one breeding female may be supported by two
caring males (Goldizen, 1987; Dunbar, 1995). Breeding pairs may receive help from the male’s brother, who may have
lile chance of siring ospring himself and for whom the opportunity costs of caring are small.
Mang systems can thus be sensive to a populaon’s ASR, as it aects both the value of parenng and mang
behaviours for each sex, as well as their bargaining power (Székely et al., 2014). However, appealing to the
preferences of members of the rare sex is not the only possible response to an unfavourable sex rao. Sexual conict
theory also predicts that males can use coercion to regain bargaining power lost through a skewed sex rao, as has
been observed in many taxa. Higher ASR is associated with increased rates of male-to-female aggression in crab
spiders (Misumena vaa, Holdsworth and Morse, 2000), common lizards (Lacerta vivipara, Galliard et al., 2005),
monk seals (Monachus schauinslandi, Johanos et al., 2010), and possibly feral horses (Equus ferus caballus, Regan et
al., 2020), although not all studies have found evidence for a sex rao eect (e.g. Head and Brooks, 2006; Baniel et
al., 2017). Aggression used to obtain sexual access to females against their will is prevalent in mulple primate species
(Smuts, 1992; Muller and Wrangham, 2009; Muller et al., 2011), but there is less evidence that this behaviour is
responsive to ASR in primates. Among chimpanzees (Pan troglodytes) at Ngogo in Uganda, females received higher
rates of aggression from males when the OSR was higher (Was, 2022), but it is unclear whether this result indicates
that males increased their aggressive behaviour towards females when they had many competors or whether
aggression received by females was proporonal to the greater number of males present. The laer seems to be the
case among mountain gorillas (Gorilla gorilla beringei) studied by Robbins (2009). Here the number of males present
was not associated with rates of male-to-female aggression, but females did receive more threatening behaviours
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(chest-beang) when they were in a mul-male versus single-male group. Similarly, the OSR does not predict how
oen female chacma baboons (Papio ursinus) are subject to aggression by males in their group (Baniel et al., 2017).
We are not aware of studies tesng for a sex-rao eect on male-to-female aggression among bonobos (Pan
paniscus). However, Fruth and Hohmann (2003) found that although female bonobos with sexual swellings are
preferred targets of male aggression, male-to-female aggression is not dependent on the number of females in the
group exhibing swellings. Overall, the current literature does not show much support for consistent sex-rao eects
on male-to-female aggression in our closest relaves.
Mate guarding is another tacc used to monopolize the reproducon of a partner when strong compeon
due to a male-biased ASR makes nding a new mate unlikely (Harts and Kokko, 2013). ASR -sensive mate guarding
has been observed among males of some populaons of soapberry bugs (Jadera haematoloma, Carroll and Corneli,
1995), water striders (Gerris buenoi Kirkaldy and Gerris lacustris, Rowe, 1992; Vepsalainen and Savolainen, 1995),
beetles (Lethrus apterus, Rosa et al., 2017), various crustaceans (Wada et al., 1999; Mathews, 2002; Karlsson et al.,
2010; Takeshita and Henmi, 2010), and Soay sheep (Ovis aries, Cluon-Brock, 2016, p. 632). Mate guarding is also
common in some primate species, such as baboons (Papio cynocephalus, Bulger, 1993), sifakas (Propithecus verreauxi,
Mass et al., 2009), mandrills (Mandrillus sphinx, Setchell et al., 2005) and chimpanzees (Was, 1998). Again there is
lile exisng research tesng whether this behaviour in primates is sensive to sex raos, and the samples in exisng
studies are small (usually consisng of one or two groups where ASR varies temporally). In one longitudinal study of
Japanese macaques (Macaca fuscata), Takahashi (2001) was able to capture variaon in male mang behaviour in
relaon to temporal changes in OSR. He found evidence that resident males more oen violently interfered with
oang males’ mang aempts in periods when the sex rao is more male biased, although this observaon could
simply mean that mate guarding was less successful when compeon is more erce. Perhaps the eect of sex raos
on mate guarding behaviour is limited in primates because other populaon- and species-level factors are more
important in determining its payos. For example, mate guarding oen prevents eecve foraging and can increase
predaon risk (Cluon-Brock, 2016), and these costs are expected to vary between environments. Mate guarding
also is more costly when ovulaon is inconspicuous; the absence of a clear signal of females’ ferlity makes it dicult
to determine when guarding will pay o (Cluon-Brock, 2016, p. 480-2). The low reliability of bonobos’ sexual
swellings as a signal for ovulaon (Reichert et al., 2002; Douglas et al., 2016) may therefore partly explain why mate
guarding and male-to-female aggressive behaviours are rare compared to chimps.
Mate guarding can pave the way for paternal care by decreasing the opportunity costs of care as acve guarding
places males in closer proximity to their ospring. Several scholars have suggested that male mate guarding preceded
the evoluon of pair bonding in mammals (Lukas and Cluon-Brock, 2013), and more specically in humans (Schacht
and Bell, 2016; Loo et al., 2017), but see Gavrilets (2012) for an alternave view. Loo et al. (2021) hypothesize that
mate guarding as a response to high sex raos may have preceded the high level of paternal care common in
callitrichids.
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This summary of current research shows that biased sex raos can drive sexual conict by aecng the
reproducve opportunies of individuals in the populaon and are an important source of bargaining power in
situaons of sexual conict. Having fewer reproducve opons changes the value of dierent reproducve strategies
for both sexes, and can lead to individuals of the more abundant sex adhering more to the preferences of potenal
mates. Alternavely, males have been shown to respond to an unfavourable bargaining posion with physical
coercion of females. Evidence for this sex rao-eect is weak in primates. This could be due to the dicules in
obtaining large sample sizes of populaons that vary in ASR but where other factors aecng mang behaviour are
held constant (or are stascally accounted for). Alternavely, sex rao eects may be completely absent when other
ecological factors play a larger role in determining the costs and benets of parenng and mang strategies.
