Intrasexual Violence and Aggression
Intrasexual violence is the intentional use of physical force to inflict harm on another, and it occurs between members of the same sex, in relation to competition over mates. Intrasexual aggression occurs between members of the same sex during competition for mates and is an intentional act to inflict harm on another. It can encompass direct and indirect acts. Aggression can have violent consequences – but not always – for example, gossip, ostracization, and scarification can intimidate without inflicting bodily harm to another.
This entry provides a concise overview of the theory and evidence regarding intrasexual selection as a driving force – shaping patterns of human aggression and violence. It tends not to dwell on human violence and aggression as being purely unique behaviors that have unique underpinnings. Rather, it attempts to discuss an ultimate framework for the evolution of human aggression and violence by putting humans into context equal with research on many taxa. While there are always within-taxa specificities, this entry attempts to find generalities. The evolution of sex roles, mating systems, and sex ratios are discussed as they relate to the distribution of human violence and aggression. As a necessity, this entry focuses on male intrasexual violence and aggression, simply because it is more prevalent among males, but does also consider intrasexual aggression and violence in women.
Male Intrasexual Violence and Aggression
Sexual selection is an evolutionary process that favors traits that lead to greater success in securing a mate and therefore greater reproductive success (Andersson 1994). It was Darwin (1871) who first referred to sexual selection whereby members of the same sex compete with each other for mates, as intrasexual selection. In vertebrates, males are typically more aggressive than females. Contest competition, when males of many taxa physically compete with each other for access to mates, has led to the evolution of a plethora of armaments – from pincers to antlers to fangs – that males use in battle with one another (Emlen 2008; Puts 2010). Although there are some exceptions, intrasexually selected armaments are rare in females (Berglund 2013). This evolved weaponry suggests that males are typically engaging in aggression during competition for mates more than females. Additionally, with females providing a greater amount of parental investment, a trade-off between parental investment and competing for mates typically exists. Fish are an exception to this because the trade-off is usually weaker or absent altogether (Jennions and Kokko 2010). Some bird taxa are another exception where the bias towards female parental investment is reduced comparative to mammals and reptiles (Goymann et al. 2015).
In humans, studies and crime statistics consistently show that violence and aggression are more prevalent among men than women. There are two caveats to this point: Adolescent girls use indirect aggression more than adolescent boys do (discussed in more detail when considering female intrasexual aggression) and women use violence in romantic partnerships as often as men do (Cross and Campbell 2011). Sexual dimorphism in aggression begins in childhood from the age of two years onwards (Campbell 1999) and peaks in young adulthood (Archer 2009). Violent crimes are much higher among men, and male on male violent crime is the most prevalent (Campbell et al. 1998). This has led researchers to the hypothesis that differences in human same-sex violence and aggression are part of a greater evolutionary pattern, which is driven by the different strategies that males and females employ to secure mates (Darwin 1871).
Underpinning this sexual dimorphism in violence are differences in potential reproductive outputs for men and women, conventional views state that men are usually capable of siring many more offspring than are women (Archer 2009). Trivers (1972) reasoned that males compete more for reproductive opportunities because they do not extend as much effort towards parental investment in offspring. Bateman (1948) showed in Drosophila that females expend more energy in gamete production, than do males in sperm production (anisogamy). It therefore would make it more beneficial for males to seek out multiple mating opportunities in situations where offspring require little parental investment, or where mothers invest a greater amount of energy into caring for their offspring, whereas when parental care is equal or greater in males the strength of intrasexual aggression in males is reduced. Furthermore, in so-called role reversed taxa, such as the polyandrous black coucals, males brood the eggs and care for the hatchlings, whereas the female birds compete and are the larger, more aggressive sex (Goymann et al. 2015).
Consideration of Sex Role Evolution
Considering sex role evolution is imperative to answering the question of whether intrasexual selection could have shaped human aggression and violence. Especially when we consider that human males follow the same general pattern of being more aggressive and violent than their female counterparts and are often the sex that uses aggression and violence to compete for mating opportunities (Carter and Kushnick 2018). Kokko and Jennions (2008) argue that explaining sex roles is more nuanced than much of the past conventional literature considers. For example, the notion that males are able to produce more offspring than females violates the “Fisher Condition,” which states that males are inevitably limited by a female’s rate of reproduction. Despite this, in humans men on average still have a greater variance in number of offspring than women do (Cross and Campbell 2011). Secondly, the assumption that females (more than males, due largely to anisogamy) cannot afford to lose their past investments in offspring violates the “Sunk Costs” fallacy (also termed Concorde fallacy), a cognitive bias that describes the tendency to think that previous effort expended in an activity makes further effort in that activity more profitable. This argument also assumes that female’s effort in gamete production is linked to greater amounts of parental care later on.
