Operational Sex Ratio and Human Mate Choice

This entry was written by Rebecca Johnston as part of a project done in BIAN 2133 ‘Human Reproductive Strategies’ at The Australian National University in 2019 Semester 2.


Reproductive choices are often dynamic, influenced by both individual and population factors. One population level and critical demographic feature affecting mating options is the availability of potential partners (Moss & Maner, 2016). The operational sex ratio (OSR) is defined as the ratio of sexually receptive males and females. When unbalanced, the OSR can have a significant impact on mating choices and behaviour (Kvarnemo & Ahnesjo, 1996). This impact has been both observed in humans and animals, in addition to experimentally tested. Therefore, within this entry, the implications of the OSR in relation to humans will be explored, including a brief overview, observational and experimental evidence and its limitations.

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It is proposed that a population of reproductively receptive men and women is similar to an economic market, operating in accordance with the principles of supply and demand (Moss & Maner, 2016). In terms of supply, the ratio of men to women in this market is not always equal, with the OSR varying between locations and time, with unbalanced ratios in humans arising from aspects such as the tradition of son preference, mortality rates, pregnancy and nursing, and migration (Noë, 2017; Pollet & Nettle, 2008). When the OSR deviates from being equal and the ratio becomes biased towards one sex, the in-demand and less prevalent sex has more bargaining power, thus can afford to have greater selectivity without losing mating opportunities and acquire the highest quality partner possible from the abundant market of potential partners. Conversely, the predominant sex has little bargaining power and will experience greater intrasexual competition for access to partners of the relatively scarce sex. When the OSR is male-biased, female scarcity means males are more willing to adopt behaviours consistent with female mating preferences including reduced promiscuity, parental care and monogamy. In contrast, when female-biased and females are abundant, males can adhere to their preferred mating system of greater promiscuity and little parental investment, resulting in women competing for male mates by offering sex with little commitment requirements (Kvarnemo & Ahnesjo, 1996; Moss & Maner, 2016).

Observational Evidence For OSR

These responses to the OSR have been both observed and experimentally tested. Pollet and Nettle (2008) found that within the US during 1910, women in male-biased states overwhelmingly married men of higher socioeconomic status (SES). In particular, as the OSR became more biased towards males, the impact of SES on marriage likelihood became greater, with the effects felt disproportionately by low SES men, whose chance of marriage was found to be significantly reduced. Similar observations were found in Uganda, where polygyny was greater in males who own land than those who were landless. Likewise, the ‘one-child’ policy and the preference for sons in China has significantly altered the local OSR. Consequently, the significantly greater amount of receptive men to women has resulted in the introduction or increase in bride prices as well as a greater number of marriages, especially at younger ages, with men likely motivated to secure a partner earlier against uncertain opportunities in the future (Noë, 2017).

Experimental Evidence For OSR

Moss and Maner (2016) tested the impact of the OSR experimentally using undergraduate students at a Florida University. Participants read articles that described either a male or female biased OSR on campus then completed the multidimensional Sociosexual Orientation Inventory, a measure of willingness towards casual sex participation. Results were found to support prior observations made regarding behaviour and a biased OSR, with participants adjusting their desire towards casual sex in response to exposure to either an unfavourable or favourable sex ratio. When in the minority, individuals typically adhered to the strategy preferred by their sex, whereas those in the majority, altered their behaviour in line with the preferences of the minority. Men were found to largely favour uncommitted sexual relationships, whereas women favoured greater commitment. Moss and Maner (2016) also explored aggression towards intrasexual rivals in relation to the OSR. Participants again read the articles but were also required to have a photo taken and write a short description about themselves. Both items were shown to other participants and told it was a competition amongst them. At the same time, aggression was tested using a noise-black test, a reaction-time task for measuring aggression. It was found that greater unprovoked aggression was demonstrated toward same-sex individuals who displayed desirable qualities, thus, represented a potential rival. Conversely, no aggression was shown against same-sex individuals with relatively undesirable qualities or between individuals when in the minority. This is unsurprising given the intent of such aggression would be to harm a competitor who represents a reproductive threat.


However, the OSR and its implications for mate choice is not without debate. Kokko et al. (2012) state that while the OSR is associated with competitive traits that improve mate acquisition, it is just a description of a pattern observed in nature that provides a general rule without creating an explanation. Additionally, population level frameworks while valuable, can be limited when factors that affect sex-specific mating strategies are not considered, such as marriage laws, religion and disease levels. Moreover, it is also argued that while the OSR can have implications for mating, it is unlikely to influence mating effort and strategies alone but rather in combination with other factors such as certainty of paternity and the degree of parental care provided by the sexes (Schacht & Borgerhoff Mulder, 2015). Therefore, testing and measuring such responses in humans represents a significant challenge.


Thus, the ratio of receptive men and women plays a critical role mating choices and behaviour. It has been both observed in the US, Uganda and China, and experimentally tested that when the ratio of receptive males and females becomes biased to one sex, those in the minority can adhere to their preferred strategy, whereas when the ratio is unfavourable, individuals may have to alter their mating strategy to that of the other sex in order to increase their probability of finding a partner. While there is supportive evidence for the influence of OSR on mating choice, partner selection and availability can be also influenced by a number of other factors. Overall, the OSR demonstrates the dynamic nature of mating and reproductive related decisions.

Literature Cited

Kokko, H., Klug, H., Jennions, M. D., & Gaillard, J. (2012). Unifying cornerstones of sexual selection: Operational sex ratio, bateman gradient and the scope for competitive investment. Ecology Letters, 15(11), 1340-1351.

Kvarnemo, C., & Ahnesjo, I. (1996). The dynamics of operational sex ratios and competition for mates. Trends In Ecology & Evolution, 11(10), 404‐408.

Moss, J. H., & Maner, J. K. (2016). Biased sex ratios influence fundamental aspects of human mating. Personality and Social Psychology Bulletin, 42(1), 72-80.

Noë, R. (2017). Local mating markets in humans and non-human animals. Behavioral Ecology and Sociobiology, 71(10), 1-18.

Pollet, T. V., & Nettle, D. (2008). Driving a hard bargain: Sex ratio and male marriage success in a historical US population. Biology Letters, 4(1), 31-33.

Schacht, R., & Borgerhoff Mulder, M. (2015). Sex ratio effects on reproductive strategies in humans. Royal Society Open Science, 2(1), 140402-140402.

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