4 Applying sexual conict theory and sex rao predicons to human populaons
Following theory and ndings in non-human animal populaons, sex raos are also predicted to inuence the payos
of dierent reproducve strategies in humans. Going one step further, sex raos have also been hypothesized to
aect the status and treatment of women, a point rst made by Guentag and Secord (1983). They argued that while
men hold ‘structural’ power in patriarchal sociees, women can gain ‘dyadic’ power from a male-biased marriage
market, thereby gaining higher status within the domain of the household and family. Guentag and Secord’s (1983)
predicons on sex rao-eects somewhat overlap with evoluonary hypotheses, but where Guentag and Secord
do not queson the origins of men’s ‘structural power’, evoluonary anthropologists have argued that patriarchal
norm systems are the result of a long evoluonary history of sexual conict (Smuts, 1995). According to this view, the
reproducve biologies of females and males result in men’s greater interest in control over women’s reproducon.
Over evoluonary me, this resulted in gender norms that have jused men’s dominance over women. Variaon in
women’s status and bargaining power within marriage however is, similar to Guentag and Secord’s concept of
dyadic power, hypothesized to depend on their alternave reproducve opons, which are partly determined by the
populaon’s sex rao.
Many studies tesng sex rao hypotheses on human reproducve behaviour have suered from
methodological issues and yielded mixed results (reviewed by Schacht and Smith, 2017). One common issue with
studies on sex rao eects highlighted by Pollet et al. (2017) is the use of aggregate naonal-level data (such as in
South and Messner, 1987; Diekmann, 1992; Barber, 2000; Kruger et al., 2010). ASR is non -normally distributed at the
naonal level, which can result in a strong but spurious eect of outliers. These analyses also oen disregard the
confounding eects of geographic clustering and shared cultural histories, which can lead to similaries between
nearby populaons that may mistakenly be interpreted as the result of shared ecological factors (Pollet et al., 2017).
More ne-grained studies have been able to avoid some of these issues by relying on localized sex rao data, and
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these studies have been more successful in convincingly idenfying cases in which biased sex raos aect
reproducve strategies in line with sexual conict predicons. We will now review empirical work that uses local ASR
data to study these topics, and then discuss how this work is limited by not seriously considering the role of cultural
norms.
Many studies nd that romanc relaonships are more stable in areas when the ASR is male-biased, supporng
the sexual conict predicon that men are more movated to stay with current partners (and perhaps invest more
in their children) when alternave partners are scarce (e.g. Pedersen, 1991; Pouget, 2017; Uggla and Mace, 2017;
Uggla and Andersson, 2018; Grosjean and Khaar, 2019). Anthropologists have also looked for evidence of increased
bargaining power of members of the rarer sex. For example, Pollet and Nele (2008) examine men’s probability of
marriage in a representave US sample from 1910. They found a posive interacon-eect between SES and local
ASR on marriage; higher SES was a more important predictor for the probability of marrying in states where the ASR
was more male-biased. This could indicate women’s beer ability to make demands on partners due to their beer
bargaining posion, although this is not universally true. In a study of cohabitaon paerns in Northern Ireland, Uggla
and Mace (2017) nd that women are more likely to cohabitate in areas with higher ASR, but do not nd evidence
that men’s SES becomes more important as a predictor of their probability to be in a stable relaonship when the
ASR rises.
Schacht and Smith (2017) test for sex rao eects in a historical sample from 19th century Utah, where the
populaon-level sex rao was male-biased due to the immigraon of Mormon men, while local dierences correlated
with variaon in male mortality rates and child sex raos, as well as the incidence of polygyny. Comparing 206 districts
with varying sex raos, their results show that men married later in districts with more male-biased ASR. Schacht and
Smith interpret this as evidence for women’s higher bargaining power when they are scarce, as women are expected
to prefer older partners who are in a beer economic posion. In a study on the relaonship preferences of women
and men in French Guyana, Schacht and Borgerho Mulder (2015) nd that men hold stronger preferences for long-
term relaonships in villages with higher ASR. Building on sexual strategies theory (Buss and Schmi, 1993), they
assume that, on average, women have a stronger interest in long-term versus short-term sexual relaonships
compared with men, and therefore argue that this result may indicate that men adjust their preferences to meet
women’s interests when they have a poorer market posion. An alternave interpretaon of their result is that men’s
varying preferences for stable relaonships may reect ASR-sensive mate guarding. Another study shows that in
Chinese regions where the ASR is higher, families with sons save more money (Wei and Zhang, 2011). These savings
may funcon as parents’ investment in the marital posion of their sons where women (and their families) can make
high demands on potenal grooms. Also in China, Porter (2016) reports that men smoke and drink less under more
male-biased sex raos, and proposes that this is an eort to make themselves more aracve as romanc partners.
Similarly suggesng men’s increased eorts to appease potenal partners, Francis (2011) nds that when the ASR in
Taiwan increased sharply aer 1950 as male soldiers and refugees arrived from China, children were more highly
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educated in regions where sex raos were higher. These ndings are explained as evidence of men’s higher parental
investment in response to women’s increased bargaining power. Finally, historical sex raos in Australia have been
found to predict current-day atudes towards women’s work. In areas that historically had high sex raos due to the
in-migraon of European male convicts, women and men today are more likely to be in favour of tradional gender
roles in which women stay at home while men are the sole breadwinners (Grosjean and Khaar, 2019). Women in
areas with historically high ASR also report more leisure me, and men provide a larger share of the household
income. The authors interpret these results as evidence of women’s greater bargaining power, arguing that these
historical bargaining dynamics are sll reected in today’s norms.
