Sex ratio

Human sex ratio is the ratio of males to females in particular ecology. It is distinct from the operational sex ratio (a concept more relevant to analyses of how sex ratio affects human mating behaviour). The operational sex ratio (OSR) is defined as either the ratio of sexually active males to fertile females in an environment or the ratio of sexually active females to sexually active males. It has been proposed that male-biased sex ratios contribute to male inceldom, and can also lead to increases in female hypergamy in environments where the sex ratio is biased towards men.

The sex ratio at birth is slanted towards males in humans by default, as there are more male foetuses conceived and born to full term than female offspring. The human sex ratio at conception and birth is furthermore influenced by a myriad of factors, including male sperm quality, environmental pollution, sex-selective abortion (typically serving to increase the amount of males in the population as practiced in India and China), and inbreeding depression (though the effects here are weak, random, and not unidirectional). Women may also have the ability to bias their production of children towards either sex depending on the resource abundance of the environment they are in and their own condition, in the context of men having more variation in reproductive value (i.e. male offspring representing a 'risky' investment in terms of the mother's reproductive success, female offspring representing a 'steady investment'). This is known as the Trivers-Williard Hypothesis. However, direct empirical tests of this hypothesis have resulted in mixed results, with inconsistent findings all-around.

Research into the effects of OSR on mating behavior indicate that the effects of an imbalanced sex ratio are chiefly exerted via an imbalanced OSR serving to make the rarer sex in a particular environment either more choosy in terms of partner choice (with the more plentiful sex more prone to compromise in terms of standards) or more prone and capable of executing their favoured sexual strategies (argued to be uncommitted sex for males and relationships with men of high mate value for women, in line with parental investment theory).

A male slanted OSR is also widely claimed to lead to increases in violent competition in men and greater social instability in general. However, evidence for this assertion is weak and inconsistent, likely being strongly modified by the specific cultural and individual contexts this violence occurs in. The evidence suggests that increases in violent behavior can be primarily limited to unmarried young men in high OSR societies (if their violence is not channeled towards an outgroup, e.g. via warfare). More men in a population does not necessarily mean more violence, with low OSR (more women) societies also presenting greater levels of violent behavior directed toward certain demographics (chiefly women).

Natural sex ratio

The average natural (uninfluenced) live-birth male to female sex ratio in the developed world is of about 105: 100. For example, this would mean that for a given population of 10 million females, there would be 10 500 000 males, and therefore 500 000 surplus males. Applying this ratio to the United States, which has a population of 285 million U.S. born citizens (excluding immigration), one could approximate that there would be more than 14 million surplus males in the country.

It has been hypothesized that the sex ratio would provide an evolutionary benefit for the spices, as younger males tend to die earlier around the age of reproduction, due to taking part in hazardous activities such as hunting or war. However, this does not seem to be the case for the modern world. Effectively, these selective pressures seem to have disappeared almost entirely (losses of young men to war is trivial in the developed world) and have been replaced by causes such as dangerous employment (ex. construction), lifestyle choices (alcoholism, drug use, whether as a result of depression or not), violent crime, and, chiefly, suicide. These new selective pressures, however, do not bring the sex ratio to parity at age of reproduction, as the latter start to wither by age 40.

Other factors that affect OSR

On top of these factors mentioned above that typically result in sex ratios being consistently biased towards males at birth, the operational sex ratio is also strongly influenced by the fact that males have a hypothetically longer reproductive career than women and tend to prefer younger women throughout their entire lifespans. This near-universal male preference for younger women would be expected to lead to greater competition among men for the relatively scarce young, fecund women in a particular population. Male immigranttion and male migration within a country may also directly increase the surplus of males.

Population aging and declining birth rates in many industrialized countries would be further expected to exacerbate this phenomenon, as this aging results in much fewer fertile age women as a portion of the population at large. However, this intensified competition for younger females would likely be limited to a certain degree by a general female aversion towards large age gaps, an aversion that is particularly pronounced in societies with greater gender equality. Though a gradually falling birth rate implies there will be a surplus of men also in only slightly older cohorts, with large effects however only to be expected during drastic changes, e.g. following the introduction of the contraceptive pill. Polygynous marriages also effectively increase the OSR, due to polygynous male's diligent mate guarding efforts and there often being harsh sanctions against adultery and extra-martial sex in such societies. A related effect is that of some men remarrying multiple times throughout their lives, engaging in de facto polygyny, with men generally remarrying more often than women. Other potential factors affecting OSR via women's sexual availability are pregnancy, lactation, menstruation and overall lower sex drive and greater coyness. For example, according to an informal survey, only 15% of women have sex during their period. That is, in any given potential courtship situation, women might more likely be mentally or physically unavailable, lowering the rate at which men can successfully enter a relationship. Since human female estrus is largely hidden, however, menstruation might play less of a role in sexual availability compared to other animals.

