Encyclopedia of Evolutionary Psychological Science

Living Edition
| Editors: Todd K. Shackelford, Viviana A. Weekes-Shackelford

Sex Ratio and Men’s Long-Term Mating

  • Andreas Filser
  • Sebastian SchnettlerEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-16999-6_3655-1



  • Sex ratios are defined as the number of men per 100 women (in pre-defined age groups).

  • We define long-term mating as a mating strategy in which men are seeking a partner for a long-term relationship.


Human mating strategies are not fixed but constitute facultative responses to developmental, individual- and population-level, social, and ecological circumstances. As a consequence, men’s preferences for and likelihood of engagement in long-term mating have been shown to be subject to variation across contexts (Schacht and Kramer 2016; Uggla and Mace 2017) “Sex Ratio” by D. Schmitt.

A growing body of literature identifies the sex ratio as such a contextual factor for mating behavior. The sex ratio is commonly defined as the number of men per 100 women, yet operational definitions vary across the literature. Variants include the adult sex ratio (ASR) which is calculated on the basis of age cohorts between puberty and menopause and the operational sex ratio (OSR) which is defined as the ratio of unmarried adult men to unmarried adult women in a population. In monogamous societies, OSR and ASR should be highly correlated. In either case, low sex ratios indicate a relative oversupply of women, whereas high sex ratios indicate a relative oversupply of men.

Mating Behavior and Sex Ratios

Evolutionary and social scientists conceptualize the local number of males and females as a mating market which is driven by supply-and-demand dynamics. Following this conceptualization, an oversupply of one sex on the mating market increases mating opportunities for the other, scarcer sex. A common hypothesis posits that both women and men’s likelihood of engaging in a sexual or romantic encounter or relationship increases with the relative number of opposite sex partners. This perspective is often referred to as the “demographic opportunity thesis” (Adkins et al. 2015). A number of scholars argue that an imbalance of mating opportunities further translates into asymmetries in bargaining power on the partner market (Arnocky et al. 2016; Schacht and Kramer 2016; Uggla and Mace 2017). When either sex outnumbers the other, the scarcer sex has a larger pool of potential partners to choose from and thus a higher likelihood to find a partner that best fits his or her own preferences. On the other hand, individuals of the more common sex face tougher competition to find a partner. In view of such intensified competition on the partner market, an optimal strategy may be to cater to the preferences of scarce partners.

Parental investment (PI) theory predicts that due to women’s higher physiological parental investment (gestation and breastfeeding), men and women have evolved different mating psychologies. Although both men and women exhibit short- and long-term mating strategies across life course stages and socio-ecological contexts (David P. Schmitt – Sex Ratio), PI theory suggests that, on average, men exhibit a higher tendency toward short-term and uncommitted mating than women (Robert Trivers: Parental Investment and Sexual Selection). Against this background, sex ratio theory suggests that the ability to realize these sex-specific mating preferences is co-determined by partner availability. When males are in short supply, they are more likely to have their preferences met because women compete for a partner by appealing to male mating preferences. Given the assumed higher average preference for short-term mating in males, this implies that, in a low sex-ratio context, males can behave more promiscuously, offer little parental investment, and still be able to find a partner. However, when women are less plentiful, men will appeal more to female preferences and be more willing to commit to a single partner. In sum, local sex ratios are theorized to shift incentives to pursue different partner market strategies and to display different mating behaviors (Arnocky et al. 2016; Schacht and Kramer 2016; Uggla and Mace 2017).

Empirical Evidence

Empirical evidence only partially supports the prediction that men are less likely to commit themselves to a single relationship when partner markets are favorable, i.e., sex ratios are female-biased. Until recently, the bulk of empirical studies analyzed indicators of female mating behavior to infer on male mating behavior. Specifically, studies analyzed associations of sex ratios with rates and timing of women’s marriage or rates of single-mother households and interpreted low rates of marriage or two-parent families as indicative of low male relationship commitment. However, recent studies undertook steps toward a more direct measurement of male mating behavior. For instance, Schacht and Kramer (2016) analyzed male marriage rates from US census data and found that male abundancy correlates with higher shares of men being married. In addition, the authors report that high sex ratios are associated with lower rates of extramarital fertility and female-headed households. Overall, the authors interpret these results as indicative of higher rates of male long-term mating strategies when women are scarce. However, inferences on individual-level correlations from aggregated data bear the risk of ecological fallacies. Testing hypotheses on individual behavior patterns should ideally involve individuals as units of analysis.

Studies in the field increasingly address this concern by analyzing individual-level data but yield contradictory results. One study shows that men in female-biased US census tracts were less likely to cohabit with a romantic partner and had a greater number of dating partners (Warner et al. 2011). Similarly, a relative abundance of women in their community increases the likelihood for Chinese men to engage in premarital intercourse (South and Trent 2010). However, there is also evidence suggesting that ASRs are inversely associated with the male propensity to form a committed relationship. For instance, Eckhard and Stauder (2018) report that education and age-specific sex ratios correlated positively with union formation among men in Germany. Moreover, Uggla and Mace (2017) report that while female cohabitation is associated with male-biased sex ratio wards in Northern Ireland, male cohabitation did not correlate significantly with ward-level ASRs in Northern Ireland. A study of union formation in Mexico finds neither significant associations between female nor between male union formation and municipality sex ratios (Parrado and Zenteno 2002).