One methodological issue in many of these studies is the comparison of only a few subpopulaons: Oen the
behaviours or atudes of people from only a few towns, regions or cies are compared, and the possibility that
factors beyond ASR confound study results is rarely considered. Cultural dierences may covary with the ASR in
consequenal ways. For example, Uggla and Mace (2017) report that in their Northern Irish sample ASR is on average
higher in rural areas as women more oen leave the countryside to move to cies. This suggests that the
cooccurrence of high ASR and higher cohabitaon rates could at least in part result from cultural dierences between
urban and rural areas, and gender norms could confound the relaonship between ASR and cohabitaon rates if
women migrate because of a preference for the qualies of urban men. In the Taiwanese study (Francis, 2011), the
observed correlaon between ASR and children’s educaon could be caused by other factors. For example, educaon
may have been more readily available in those areas that aracted more migrants.
Another important issue is the interpretaon of various measures of bargaining outcomes. For example, social
sciensts oen regard women’s lower ferlity as evidence of their higher bargaining power, drawing from the
assumpon that women tend to prefer fewer children, and when they have greater autonomy are beer able to exert
preferences over ideal family size (Bankole and Singh, 1998; Borgerho Mulder, 2009; Snopkowski and Sear, 2013).
Adapve logic used to explain this dierence is that women face greater reproducve costs than men do, and
therefore men can aord to have higher ferlity preferences (Penn and Smith, 2007; Borgerho Mulder and Rauch,
2009). However, Moya and colleagues (2016) point out that this assumpon may be faulty, as men and women do
not necessarily have conicts over ideal family size, and men also suer costs to reproducon. One of the key costs
that Moya et al. highlight relates to sex raos and bargaining power. The assumpon that men can easily replace a
wife who dies in childbirth does not take into consideraon that men compete for access to wives, and this
compeon will be parcularly strong when the ASR is male-biased. Men may be beer o adjusng their ferlity
preferences to match their wives (e.g. with longer interbirth intervals or lower overall ferlity) in order to reduce her
reproducve costs.
In another example, men’s higher paternal investment has also been aributed to women’s increased
bargaining power (Francis, 2011). While this is one possibility, the opportunity costs of paternal care decrease when
partners are scarce, so this behaviour is therefore not conclusive evidence of women’s higher bargaining power. A
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similar point complicates the nding of higher endorsements of tradional gender roles in Australian regions that
experienced high sex raos in the past (Grosjean and Khaar, 2019). Grosjean and Khaar understand men’s higher
contribuons to the household income and support for tradional gender roles as a result of women’s high bargaining
power. We suggest another, and perhaps more likely, explanaon: that men’s willingness to be the sole breadwinner
reects a mate guarding strategy. Men may have preferred for their wives to stay at home when they were faced with
strong compeon from other men in an unfavourable marriage market.
These examples illustrate how the interpretaon of bargaining outcomes as favouring men or women is not
always clear-cut. This is important, because theory and ndings from the animal literature suggest that partner
scarcity can lead to either coercive or conciliatory behaviour. Currently the most conclusive evidence for people’s use
of coercion in response to unfavourable sex raos comes from studies on inmate-partner violence (IPV) against
women. IPV has been theorized as a male strategy to seek control over a female romanc partner in situaons where
she has a higher market value than he has (Macmillan and Gartner, 1999; Kilgallen et al., 2021). Research in India
(Bose et al., 2013) and several US subpopulaons (Avakame, 1999; D’Alessio and Stolzenberg, 2010; Vanterpool et
al., 2021) indicate that IPV is more prevalent in regions with male-biased sex raos. Here the work of Bose et al.
(2013) is most convincing. They use survey data from married women in the 2005 -06 Indian Naonal Family Health
Survey to test the eect of ASR (measured at the level of the village or neighbourhood block) on their reports of
husband-to-wife violence. India’s male-biased ASR is the result of a strong son bias that has led to biased sex raos
at birth and gender dierences in child mortality in many parts of the country. Bose et al. (2013) nd a small posive
associaon between ASR and physical, sexual and psychological IPV. Other studies have found similar results using
data from the US. For example, D’Alessio and Stolzenberg (2010) use city-level data on male-to-female inmate-
partner violent crime rates in the US (which include murder, abducon, rape, sexual assault, etc.) as their dependent
variable. They nd that more IPV crimes occur in cies with male-biased sex raos, although the very high coecien ts
in their model results are dicult to interpret and suggest their models may be overed or lack appropriate controls
for variables correlated with ASR (D’Alessio and Stolzenberg, 2010, table 2). Furthermore, it is unclear whether their
results could be explained by a higher prevalence of marriage and lower divorce rates generally, factors that have
been associated with high sex raos (e.g. Angrist, 2002; Abramitzky et al., 2011; Brainerd, 2017), rather than an
increase in the rate of IPV in romanc relaonships. In a similar study, Vanterpool et al. (2021) recruited black women
from the U.S. on MTurk, as black communies in the U.S. experience wide variaon in ASR resulng from high
incarceraon rates in some areas. They too nd an associaon between percepons of high sex raos and
experiences of IPV, but their study is limited by the use of percepons of both variables rather than demographic
data.
As these studies show, it is dicult to ascertain what causes the observed relaonship between the ASR and
IPV, especially in sengs where biases in ASR result from patriarchal norms. In India for example, regional dierences
in ASR exist due to the occurrence of cross-cousin marriage, matrilocality and matrilineal inheritance in some parts
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13
of the country. These cultural traits are associated with a lack of son preference as well as with higher status and
beer treatment of women (Dyson and Moore, 1983; Chakraborty and Kim, 2010), leaving the possibility that
dierences in atudes towards women’s status explain variaon in both ASR and the prevalence of IPV, without a
causal link between the two.