Due to the work of Kokko and Jennions (2008) and others (for examples see: Goymann et al. 2015; Schacht et al. 2014), models that attempt to explain sex role evolution now take into account not only Operational Sex Ratio (OSR) or the ratio of males to females in a population that are of reproductive age, but also the availability of males and females. This highlights the need to consider Adult Sex Ratios (ASRs) and take into account the effect ASRs have on mate competition; this is discussed further in the next section. Other things to consider in sex role models are whether young born are altricial, i.e., underdeveloped at birth and requiring more care such (e.g., humans) or precocial, i.e., more developed and independent at birth (e.g., giraffes), as this will dictate the amount of parental care required. Additionally, mating system, paternity certainty, the strength of sexual selection, and even possible sexual conflict are all interrelated factors that need to be taken into account when attempting to explain sex role evolution (Kokko and Jennions 2008).
The social role theory proposes an alternative to sexual selection theory in explaining sex roles and their relationship to aggression. According to the revised social role theory, the differences in aggressive behavior exhibited by men and women are due to societal divisions of labor, rather than sexual selection (Eagly and Wood 2011). Although the proponents of the social role theory do allow that the social requirements of the sexes are interacting with ecological and environmental factors, they disagree that sexual selection can explain patterns of human aggression for two main reasons. Firstly, that explaining aggression using intrasexual selection relies mainly on the assumption of humans employing a polygynous mating system. Secondly, that sexual dimorphism is lower in humans comparative to other primates (Archer 2009). While most researchers do not deny that social and cultural norms play a role in explaining human violence and aggression, the evidence shows them unlikely to explain the entire pattern. Putting humans into evolutionary context with other taxa – it would be unparsimonious to assume human exceptionalism (Archer 2009).
Nevertheless, it is valuable to address the criticisms of the intrasexual selection explanation. Firstly, as this section addresses, explaining sex roles in all taxa is a complex interplay of factors, of which mating system is only one. Although serial monogamy is common in humans, evidence still shows that polygynous mating, or extra-marital couplings are commonplace and contribute to men having a larger variance in number of children than women (Cross and Campbell 2011). Secondly, it is accurate to say humans are generally less sexually dimorphic in say body size than our ancestors, although issues with gauging this are discussed in the next section. Nonetheless, it is not entirely accurate to say that humans are less sexually dimorphic compared to all primates, even if one only considers apes. For example, the family Hylobatidae (gibbons and siamangs) are monogamous (although sneaky mating does occur) and they are much less sexually dimorphic in both body and canine size than humans are – and are the only nonhuman primate to employ monogamy (Kramer and Russell 2015). The relationship between mating system and intrasexual selection is discussed in the following section.
Mating System can Predict Strength of Intrasexual Selection
Phylogenetic studies show past ancestral mating system can often predict future mating strategies, and in turn mating system can affect the strength of intrasexual selection. A recent cross-cultural study showed that the intensity of intrasexual aggression is highest in societies that sanction polygyny and where the variance in number of wives is the greatest (Carter and Kushnick 2018). Although many of these societies practice serial monogamy more commonly (one partner at a time, or for a number of years), opportunistic polygynous mating is still occurring frequently, as it does even in societies where polygyny is outlawed (Cross and Campbell 2011; Kramer and Russell 2015). This opportunistic polygyny could be driving the intensity of male intrasexual violence and aggression. Granting modern humans are generally considered less sexually dimorphic in size than both the Australopithecines and early Homo sapiens fossils from the mid-Pleistocene (Kramer and Russell 2015).
