Too Many Males or Too Many Females? Classroom Sex Ratio, Life History Strategies and Risk-Taking Behaviors

Sexual selection, along with natural selection, is one of the driving forces for the evolution of species. Sexual selection occurs through competition between same-sex individuals, generally males, in order to access the opposite sex (intrasexual competition), and selection by one of the sexes, generally females, of individuals of opposite sex (intrasexual selection) (Darwin, 1871). To explain these sex differences in reproductive strategies, the parental investment theory is suggested (Trivers, 1972). This theory defines parental investment as “any investment by the individual in its offspring that increases the offspring’s chances of surviving (and hence reproductive success) at the cost of the parent’s ability to invest in other offspring” (p.136). More specifically, human females are the sex that provides the most minimum physiological demands for reproduction and offspring survival (e.g., gestation, lactation or infant care in mammals). Furthermore, females have lower potential reproductive capacity compared to males and represent a limited resource (Trivers, 1972). Offspring survival also plays a key role for ultimate reproductive success in females, which promotes higher avoidance of risk-taking behaviors in females that could imply potential injury or death (Campbell, 2013). Alternatively, males’ potential reproductive capacity depends on the number of females they can access, thus leading them to compete for female partners in an effort to secure their individual reproductive success (Geary et al., 2004). This male-male competition occurs mainly through agonistic behaviors (fighting, chasing or displaying) aimed at deterring rivals, and courtship behaviors to attract potential mates (Weir et al., 2011). This provides successful males with higher potentiality to have larger numbers of offspring. In fact, since ancient times, males have fought to gain access to mates thus developing traits that contribute to their success in contest competition (Hill et al., 2013).

An evolutionary approach on risk-taking behaviors can help explain the adaptive value of these behaviors, as well as their higher expression in males. Additionally, there is an evolutionary explanation known as the “good genes hypothesis” (Zahavi, 1975), which proposes that females choose mates based on males’ genetic quality. Selecting mates with good genes will provide the offspring with relevant evolutionary mechanisms such as adaptability, good health or genetic variability (Gangestad et al., 2007). This is why risk-taking behaviors, despite the potential negative costs for individuals who exhibits them, can also contribute to the bearers’ fitness. In this sense, risk-taking behaviors can act as an honest indicator of good genes in males and which is highly valued by females (Kelly and Dunbar, 2001). Furthermore, females also choose their partners based on their ability to be good providers (Marlowe, 2003). In fact, the selection of mates considered good providers promotes higher intrasexual competition in males, with some studies suggesting that this contest competition could have been the main sexual selection mechanisms in males (Puts, 2010). Successful males in such contest competition will gain social status and access to resources, which will in turn contribute to their higher reproductive success (Hopcroft, 2006). It is therefore in this context of contest competition that risk-taking behaviors become a means through which males can exhibit formidability and gain social status against their potential rivals (Fessler et al., 2014). Finally, the choice of good fathers for helping in raising the offspring is another valuable factor considered by females, and which contrasts traits attributed to good provisioning skills, mainly based on intrasexual competition (Lu et al., 2015). This means that both contest competition and risk-taking behaviors would be traits opposed to good father attributes. In fact, good father attributes tend to show in species where males have higher reproductive success when involved in the care of their offspring than when they compete with other males (Lu et al., 2015). However, it is possible to see both reproductive strategies combined in one species, such as in humans (Geary et al., 2004).

Despite mate choice being mainly females’ decision, males also show preferences in their mates’ choices. More specifically, fertility and good genes in females are highly valued characteristics by males and are assessed based on physical traits related to females’ attractiveness (Thornhill & Gangestad, 1999). Fidelity and being a good mother are also qualities that male wish for in their partners, both addressed to easing paternal uncertainty and ensuring maternal investment (Thornhill & Gangestad, 1999). Finally, the ability to provide resources is also a highly valued trait in females, particularly in modern economies where women are fully involved in paid jobs (Lu et al., 2015). As a result, females also compete socially between them, particularly when it comes to breeding sites, reproduction opportunities, belonging to breeding groups and social status within these groups (Clutton-Brock & Huchard, 2013). That is, both reproductive strategies of contest competition as well as mate choice are highly widespread in males and females, with differences in these strategies being quantitative rather than qualitative (Campbell, 2013).