While locally contextualized studies have shown the potenal for ASR to interact with reproducve strategies,
we nd that they rarely address the complex gender dynamics that form a pathway between ASR and resulng
reproducve strategies. Examining the relaonship between ASR, gender norms and reproducve strategies may
help us to understand why simple mang market predicons somemes fail to explain variaon in reproducve
strategies and bargaining power, as is also suggested by Schacht and Smith (2017). In the remaining part of this review,
we revise exisng theory to incorporate the role of gender norms and cultural evoluon in human sexual conict. We
focus on ve important points that address how gender norms interact with sex raos to aect mang market
dynamics, reproducve strategies and gendered bargaining posions: 1) Gender norms do not always reect
bargaining outcomes but are subject to their own selecve processes; 2) gender norms create bounds on who can
reproduce with whom and what is valued on the marriage market; 3) gender norms have the potenal to constrain
people’s reproducve decision-making power in a way that is unequal between genders; 4) biases in local ASRs
themselves are oen the result of gender norms; and 5) gender norms aect how individuals respond to biased sex
raos. Collecvely these points show that sex rao dynamics - and sexual conict more broadly - in humans are
deeply interlinked with gender norms (see g. 1), and that the interpretaon of sex rao biases as favouring men or
women is not always clear-cut. This ts with theory and ndings from the animal literature, which suggest that
partner scarcity can lead to either coercive or conciliatory behaviour. Here we have shown some examples that are
clearly coercive (e.g. IPV) and examples that are more conciliatory in nature (e.g. paternal investment), but most of
the studies we have highlighted have data that can be interpreted mulple ways, and reect the need for deeper
contextualizaon of history, culture and demographics.
We conclude this paper by arguing that human sexual conict may be beer termed ‘gendered conict’
(Lawson et al., 2023), because it can only be understood by explicitly modelling the role of gender norms and by
integrang models of cultural evoluon.
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14
Figure 1: Schemac representaon of the relaonship between ASR and gender ideology.
4.1 Gender norms are a product of sexual conict as well as other evoluonary processes
Evoluonary anthropologists have argued that gender norms are the result of a long evoluonary history of sexual
conict (Smuts, 1995; Hrdy, 1997). According to this view, the reproducve biologies of females and males result in
male’s higher interest in control over females’ reproducon than vice versa. Smuts (1995) argues that patriarchal
norms that jusfy men’s dominance over women have evolved as a connuaon and extension of male eorts to
control female reproducon, common in the animal kingdom and specically in mammals and primates. Importantly,
“human males are not ‘genecally programmed’ to coerce and control women, and [...] women are not ‘genecally
programmed’ to accept subordinate status.” (Smuts, 1995, p. 21). The evoluon of patriarchy refers to the cultural
evoluon of social norms prescribing male dominance, and Smuts argues that human residence paerns, male
control over resources, and the uniquely human ability to convey informaon through language have enabled greater
control of men over women than is possible in other animals (Smuts, 1995). Gender norms are socially enforced by
group members (and oen by the state) through punishment or social exclusion (Egan and Perry, 2001; Blakemore,
2003; Parro, 2009; Skočajić et al., 2020). Like other social norms, gender norms may adjust to changing community´
members’ interests in norm-governed behaviours, as well as to their bargaining posions (Singh et al., 2017). Sex
rao dynamics can therefore inuence gender norms by changing individuals’ interests. For example, extreme sex
raos can movate a tolerance of polygamy (Starkweather and Hames, 2012), and in other cases have led to the
relaxaon of exogamy rules (Mishra, 2013; Larsen and Kaur, 2013). However, there are vari ous reasons why norms
do not always reect a bargaining outcome of the opmal strategies of individuals in a social group.
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15
First, social norms are oen subject to a period of mismatch when adaptaons have not yet caught up with
current selecon pressures. Such ‘adapve lags’ may be caused by a number of processes. One example is group
members’ eorts to coordinate behaviour by adhering to social norms. Social norms structure human behaviour in
such a way that makes it predictable and facilitates smooth coordinaon between group members. People who
deviate from the norm could experience costs simply from miscoordinaon (Young, 1993; Centola and Baronchelli,
2015; O’Connor, 2019). In China, women who pursue higher educaon risk not nding a partner, as they oen seek
men who are willing to leave behind tradional roles of the husband as breadwinner and the wife as homemaker,
while many men prefer women who are less educated than themselves and will take on the role of homemaker (Ji,
2015; Hong Fincher, 2016). Such mismatches in partner preferences may occur when one tries to depart from
common gender norms. In addion, once a norm becomes an important part of the moral code of a group, deviance
from the norm can lead to important social costs, such as exclusion or punishment by other group members. Deviance
from culturally dominant gender norms also can result in negave health outcomes via negligence or violence
(Macmillan and Gartner, 1999; Weber et al., 2019; Kilgallen et al., 2021). The coordinaon benets of norm
adherence as well as the social costs of deviance therefore can lead to cultural inera that results in a disconnect
between a group’s norms and group members’ opmal behaviour. Because of this, cultural phylogeny constrains
changes in social norms, as is likely the case for marriage norms: In a phylogenec analysis of cultural groups included
in the Standard Cross-Cultural Sample, the cultural history of populaons explained twice the variance in local rules
on polygyny and in the occurrence of polygyny as relevant ecological predictors (Minocher et al., 2019). Here adapve
lag is one possible explanaon for the lack of divergence between related groups.
Second, norms themselves are subject to evoluonary processes that are separate from the payos they
provide to group members. Cultural evoluon theory hypothesizes that cultural traits, among which social norms,
are subject to processes similar but not idencal to Darwinian evoluon, as traits vary and some are more likely to
spread than others. At the micro-level, this theorecal framework considers how cultural traits are passed between
individuals, such as through biased social learning (Boyd and Richerson, 1985; Boyd et al., 2011; Mesoudi, 2011a;
Richerson and Boyd, 2005). For example, the rate at which norms spread depends on the cultural ‘models’ displaying
those norms. Various studies have shown that people are more likely to adopt cultural traits held by high-status
individuals (Henrich and Gil-White, 2001; Atkisson et al., 2012; Chudek et al., 2012). Common norms may also spread
more easily by virtue of their popularity when people conform to the majority (Henrich and Boyd, 1998; Eerson et
al., 2008; Muthukrishna et al., 2016; Hagen and Scelza, 2020). As an example, pracces of female genital cung (FGC)
are thought to have culturally evolved from conicng interests between women and men by liming women’s desire
for extra-pair sex while increasing their husbands’ paternity certainty. However, recent research on this topic shows
that while FGC is clearly harmful to women, its eect on women’s sexual behaviour and thereby men’s paternity
certainty may be limited (Howard and Gibson, 2019). Rather than being an outcome of connued sexual conict,
frequency-dependent copying may drive the persistence of FGC (Howard and Gibson, 2017). Specic to marriage
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16
norms, Henrich et al. (2012) have suggested that group-level benets to monogamous marriage drove the spread of
monogamy to many populaons around the world. In their argument, social enforcement of monogamous marriage
is argued to reduce intrasexual compeon, result in more men being married and higher paternal investment.