One explanation for these contradictory findings could be that union and marital formation are only weak indicators for long-term mating strategies when social costs and stigma associated with separation are low. Consequently, associations of sex ratios with indicators such as marriage or premarital intercourse might vary across time and space. Also, associations should be sensitive to different operationalizations of similar indicators. For instance, marriage and cohabitation might reflect different levels of commitment across societies.

Another point to be made is that studies of long-term mating strategies should seek to include larger portions of individuals’ relationship trajectories beyond just the transition to first marriages, cohabitation, or parenthood. Besides these early transitions, for instance, the stability of pair bonds, or the likelihood for infidelity to one’s partner, might be relevant correlates of sex ratios as well. Men might at first commit to a partner but only later choose to opt out of the relationship to that partner. This follows from the higher opportunity costs men in a numerically favorable partner market have for staying in a relationship as compared to men in a less favorable partner market. Empirically, Arnocky et al. (2016) could show, for instance, that partnered men exhibited stronger infidelity intentions in an experiment when led to believe that mates are abundant instead of scarce. Similarly, female-biased sex ratios correlated with greater risks of separation for both men and women in an analysis of register data from Northern Ireland (Uggla and Mace 2017).

A further explanation for contradictory findings could be that regional or local sex ratios do not correspond closely with individual experiences of partner availability. Experimentally, participants have been shown to adapt in response to clues of local sex ratios. For instance, men were less favorable to engaging in casual, uncommitted sex when treatments suggested that potential female partners are scarce versus plentiful in an experiment (Arnocky et al. 2016). However, explicitly providing participants with information on local sex ratios in a lab experiment might not have high external validity for how individuals normally perceive partner availability on the partner market. Social everyday interactions in modern societies are structured into social foci, such as workplaces, schools, colleges, hangouts, or friendship networks. Sex ratios in these local contexts might not necessarily be representative of the local proportion of men and women: even in a high-sex-ratio society, for instance, male and female networks may be highly segmented by sex and gender. Consequently, individuals likely perceive partner pools selectively based on their respective social foci. This is supported by a recent study of nationwide survey data which finds no correlation between a variety of local sex ratio measures and subjective partner availability in Germany (Filser and Preetz 2019). If indeed local sex ratios are only loosely, if at all, correlated with individual perceptions of sex ratios, this may be one explanation for the contradictory results on the purported association between sex ratios and male mating behavior. In general, operationalizing sex ratios more closely based on what individuals actually perceive as their partner market might yield more consistent results when studying social outcomes of sex ratios.

Observational studies of sex ratios in social foci such as campuses seem to be a promising approach to circumvent such problems. For instance, sex ratios of colleges might be a closer approximation of students’ peer group sex ratios and therefore capture subjective experiences of partner availability more accurately. Yet, empirical evidence on campus sex ratios remains contradictory as well. For instance, female-biased college campus sex ratios have been reported to correlate with more frequent hookups and a higher number of sexual partners. Moreover, when female students outnumbered men on campus, both men and women were less likely to consider love a necessary prerequisite for sex, an indication for a less restrictive socio-sexual orientation (Adkins et al. 2015). However, this is at odds with a recent study which finds that in high schools in which girls outnumber boys, students report a less sexually permissive normative climate and girls report less casual sex when compared with their counterparts at schools in which boys outnumber girls (Harknett and Cranney 2017). Of course, both studies differ in the age structure of their study populations. However, the opposite associations suggest that more research on school and campus sex ratios is warranted.


Sex ratios have been theorized to affect men’s propensity for long-term versus short-term mating behavior. Specifically, men are predicted to be more committed to a single partner when alternative options are scarce, i.e., sex ratios are male-biased. While there is some experimental and observational evidence to support these predictions, the literature remains contradictory and fragmented across the social and biological sciences.

More research is needed to establish a better understanding of how sex ratios affect mating. Specifically, future research should focus on individual-level data instead of studying population-level indicators to avoid ecological fallacies. Analyses of individual-level data further enable researchers to study potential variation of behavioral responses to imbalanced sex ratios across social status groups or other individual characteristics (Uggla and Mace 2017). In addition, more longitudinal analyses are necessary to substantiate how sex ratios affect mating strategies across the life course. Finally, paying closer attention on how responses to comparable sex ratios vary across cultural contexts seems indispensable. Social norms concerning sexual activities and union formation, particularly social sanctions against pre- or extramarital intercourse, may limit the viability of mating options.

While both theoretical and empirical clues that sex ratios are associated with men’s long-term mating exist in the literature, an integrated and empirically well-substantiated model has yet to be established.



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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Department of Social SciencesCarl von Ossietzky University of OldenburgOldenburgGermany

Section editors and affiliations

  • Tara DeLecce
    • 1
  1. 1.Department of PsychologyOakland UniversityRochesterUSA