When OSRs are biased towards women, the primary cause is typically large scale violent conflict, as was seen in the sex ratios in Western societies after the two World Wars (with up to 30% of the males population of certain involved nations perishing in these conflicts). These wars resulted in large amounts of excess females due to enormous proportions of the military service aged males being killed in many of the participant nations. This paucity of males, together with social norms that demanded female chastity and that also punished extramarital sex, resulted in many women in these countries becoming involuntarily celibate due to them being unable to marry. On the other hand, certain authors have argued that this dearth of marriageable men also led to large increases in female promiscuity, with these authors arguing that these women were forced to seek less investment in their relations with males due to their lower bargaining power on the mating market. These increases in female promiscuity after major wars perhaps contributed to the 'flapper' subculture of the 20s after the Great War and the cinematic depictions of the seductive and lethal 'femme fatale' after the Second World War.

Other contextual factors such as local economic conditions, immigration trends by sex, dating app usage by sex, and sex differences in employment in specific workforce sectors plus sex-biased enrollments in certain university majors can also result in lopsided operational sex ratios in particular locales, causing them to be skewed to favor either sex depending on the specific environmental context examined. For example, dating apps are consistently found to be slanted towards male users, and university campuses in many developed countries consistently have more women than men.

Effects of OSR on mating behaviors

The effects of a slanted OSR on human sexual behaviors appear to be multifaceted and often contradictory, likely varying a lot by cultural and environmental context. The evidence is broadly supportive of parental investment theory, that is, the prediction (grounded in evolutionary psychology) that due to women's greater investment in childrearing they prefer men with resources and the propensity to invest heavily in them, at least in the context of long-term relationships. Several lines of evidence suggest that a higher sex ratio (more men) results in women being more able to translate their ideal mate preferences into reality in their relationships. Among the evidence supporting this view; it has been found women report more satisfaction in their marriages (though this appears to be moderated by race, with this effect only holding among Caucasians in the US), are more likely to be married at a younger age, and are more likely to marry wealthier men in environments with a higher sex ratio. There is also preliminary evidence that a higher sex ratio may serve to make women choosier in terms of the level of physical attractiveness they demand in a male partner, as it has been found that women primed in laboratory experiments to believe that men are more plentiful display greater preferences for more symmetrical (and thus more attractive) males.

Conversely, when sex ratios are low (more women), there are both more unmarried young men and more married older men, consistent with the theory that a low sex ratio is increasing male 'bargaining power' in the sexual market in general, leading to young men scrambling to maximize their casual sexual opportunities, while older men find it easier to find wives when they do desire to settle down. Paradoxically, a low sex ratio (more women) both seems to result in higher stated standards in male partners among women but lower standards when it comes to revealed preferences (i.e., by examining the income of the men they marry). This may be due to low sex ratios causing more flawed perceptions of men and relationships in general in areas with low sex ratios, potentially due to these low ratios resulting in more women being 'pumped and dumped' by certain men taking full advantage of the plethora of options a surplus of women affords them.

Providing further support for this idea that low sex ratios increase male promiscuity and frequently result in jaded women, there have also been claims that the contemporary campus 'hook up culture' is driven in whole or in part by low sex ratios at many universities, mainly due to women now dominating higher education. Research indicates that women at campuses with lower sex ratios are more likely to have sex in relationships, to have a more negative view of men, and to be more willing to have sex without commitment. Given female hypergamy and choosiness, it is not clear whether a plurality of men or only a select few benefit from these shifts in female dating behavior that occur due to a low sex ratio.

Sex ratio and violence

A male-biased (high) sex ratio seems to generally result in greater levels of intersexual competition among men, with the overly violent aspects of this increase of intrasexual competition being reflected in greater levels of violence among childless men directed towards other men in regions with a higher OSR, but there are conflicting findings related to violent behavior overall in environments with high OSRs.

Part of the reason for these inconsistent findings may be that environments with a lower sex ratio (more women) may result in more intrasexual competition among certain males, possibly due to these males taking advantage of the low sex ratio to maximize their casual sexual opportunities, therefore resulting in increased intrasexual conflict among these men. A low OSR is also predicted to lead to increases in mating effort among men overall, and this greater effort overall mating effort may include more opportunities for certain men to employ sexual coercive behavior when combined with the larger pool of non-attached females.

Supporting the hypothesis that a low sex ratio leads to increases in overall mating effort among men, some research has found lower rates of rape in areas with high sex ratios (more men) and higher rates of domestic violence in areas with low ratios (more women). It is important to note that, in humans, intrasexual competition also takes on several forms, including financial displays, self-enhancement and gossip (in both sexes), and is not necessarily reducible to brute force.

Several researchers have also claimed that a high OSR in a particular population can be predictive of violence directed towards the outgroup, mainly due to the plentiful amount of incel men created by such high OSRS, leaving these men with little hope of marriage. High OSRs in a country (combined with high rates of polygynous marriages) can be used to predict that countries likelihood of producing suicide attackers, and it has been proposed that one of the main drivers of the Viking raids in the early medieval era was that the high prevalence of polygyny in this culture caused a high OSR, resulting in many young men being unable to marry. Combined with other factors such as the technological advances in shipbuilding made during the era, plus the increasing political consolidation that took place in Scandinavia during this period, the argument is that this high OSR led to these young Scandinavian men to increasingly look overseas, leading to them attacking other groups like the English to gain resources that would improve their standing on the mating market. The possibility of finding young women to rape would also be a clear incentive to engage in these raids.

See also

• Sex drive