Henrich et al. (2012) argue that norms prescribing monogamy may have increased the economic prosperity of
monogamous people, and that this prosperity helped spur its spread to other populaons. If correct, this implies that
marriage rules, like other gender norms, can spread as a result of biased cultural transmission.
In summary, gender norms are thought to result, in part, from an evoluonary history of sexual conict. While
the norms in a populaon are likely subject to change depending on the interests and bargaining posions of its
members, they are also subject to other processes and are therefore unlikely to perfectly reect sexual conict-
derived bargaining outcomes.
4.2 Gender norms constrain someone’s value on the market
Gender norms both constrain who is on the market, and regulate what their value is. Concerning the rst, marriage
systems are a well-known constraint on reproducve strategies, and interact with ASR in important ways. Whether
socially or legally enforced, norms about who is ‘on the market’ and about how they are valued are oen not gender-
neutral. For example, gender dierences may exist in the social acceptance of remarriage aer divorce or aer the
death of a spouse (Whyte, 1978), and the legal minimum age for marriage is lower for women than for men in 43 out
of 201 countries (UN Stascs Division, 2013). Furthermore, the acceptance or rejecon of polygamy plays a large
role in determining the ‘market value’ of women and men. In populaons where polygyny is culturally normave,
some men have the ability to monopolize the reproducon of mulple women. In populaons where the ASR is
female-biased, polygyny can increase the demand for women, which – all else equal - could improve their bargaining
posion (Becker, 1981). The costs to men in this situaon may also be minimized, as a female-biased sex rao can
lead to most men being able to have at least one wife (see g. 2). This is very dierent from cultures where polygyny
is allowed and the sex rao is equal or male-biased. In these cases, reproducve skew would be exacerbated by the
concurrent demographic and cultural restricons on access to women. In 77% of cultures in the Standard Cross-
cultural Sample, men are allowed to have mulple spouses, whereas only 6.5% allow women to have mulple
spouses (Whyte, 1978). Today polygyny is legal in 50 countries, and in these countries between 2 and 36% of people
live in polygynous households (Pew Research Center, 2019). At least one study examines the level of polygyny in
relaon to local sex raos: Pollet and Nele (2009) nd that in Uganda, the percentage of men in polygynous unions
tracks the regional adult sex rao, with more men in polygynous unions when the sex rao is female-biased.
Polyandry can also be aected by sex raos. Starkweather and Hames (2012) review the literature on non-classical
polyandry and nd that it is somemes pracced when populaons are faced with a parcularly male-biased sex
rao. However, strong cultural norms prohibing polygamy prevent this from happening in many cultural groups. For
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17
example, the scarcity of French men aer World War II led to higher bargaining power for men, but not to a tolerance
for polygynous marriage (Abramitzky et al., 2011). Therefore, while it is possible for polygamy to dampen sex rao
eects on marriage market dynamics, strong cultural norms and marriage rules can limit this potenal.
When the sex rao is skewed but all else is equal, does polygamy result in higher bargaining power for the
abundant sex by increasing their demand, as follows from a marriage market approach (and is suggested by, among
others, Becker, 1981)? Importantly, all else is usually not equal. As we argue in this paper, marriage rules, gender
norms and sex raos are all interlinked. Polygyny is oen accompanied by a strong gender hierarchy, male-controlled
access to resources and reduced decision-making power for women. This issue is further discussed by Grossbard
(2015).
Figure 2: This diagram illustrates the eect of polygyny on partnership opportunies in social groups depending on
the local ASR. Green circles indicate women, orange triangles refer to men, and lines around individuals indicate
marriages or pair bonds. Rows represent dierent ASR, and columns vary in their marriage rules. The ASR and local
marriage rules interact to determine marriage opportunies. For example, in the upper row the populaon ASR is
female biased. When monogamy is the norm, some women remain without a partner. When men can have mulple
partners, this dampens the eect of a female-biased sex rao (and could even result in some men remaining
partnerless). If polyandry occurs, this exacerbates the scarcity of partners for women.
Norms regarding beauty standards and gender roles have a strong inuence on partner preferences and
thereby aect how people are value on the marriage market. Norms regarding gender roles and divisions of labour
can also aect bargaining power in a special way: they can create a stronger interdependence among romanc
partners who have a family to care for. This is true for gendered divisions of labour common in subsistence-based
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communies, such as in hunter-gatherer populaons where men and women have complementary foraging tasks, as
well as the breadwinner-homemaker division of labour that is common in urbanized populaons around the world,
where men more oen work outside the home while women perform more of the childcare labour and other
household dues. The types of labour and the roles prescribed to women and men have far-reaching consequences
for women’s and men’s relave dependence on another. Earning an income by wor king outside the home is a source
of power for a variety of reasons, an important reason being decreased dependency on a spouse by improving
someone’s opons outside of a relaonship, as is discussed extensively in a large literature in anthropology, sociology,
feminist economics and development studies (for example see Schlegel and Barry, 1986; Lundberg and Pollak, 1996;
Agarwal, 1997; Kantor, 2003; Vyas and Was, 2009). On the other hand, men are also reported to respond to women’s
employment with violent backlash, when women’s work conicts with their perceived noons of tradional gender
roles (Atkinson et al., 2005; Weitzman, 2014; Cools and Kotsadam, 2017; Kilgallen et al., 2021).