Debate surrounds this point about variation between modern humans and/or hominid ancestors given sex determination and dimorphism are very difficult to gauge from the small sample sizes of fossils available, especially when these fossils do vary in size and are often fragmentary. It is also problematic to determine correlations between mating behavior and fossil size in extinct species (Plavcan 2012). Nevertheless, body size dimorphism in humans is commonly greater than in other monogamous species of apes (e.g., Gibbons), but smaller than in polygynous species (e.g., Gorillas) suggesting some degree of male antagonistic competition (Kramer and Russell 2015). Males (and females) can also compete for mates in aggressive yet nonviolent ways such as scarification for intimidation, which can act as an armament or an ornament. These nonviolent behaviors are unlikely to reduce the strength of intrasexual competition but are likely to exhibit different selection pressures upon it (Carter and Kushnick 2018).
The Relationship between Sex Ratios and Violence
As touched on earlier, there is a relationship between sex ratios and violence that is mediated by mating behavior. As violence and aggression are usually more prevalent in men, a long commonly held belief is that an ASR skewed towards men will result in more violence. Additionally, due to females providing a greater parental investment, conventional sexual selection theory assumes this skews the OSR towards males. Schacht et al. (2014, pp. 214) termed this the “more men, more violence expectation.” This notion fits with long held views on sexual selection, such as the influential work of Emlen and Oring (1977) who coined the term OSR as a key measure of the potential intensity of sexual selection. The conventional view has been that male biased sex ratios lead to an increase in male-male competition. This explanation seems intuitive, and mathematically if one considers again that males are usually the victims and perpetrators of violent crime, more men additively equal more violent crime. This was the explanation given for the increasing violent crimes in China and India, due to a preference for sons – both countries had a male biased ASR (Oldenburg 1992; Dreze and Khera 2000; Edlund et al. 2007).
Although these notions appear intuitive, they have been challenged recently. Both mathematical and empirical evidence for such commonly held assumptions are not met (Kokko and Jennions 2008; Schacht et al. 2014; Carter and Kushnick 2018). In their review of data from 15 preindustrial societies, Schacht et al. (2014) demonstrated that there is more male-male competition when sex ratios are female biased. Further evidence for this comes from a recent cross-cultural study that found the strength of intrasexual selection was greater in female skewed sex ratios (Carter and Kushnick 2018). Kokko and Jennions (2008) have demonstrated that, under certain circumstances, male biased adult sex ratios can lead to a decrease in competition. This is because some males will shy from competition when costs are high or probable benefits low – leading to an ASR that is a poor measure of OSR. That the strength of intrasexual competition among males may be higher when females are in excess does not necessarily negate the hypothesis that this could have shaped human male aggression, but it does illustrate a need for further evaluation of the relationship between human violence and mating.
To consider the relationship between human male violence and mating behavior, several points need to be taken into consideration. Firstly, competition for mates is not always violent; it can be aggressive without resulting in physical violence, such as intimidation tactics (Carter and Kushnick 2018). Male tactics depend on female mate choice, in situations where females prefer males who demonstrate their status, wealth, and resources nonviolently – nonviolent tactics will be the prevailing mode of competition. Crime statistics often do not delineate between intra- and intersexual violence and aggression, making gauging accurate relationships difficult. In a recent review of crime statistics worldwide and their relationship to male-skewed sex ratios, results were mixed and dependent on society, type of violence measured, and rate of violence (Schacht et al. 2014). For example, studies of India and China found positive relationships between male-skewed sex ratios and homicide rates, violent and property crime rates (Oldenburg 1992; Dreze and Khera 2000; Edlund et al. 2007). However, studies considering US cities found negative or mixed relationships between male-biased sex ratios and homicide rates (Messner and Sampson 1991). This is further evidence that there is no simple relationship between sex ratios and violent crime (Schacht et al. 2014).
Intrasexual Violence and Aggression in Women
Women consistently use violence less than men do, with the exception of violence within intimate partner relationships (Campbell 1999). One possible reason for this disparity in intrasexual violence is that in humans it is the largely the female who expends energy towards enhancing their physical appearance. It is in this way females attract a mate and attempt to out-compete other females (Buss 2003). The trend toward monogamy or at least serial monogamy may have meant that men sought higher quality mates than under a polygynous mating system (Cross and Campbell 2011). Due to an expanding hominin brain, human babies became more altricial than previously seen in our ancestors; this meant human babies required more resources and energy to survive – this preceded the emergence of bi-parental care, monogamy, and co-operative breeding. Put simply, women required help to raise a child and therefore had to attract and compete as men do for mates – something that is quite rare in other species (Buss 2003; Cross and Campbell 2011).