One important aspect from evolutionary theory is the analysis of individuals’ survival and reproduction strategies, which relates to life history strategy, a mid-level evolutionary theory that analyzes how organisms allocate efforts to meet a variety of survival and reproduction demands throughout their life cycles (Del Giudice et al., 2016). According to this theory, individuals must perform trade-offs when allocating finite energy and resources towards growth, maintenance, reproduction, and parental care. Moreover, using these trade-offs varies based on the demands of the closest environment, mainly in the stages of growth and development during childhood (Belsky et al., 2012). Throughout evolutionary history, the relation between the conditions of environments and life history strategies settled and this relation still determines organisms’ responses to their current environment (Pepper & Nettle, 2017). These environmental constraints which determine life history strategies are environmental unpredictability, related to morbidity-mortality (e.g., diseases, wars and violence), and environmental harshness, derived from resource scarcity (e.g. famine and economic recession) (Ellis et al., 2009). Due to individual differences in the exposure to the environmental cues of unpredictability and harshness, individuals’ trade-off decisions will differ, and can broadly be divided into those who allocate bio-energetic resource towards reproduction or early reproduction (i.e., fast life history strategists) and those who invest mainly in development and maintenance, including parenting (i.e., slow life history strategists) (Ellis et al., 2009). In general, unpredictable and harshness environments promote the development of fast life history strategies, where present fitness is prioritized and can be seen in a variation of physiological and psychological characteristics such as faster development, earlier sexual initiation, shorter-term partners, higher number of offspring that receive less investment, shorter-term approaches, higher immediate pleasure seeking, and higher risk engagement (Ellis et al., 2012). By contrast, predictable and controllable environments promote the development of slow life history strategies, which are characterized by the opposite pattern.

In essence, fast life history strategists tend characteristically to invest more time and energy in mating (reproduction), compared to slow life history strategists, who tend to invest more in growth, development and parental investment (Del Giudice & Belsky, 2011). As a result of such strategies, those individuals whose investment in health and longevity is more adaptive than the investment in reproduction (slow life history strategy) will be risk-averse, since they will have high future expectations and value potential losses; conversely, those individuals for whom it is more adaptive to invest in reproduction due to their lower expectations on a long and healthy life (fast life history strategy) will more prone to take risks since they have less to lose (Sear, 2020). Engaging in risk-taking behaviors would therefore be a trait of fast life history strategists, which has been previously observed in risky sexual behaviors (Belsky et al., 2010), aggression (Figueredo et al., 2018), and gambling and criminality (Mishra et al., 2017). These same findings have been observed in adolescents, where both males and females with fast life history strategies tend to show higher levels of engagement in risk-taking behaviors (Lehmann et al., 2018), and aggression and criminal conduct (Simmons et al., 2019).

Despite the key role of environmental harshness and unpredictability, there are other internal and external factors that can interact and impact the development of life history strategies. Both harshness and unpredictability can interact with internal body states and reinforce the individual’s development of life history strategy in the same direction (Chang et al., 2019). For instance, individuals who had been exposed to harsh and unpredictable environments during childhood and who had experienced adverse internal body states adjusted their pattern towards fast life history strategies in adolescence (Chang et al., 2019). It has also been seen that environmental unpredictability and harshness can interact with attachment between mother and child in the development of life history strategies (Lu et al., 2022). Results showed that insecure attachment strengthened the negative effect of childhood adversities over the development of a slow life history strategy; while in environments of secure attachment, such negative effect was mitigated (Lu et al., 2022).

It has also been seen that the development and expression of individuals’ life history strategies are not stable and may change throughout the life cycle due to changes in biological and social capital (e.g., offspring, relationships) as well as due to volatile information from the individual’s closest environment (Kubinski et al., 2017). This would be in line with the concept of “phenotypic plasticity”, which refers to individuals’ ability to adjust their life history strategies based on external and internal signals (Sear, 2020). When individuals’ closest environment lacks resources (harshness condition), they adjust their response and tend to spend more, in the case of fast life history strategists, or towards saving in the case of slow life history strategists (Griskevicius et al., 2013). Likewise, exposure to signs of mortality in adults (unpredictability condition) led individuals to feel an urge to postpone reproduction in slow life history strategists, as well as to invest more in their own embodied, and in fact, the opposite effect was observed in fast life history strategists (Griskevicius et al., 2011). Even females with fast life history strategies, when faced with signs of harshness, felt an urge to eat more, without restricting calories intake, thus promoting weight gain for the purpose of successful reproduction, while the opposite effect was seen in females with slower life history strategies (Hill et al., 2013).