Marriage rules, beauty standards and gender roles are thus two examples of ways in which gender norms have
a strong eect on people’s value on the marriage market.
4.3 Gender norms aect freedom of choice in a way that can override marriage market opportunies
In many populaons, women and men do not have the same degree of freedom in reproducve decision-making.
Somemes individuals of only one gender can approach the other with a marriage proposal, allowing the other party
only a choice between refusal or acceptance. Parents oen inuence the choice of spouse for children of both women
and men, but cross-culturally men have a say in their own marriage choices more oen than women (Whyte, 1978).
Parents’ and children’s interests oen do not completely overlap, especially when high marriage payments are
involved or marriage is in another way consequenal to a woman’s family (Borgerho Mulder, 1998; Apostolou, 2009).
Bridewealth is subject to market demands for women and can rise under male-biased sex raos (Francis, 2011),
increasing parental interests to control a daughter’s marital decisions and causing further divergence of parents and
daughters’ interests. This can have the eect of liming women’s autonomy where their value on the marriage market
is high. Here the funcon of the bridewealth is important. For example, in China bride wealth is skyrockeng due to
male-biased sex raos (Wei and Zhang, 2011), but much of the bride wealth devolves to the couple (for example in
the form of real estate, cars or other valued goods), making it more similar to dowry (Yan, 2003; Wei and Zhang,
2011). Dowries can also be sensive to the demand for men (Rao, 1993, e.g.), but high dowry does not lead to the
same level of parental control because dowries oen funcon as pre-mortem inheritance that will be owned by the
married couple rather than as a ‘payment’ to the groom’s family (Goody, 1976; Gaulin and Boster, 1990).
Second, normave restricons on pre- or extra-marital sex constrain people’s ability to benet from a
favourable marriage market. Premarital sex allows people entering the marriage market to learn about potenal
partners available to them while extramarital sex can make it easier to switch partners (Buss et al., 2017; Scelza and
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19
Prall, 2023). A double standard where men have more freedom to engage in both premarital and extramarital sex is
common throughout the world, liming women’s access to informaon about available partners (Broude, 1980).
Once again, however, gender norms can interact with ASR to aect these pracces. Himba have a strongly female-
biased ASR, but this co-occurs with a long history of matriliny and a pastoralist producon system that requires long
periods of spousal separaon (Scelza et al., 2021). In this seng, while the female-biased ASR may be leading to a
marriage market that is more favourable for men, the social history and ecological circumstances have contributed
to gender norms that allow women signicant sexual autonomy, so that both premarital and extramarital sex are
common for women.
Lastly, gender-based dierences in the right and ability to iniate divorce (and remarry) potenally have a
strong eect on women and men’s bargaining power in marriage (Scelza, 2013). Bargaining power derived from a
favourable mang market can be nullied if the costs of divorce are high, for example because of legal restricons or
social sgma on divorce (Bargain et al., 2020), and normave restricons on women’s ability to iniate divorce are
more common than on men’s (Broude and Greene, 1983). In most countries women and men legally have equal
access to divorce, but restricted unilateral or fault-based divorce laws in some pose limits on divorce that may
disproporonately aect women’s ability to leave their husbands. Furthermore, women’s prospects aer divorce
may be further limited by their nancial dependence on husbands (Leopold, 2018). Gender-based restricons on
divorce could lead to counterintuive situaons in which a biased sex rao results in a good bargaining posion of
one gender when looking for a marriage partner, but very lile bargaining power in the marriage itself. These double
standards on women’s and men’s freedom of choice in marriage are expected to lead to an asymmetric relaonship
between their market value as derived from the ASR and women’s and men’s bargaining power, where women are
oen more constrained in their ability to gain leverage from a favourable sex rao due to stronger limits on their
freedom of choosing partners, iniang divorce and having pre- and extramarital sex.
4.4 Gender norms directly aect sex raos
To test mang market predicons, many studies understandably focus on populaons in which the factors resulng
in biased sex raos are exogenous to bargaining dynamics. Many researchers purposefully select populaons in which
sex-biased migraon (Angrist, 2002; Schacht and Borgerho Mulder, 2015; Uggla and Mace, 2017; Grosjean and
Khaar, 2019), excess male mortality (Jones and Ferguson, 2006; Abramitzky et al., 2011; Brainerd, 2017) or high
male incarceraon rates (Fosse and Kiecolt, 1993; Cready et al., 1997; Vanterpool et al., 2021) are the main cause
of bias in the ASR, rather than the respecve status of women and men. However, gender norms themselves in many
cases directly inuence local adult sex raos. This can happen through various pathways. Norms enforcing a gendered
division of labour can aect the adult sex rao when some tasks are associated with increased mortality. Hunng and
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warfare are examples of work typically assigned to men that carry a high mortality risk and that can contribute to
biased adult sex raos (Abramitzky et al., 2011; Brainerd, 2017). Labour migraon unl recently was more oen male-
biased, but women constute a growing part of the world’s main migraon ows, either following their partners or
seeking beer employment or educaon by themselves (summarized by Clarke, 2003).
There are various pathways through which gender norms, and the dierenal valuaon of boys and girls, can
aect sex raos. A preference for either girls or boys can be a strong movator for parity progression and thereby
have important demographic consequences. For example, among Mosuo people in Southwest China, where descent
pracces vary between communies, matrilineal descent pracces are associated with an increased likelihood of
connued ferlity aer a son, and patriliny is linked with connued ferlity aer a daughter (Mason et al., 2016).