While it is true that men’s reproductive output will always be limited by female’s reproductive capacity (Fisher’s Condition), resulting in a reduction in males potential reproductive capacity – evidence still shows that men, even in modern society, have greater reproductive variance than do women (Kokko and Jennions 2008; Jokela et al. 2010). This discrepancy is largely due to opportunistic polygynous mating in societies where serial monogamy is considered the norm. Many traditional societies today still sanction polygyny, (even if serial monogamy is more common) and the strength of intrasexual violence in men co-varies with the relative presence of polygyny, meaning that intrasexual aggression is higher in societies where polygyny is sanctioned (Carter and Kushnick 2018). It therefore follows that men have a collection of adaptations related to competing for mates, while women tend to have a suite of adaptations to attract a mate and for this reason may compete less violently with one another. Women tend to compete with each other by making further changes or enhancements to physical appearance using: makeup, nail polish, tanning, plastic surgery, bras, corsets, and clothing. Men also enhance their appearance but not at the same rate and prevalence as women do (Puts 2010; Cross and Campbell 2011).
When women do engage in intrasexual aggression, it usually results in less violent consequences than it does in men. For example, gossip and ostracization can stigmatize rivals without escalating to physical violence. These acts do however inflict stress on the rival and act to besmirch the competitor’s reputation – usually such gossip takes the form of belittling a competitor’s physical appearance or casting doubt on her sexual reputation. Young women are more likely than men to question a rival’s loyalty and draw attention to her infidelity (Milhausen and Herold 1999). Such tactics act to cloud paternity certainty and may give women who use them an advantage, especially if they do so while highlighting their own fidelity (Cross and Campbell 2011). These encounters can escalate to physical violence, when a female retaliates due to the besmirching of her sexual reputation. This escalation to violence is most likely to occur when resources are scarce, and women are impoverished (Cross and Campbell 2011).
When women do engage in violent intrasexual confrontation, it is usually less grievous than it is in men. As with males, intrasexual violence among women is most common in adolescence and young adulthood: US official crime statistics report 73% of attacks carried out by adolescent girls were on other girls (Cross and Campbell 2011). In a British study, 7% of women reported being attacked by another female in the preceding 5 years (George 1999). In both the UK and USA, female-female assaults occur most commonly between the ages of 15 to 24 year olds (Cross and Campbell 2011). The reasons for these attacks fall (either directly or indirectly) into these three categories: jealousy, retaliation for slander against sexual reputation, and competition for desirable partners (Ness 2004; Cross and Campbell 2011). Again, skewed sex ratios are implicated to explain rates of violent crimes. In African-American communities where the adult sex ratio is heavily skewed towards females, some argue that this ramps up competition between females for mates and thus results in more assaults among women (Cross and Campbell 2011). As presented in the previous section however, considerable evidence suggests that a female-biased sex ratio may result in higher male-male competition (Schacht et al. 2014; Carter and Kushnick 2018). Though the two are not mutually exclusive and it could be that in certain societies, female biased sex ratios result in increased intrasexual violence in both sexes, or other ecological conditions are at play.
Other such ecological factors are resource availability, and men’s resources seem to predict female intrasexual violence. In poorer communities, fighting for a well-resourced mate may be worth the risk, as the variance in income is much larger than it is in the middle classes. Where the differences in resources are negligible, such as between a Doctor and a Lawyer, the costs of engaging in violence may be greater. Overall, rates of intrasexual violence among women are much lower and less serious than that of men. Weapons are rarely used, and punching and kicking are the main modes of violence (Cross and Campbell 2011).
A re-evaluation of sexual selection theory in recent years has challenged some long-held assumptions, stemming further consideration of a multitude of factors – resulting in models that are more sophisticated (Kokko and Jennions 2008). Intrasexual violence and aggression reflected upon here make up only half of the sexual selection story, as intersexual violence or violence between mates occurs frequently (Cross and Campbell 2011). In addition, other adaptive and ecological problems may be solved by aggression that are still rooted in evolutionary theory but fall outside of sexual selection (Buss 2009). Social and cultural roles should be taken into account, but researchers generally agree – they are unlikely to explain the majority of sex differences in aggression – to assume so is committing human exceptionalism (Archer 2009). Intrasexual violence and aggression in humans, when contextualized with other taxa, can elucidate differences between males and females use of violence and aggression and can go some way to predict the patterning of violence and aggression generally.
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