Although risk-taking behaviors are generally linked with the development of fast life history strategies, the expression of these behaviors can be different in males and females. Variation in risk-taking behaviors due to life history strategy was found in young males but not in young females (Salas-Rodríguez et al., 2021). More specifically, engagement in risk-taking behaviors related to faster life history strategies in young males, while no differences in risk-taking behaviors based on life history strategy were found in females. These findings might imply differences between males and females when it comes to elements of life history strategy but also when it comes to the perception and response to the environment and which can affect the expression of certain behaviors, in this case risk-taking behaviors (Copping et al., 2017). In fact, although males are more responsive to contextual cues of mating and competition compared to females (Ellis et al., 2012), the adjustment of life history strategies to external cues seems to be more sensitive in females than in males (Richardson et al., 2017). Consequently, some authors recommend estimating the effect of life history strategy separated by sex, since sex differentiated pathways allow to obtain more information for a joint estimation in both sexes (Copping & Richardson, 2020).

So far it has been seen that trade-off decisions between the basic needs of survival and reproduction, encompassed in life history strategies can depend in part on individuals’ immediate ecological conditions, and that males and females may respond in different ways to these proximate external cues. The number of individuals competing for breeding partners is one of those ecological conditions that can moderate the intensity of reproductive competition in males and females, and which results from operational sex ratio (Clutton-Brock & Huchard, 2013). Operational sex ratio is defined as the ratio of sexually active males to sexually active females in a population at a given time and place (Emlen & Oring 1977). The theory of operational sex ratio proposes that sexual selection forces will be more frequent and intense in the sex with lower parental investment, since more individuals of that sex are ready and will compete for reproduction; at the same time, opposite-sex individuals become temporarily unavailable, given their higher parental investment (Emlen & Oring, 1977). Since skewed sex ratio affects the availability of potential mates, a competition between the members of the most abundant sex will increase (de Jong et al., 2012). However, other authors suggest that in skewed sex ratio contexts, intrasexual competition can be higher amongst the minority sex, given their potential benefits of accessing higher numbers of potential partners (Schacht et al., 2014). Furthermore, since males compete more intensely than females for mating, a skewed sex ratio results in a more dramatic intrasexual competition in males (Weir et al., 2011). Therefore, in more male-skewed sex ratios, males will invest more in competitive behaviors such as risk-taking behaviors, given their higher difficulty to access mates (Sng and Ackerman, 2020). In any case, sex ratio can also determine the intensity of mate competition in females and therefore affect their engagement in risk taking-behaviors (Campbell, 2013).

Some authors have suggested that operational sex ratio does not always determine the mating system of species and that both phenomena can be independent results from sexual selection (Willson & Pianka, 1963). Some recent models suggest that the intensity of intrasexual competition between males does not always depend on operational sex ratio but depends mainly on the potential costs linked to investing in a specific trait aimed at increasing mating rate (Kokko et al., 2012). This means that intrasexual competition may depend to a greater extent on how much individuals invest in a trait that increases mating success, regardless of the costs that such trait can cause (Kokko et al., 2012). As a result, intrasexual competition would be more intense when potential gains—number of available opposite-sex individuals—increases faster than the costs caused by a specific trait—the possibility of dying in competition.