In India the last-born child is more likely to be a boy than a girl, and Chaudhuri (2012) esmates that 7% of the births
are the result of son preference. In addion, a preference for one gender can lead to inferior treatment of the other,
resulng in biased mortality rates. Under-ve mortality is biased against girls in the Caucasus and Central Asia, and
biased against boys in Uganda, Guinea-Bissau, Uzbekistan and Mongolia (Alkema et al., 2014). At the extreme, sex
preferences can result in sex-biased infancide, with evidence suggesng this pracce was once widespread in South
and East Asia, Europe and small-scale sociees across the globe (Sen, 1990; Divale and Harris, 1976; Smith and Smith,
1994; Mungello, 2008; King, 2014; Dong and Kurosu, 2016). On top of these sources of bias in child sex raos, access
to sex-selecve aboron in combinaon with child-liming and family-planning policies have led to skyrockeng sex
raos at birth in India and China (Banister, 2004; Zhu et al., 2009; World Bank, 2022).
Although the eects can be somewhat migated through sex-biased migraon, biased sex raos at birth
generally result in biased ASR in the next generaon. Countries where the sex rao at birth is stascally higher than
would be expected in the absence of son-preference account for 38% of today’s world populaon and for 91% of all
people living in countries with an ASR over 1.05 (calculaon based on populaon esmates from World Bank, 2022;
Chao et al., 2019). These numbers underline the signicance of gender norms and their role in the origin of biased
sex raos. This relaonship cannot be ignored, especially because of our nal point: gender ideology can aect how
people may respond to their market value and how market value is translated into gendered bargaining posions.
4.5 Gender norms aect how people respond to gains or decreases in their market value
Theory and ndings from the non-human animal literature suggest that men have two opons in their treatment of
women when faced with high compeon on the marriage market. They can increase their chances by moving
towards the preferences of a (potenal) partner, which would indicate a true improvement of women’s bargaining
posion and result in women’s higher status. Alternavely, when men’s market value is low because of a biased ASR,
they can force their bargaining posion through coercive taccs such as mate guarding or (threats of) social or
physical coercion. Which of these strategies becomes prominent in a parcular place is likely to reect exisng gender
https://doi.org/10.1017/ehs.2024.3 Published online by Cambridge University Press
21
ideologies. For example, in a populaon that already strongly enforces men’s dominance over women, the laer may
be more prominent, while in more gender egalitarian sengs conciliatory taccs might be more common. Guentag
and Secord (1983) were among the rst to predict a role for gender ideologies in shaping people’s responses to biased
sex raos. They argue that under a biased sex rao, members of the rare sex gain ‘dyadic’ power in romanc
relaonships. However, they also state that the eect of high sex raos on women’s status will not have the same
eect as low sex raos on men’s status, as men disproporonately hold ‘structural power’ in human sociees. The
combinaon of these two factors means that the ways that sex rao imbalances play out are not mirror images of
one another. According to Guentag and Secord, a male-biased sex rao will lead to an increased valuaon of
women’s reproducve value, but not women’s empowerment. Under a male-biased sex rao, men “must treat [their
partner] well or run the risk of losing her to another man. But their structural power is sucient to allow them to
place constraints on women’s freedoms and impose a sexual morality on them” (Guentag and Secord, 1983, p. 28).
This can be seen in many cross-cultural instances of restricons on female autonomy and sexuality. But more
conciliatory responses to a male-biased sex rao might be more common in places where gender norms are more
egalitarian. For example, in a historical US study, Pollet and Nele (2008) found that in states with a more male-
biased ASR, women were able to leverage greater demands, with higher SES men being more likely to be married.
Here, men were winning the mang compeon by
invesng.
When it comes to female-biased sex raos, Guentag and Secord predict a devaluaon of women in society
and the relegaon of women as mere sex objects. Several studies suggest that men may a higher preference for
short-term relaonships and uncommied sex in places where there is a more female-biased ASR, including among
US college students (Adkins et al., 2015), in US cies (Kruger and Schlemmer, 2009) and in rural southwestern Guyana
(Schacht and Borgerho Mulder, 2015). However, gender norms can temper this eect. For example, Himba
pastoralists have a strongly female-biased ASR, but also a history of matriliny and strong norms for female sexual
autonomy and freedom of movement. Their labour both in producon and reproducon, is also highly valued. This
means that although women have lower market value because they are more plenful, gender norms that allow for
easy divorce and concurrent partnerships mean that women are sll able to exert partner choice and leverage the
market value they do have in ways that benet them (Scelza et al., 2019, 2021). The ways in which women respond
to unfavourable market condions may also relate to other factors like post-marital residence paerns and the
availability of allocare. For example, where women can rely on others to help care for their children, the costs of
reduced male support will not be as great as they are in patrilineal, patrilocal sociees where women are more
separated from their kin and have less access to alternave forms of support if their partner leaves them.
Where Guentag and Secord do not queson the origins of men’s ‘structural power’, evoluonary
anthropologists understand patriarchy to be the result of a long history of sexual conict (Smuts, 1995; Hrdy, 1997).
Our perspecve is similar in that it predicts that the common gender ideology in a populaon inuences how people
https://doi.org/10.1017/ehs.2024.3 Published online by Cambridge University Press
22
respond to their market value as derived from the sex rao. As reviewed above, responsivity to biased sex raos has
been documented in human populaons for both conciliatory and coercive behaviours. We further show that gender
norms can aect mang market dynamics. However, more work in this area is needed. In parcular, more empirical
studies looking at how market dynamics may be responsive to gender ideology would help to test and rene our
predicons.
5 Discussion
Currently much of the research in the human literature is limited by its reliance on samples from only a few areas
with a limited ecological range, making it dicult to separate sex rao eects on gender norms from other dierences
between subpopulaons. For example, environmental factors can correlate with sex raos in a non-causal way and
lead to false esmates of sex rao eects. Importantly, not all variables interpreted as bargaining outcomes are
conclusively indicave of women’s or men’s preferred bargaining outcome, further complicang this research. In
order to eecvely study this relaonship between sex raos and bargaining over reproducve strategies,
anthropologists must reckon with the endogenous role of gender norms in sexual conict dynamics. Gender ideology
can be an important cause of biases in sex raos, and likely aect how individuals respond to biased sex raos. Gender
norms thereby have the potenal to inuence how sex raos translate to gendered bargaining dynamics, and future
work should explicitly take this into account in their predicons as well as in the interpretaon of their ndings.
In this paper we have argued that the interacon between gender norms and sexual conict adds a layer of
complexity that cannot be ignored when studying sexual conict in humans. To explicitly foreground this important
role of gender norms, we agree with Lawson et al.’s (2023) proposal of the term ‘gendered conict’ when applying
sexual conict theory to human behaviour. We have provided evidence and suggest hypotheses for some of the
myriad of ways in which gender norms and sexual conict interact specically in the context of sex raos. As Lawson
et al (2023) discuss, referring to gendered conict does not negate the central role of sex and reproducon in conict
over evoluonary me, but is useful in that it helps underline the role of social and cultural inuences. The term
gendered conict works to disessenalize dierences between women and men by underlining the fact that women
and men’s conicng interests cannot be reduced to sex-based biological dierences, but are very much shaped by
gender norms. We further argue that models of cultural evoluon are crucial in understanding gendered conict,
while gender norms as well as bargaining and power dynamics are somewhat neglected in cultural evoluon theory
itself (Singh et al., 2017; Lawson et al., 2023). Sexual conict theory has been used quite extensively to model and
measure bargaining over gender roles and over women’s autonomy and decision-making power (e.g. Smuts, 1995;
Käär et al., 1998; Borgerho Mulder and Rauch, 2009; O’Connor, 2019; Kilgallen et al., 2021). This work centres on
the queson of how tness payos can aect the spread and maintenance of gender norms and is mostly separate
from the literature on cultural evoluon theory, which has long studied the evoluon of social norms but rarely looks
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23
explicitly at gender. Some notable excepons include a theorecal study of the evoluon of sex-biased transmission
(Zeerman, 2016) and a study showing female genital cung (FGC) as a frequency dependent behaviour that is
associated with increased tness (Howard and Gibson, 2017). Future studies could go further to use models of
cultural evoluon will be required to understand how gender norms spread, persist and change over me. These
models will need to consider how power dynamics and gendered tness interests aect these dynamics. At minimum,
future studies on sex rao eects need to consider the processes leading to biased sex raos and their relaonship
to gender ideologies. Where previous research has somewhat neglected the causal role of gender norms or simply
avoided populaons in which gender norms are an obvious source of variaon in ASR, there is opportunity for future
research to study these topics in populaons where there is a direct link between gender ideology and ASR.
Disentangling the complex links between sex raos, gender norms and gendered bargaining power will be no easy
feat. Finding suitable cases to test the proposed hypotheses is challenging, precisely because of this complex
relaonship, but exogenous factors that aect ASR and marriage rules may oer a way out. For example, to our
knowledge there is currently no research that empirically tests whether levels of polygamy dampen the eect of sex
raos on the relave bargaining power of women and men by changing the demand for partners. Future work may
address this queson by studying gendered bargaining dynamics in a populaon where the level of polygamy
regionally varies due to ecological factors unrelated to gender ideology. Legislave changes of legal dierences
between regions can further provide natural experiments for studying how marriage rules or freedom of choice in
marriage and divorce can constrain people’s ability to gain leverage from a biased sex rao. More broadly,
understanding the causal role that gender norms might play would benet from more longitudinal work, parcularly
from areas undergoing rapid cultural and economic change. Alternavely, experimental norm-change studies that
specically posit causal direcon in the study design can be another way to see how gender norms might alter beliefs
about processes that aect the ASR (e.g. sex-biased investment).
6 Conclusion
A growing body of research addresses the eect of sex raos on reproducve behaviour in humans. This research
follows predicons from sexual conict theory and ndings from the non -human animal literature, where sex raos
have been shown to aect mang strategies and bargaining in many species in line with predicons. When a
populaon’s sex rao is biased, this changes both the interests of each sex in parenng and mang strategies, as well
as individuals’ bargaining power in sexual conicts of interest. Furthermore, the costs and benets of dierent mang
strategies in turn can inuence sex raos, resulng in a dynamic relaonship between sex raos and mang and
parenng behaviour. The tness payos of these behaviours are also dependent on environmental factors, such as
mortality risks associated with parenng or mate search, which can constrain the eect of sex raos. In humans,
sexual conict dynamics are even more complex due to the role of cultural norms regarding gender, and here we
https://doi.org/10.1017/ehs.2024.3 Published online by Cambridge University Press
24
believe the term ‘gendered conict’ is useful in direcng researchers’ focus to this addional layer of complexity
(Lawson et al., 2023).
In this paper we argue that sexual conict theory must be revised in order to understand these processes in
human populaons, where culture adds a layer of complexity that cannot be ignored. Gender norms themselves are
in part a product of conicts of interest between women and men. However as cultural traits they are also subject to
cultural evoluonary processes, resulng in their detachment from expected sexual conict outcomes and their
potenal to in turn inuence conict dynamics. Gender norms play a central role in marriage market and bargaining
dynamics by altering who is on the market, how individuals are valued and how much freedom of choice they have,
and by directly aecng sex raos. Crucially, gender norms may also structure how individuals respond to market
value gained or lost through biased sex raos. Integrang sexual conict theory and cultural evoluon theory is crucial
to understanding gendered conict dynamics.
7 Acknowledgements
We would like to thank the Center for Behaviour, Evoluon and Culture, and specically Clark Barre, David
Lawson, Nancy Levine and Brian Wood for fruiul discussions and thoughul comments on the manuscript. We
would like to thank the four anonymous reviewers for their helpful suggesons.
8 Author Contribuons
RVH led and BAS supported the conceptualizaon of this paper. RVH wrote the original dra, and RVH and BAS equally
contributed to eding and revising.
9 Financial Support
This work was supported by the Hiroshi Wagatsuma Memorial Fellowship.
10 Conict of Interest
The authors declare no conicts of interest.
11 Research Transparency and Reproducibility
n/a
https://doi.org/10.1017/ehs.2024.3 Published online by Cambridge University Press
25
12 Data Availability Statement
n/a
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