Despite some theoretical objections, the role of sex ratio at local level (i.e., states, counties, municipalities) has been analyzed for a wide range of risk-taking behaviors. For example, US communities with male-skewed sex ratios showed a relation with higher levels of binge drinking in males but not in females (Aung et al., 2019). Additionally, countries with male-skewed sex ratio showed higher tendency towards extreme risk-taking behaviors such as male suicide attacks (Gibson, 2011). In fact, male-skewed sex ratio could be linked to historic Vikings raids, where low-status single men were more prone to risky behaviors in order to increase their wealth and status (Raffield et al., 2017). As a matter of fact, male-on-male violence and non-violent offending has been found to be more prevailing in male-skewed sex ratio municipalities in Stockholm metropolitan area, in Sweden (Filser et al., 2021) and campus with female-skewed sex ratios were associated with higher sexual activity in females (Uecker & Regnerus, 2010). In adolescents, higher propensity towards unprotected sex in females was related to female-skewed sex ratios at city-level in Brazil (Ramos et al., 2017). These studies are in line with the suggestion of higher intrasexual competition between the members of the majority group and which would lead to higher engagement in risk-taking behaviors. However, other studies that analyze the effect of sex ratio at local level found that it was the minority sex the one who exhibited more risk-taking behaviors when sex ratios were skewed. For example, at US county-level, males showed higher risk of infection by sexually transmitted diseases in female-skewed populations (Pouget, 2017), as well as higher rates of violent behavior (Schacht et al., 2016). Moreover, females who lived in male-skewed sex ratio wards in Northern Ireland showed higher risk of death by accidents, suicide, or alcohol abuse (Uggla & Mace, 2015). Finally, higher sexual activity and presence of sexually transmitted disease in male-skewed sex ratio communities in China was also found in females (Trent & South, 2012). In the case of adolescents, females showed higher sexual activity in male-skewed sex ratio communities in the US (Billy et al., 1994).

Most studies that analyze sex ratio base their analyses on local sex ratio, with data aggregated to higher levels (i.e., nations, regions, or cities) (Pollet et al., 2017). However, analyzing sex ratio aggregated to higher levels can hide significant variations at a more proximate level. In other words, local sex ratio would not reflect the proportion of males and females in closest environments in a precise manner (Uggla & Mace, 2017). As a result, studies based on aggregated data might have fallen into the phenomenon of the “ecological fallacy”, where deductions about individuals are made based on data aggregated to higher levels (Pearce, 2000). Additionally, relations found between variables can be completely different based on each analysis level, that is, the same relation at individual level can show opposite results at national level (Kuppens & Pollet, 2014). Data collected from individuals’ situational sex ratios (e.g., workplaces, classrooms, hangouts) relates to individuals’ interactions with each other at its closest level and hence provides more accurate information on partner supply and demand, compared to local sex ratios (Filser & Preetz, 2021). In essence, operationalizing sex ratio in closely circumscribed social context levels is a more precise measurement of this variable compared to sex ratios operationalized at larger levels.

Despite its advantages, the effect of situational sex ratio has been analyzed to a lesser extent. However, there are some interesting results, such as the following cases: in the US, adolescent females showed higher probability of participating in serious violence when they were part of male-skewed friendship networks, while male adolescents who were part of female-skewed friendship networks showed lower probability (Haynie et al., 2007). In Germany, both male and female adolescent students showed higher risk of school violence in male-skewed classrooms, mainly in relation to injuries attributable to other students (Filser et al., 2022); and in the Netherlands, a higher relation between status and relational aggression in adolescent male students was found in classrooms with male-skewed sex ratios (Zwaan et al., 2013). However, in Israel, it was found that female-skewed classrooms sex ratio promoted academic improvement in both females and males, which was mediated by lower classroom disruption and violence (Lavy & Schlosser, 2011). Finally, in an experimental context, being part of the majority sex promotes aggressive behaviors towards desirable same-sex individuals and higher intrasexual competition, both in males and females (Moss & Maner, 2016).

At all events, even the studies that analyze sex ratio at situational level show contradictory results, thus making it difficult to stablish a clear relation between sex ratio and risk-taking behaviors. The absence of a clear pattern in the relation between risk-taking behaviors and sex ratio might be due, in part, to the existence of individual differences in the adjustment of sex ratio (Schacht et al., 2017). It is therefore necessary to analyze which factors are involved in individual differences in the adjustment of competition strategies between individuals based on sex ratio (Schacht et al., 2017). Likewise, life history strategy, given its role in individual differences in reproductive and competitive adjustment, as well as its ability to adjust to contextual variables, might be a relevant individual variable when it comes to explaining the association between sex ratio and risk-taking behaviors.