Evolutionary and Life History Insights into Masculinity and Warfare
2020; University of Chicago Press; Volume: 62; Issue: S23 Linguagem: Inglês
10.1086/711688
ISSN1537-5382
Autores Tópico(s)Evolutionary Game Theory and Cooperation
ResumoPrevious articleNext article FreeEvolutionary and Life History Insights into Masculinity and Warfare Opportunities and LimitationsRichard G. BribiescasRichard G. BribiescasPDFPDF PLUSAbstractFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinked InRedditEmailQR Code SectionsMoreAbstractIn this paper I present, evaluate, and discuss evidence that evolutionary and life history theory can inform our understanding of violence and warfare as well as how these human challenges are influenced by the evolution of male life histories and our perceptions of masculinity. While other anthropological perspectives have interrogated the evolution of war from human and comparative contexts, few have addressed the influence of male life history variables such as age-specific mortality and reproductive constraints on the emergence and conduct of war. Other evolutionary factors worthy of consideration include sexual dimorphism, phenotypic plasticity, and paternity uncertainty. I initially present a contextual examination of the evolutionary science that underlies the engagement of human males with violence and warfare, casting particular attention on the opportunities and constraints of deploying evolutionary and life history theory. I later incorporate other variables including human variation, behavioral endocrinology, and comparative perspectives. In the latter half of this paper I suggest a novel perspective for understanding males, war, and violence by drawing on male-specific patterns of mortality, paternity uncertainty, and the evolution of male aging. While evolutionary and life history theory can provide an informative perspective to understand how males contribute to violence and warfare, there are limitations that set the stage for intellectual partnership opportunities with cultural anthropologists, psychologists, and other social scientists.Our species, Homo sapiens, is characterized by a number of traits that make us unique compared with most other great apes, primates, mammals, and vertebrates. Language, habitual bipedal locomotion, artistic endeavors, creativity, and social norms guided by moral principles, as well as advanced tool creation and use, set us apart from most other organisms (Fry 2013; Fuentes 2017). Another trait that is unique to humans is warfare, that is, the organized coalitional group effort to inflict harm or death on other conspecific groups. Chimpanzees and other organisms have been suggested to engage in warfare and lethal conspecific violence (Gomez et al. 2016). But within historical times, no other species has employed war on the scale, complexity, or lethality as humans (Glowacki, Wilson, and Wrangham 2017; Majolo 2019; Wrangham 1999; Wrangham and Peterson 1996). Indeed, modern human warfare threatens not only all people but the viability of all life on the entire planet (Ehrlich et al. 1983). The uniqueness of this human characteristic and the male-biased role in warfare therefore merits deep investigation and interrogation.Human males play a predominant role in the planning and participation of war, although females and others with nonbinary gender self-identification also partake in warfare but on a more limited scale compared with males (Adams 1983; Feinberg 1996; Tepe et al. 2016). The predominant role of men in warfare may simply be an extension of male privilege and power that is common across many modern societies independent of lifestyle and ecology (i.e., foragers, industrialized). That is, military power and influence can translate into greater access to resources or social capital (Glowacki and Wrangham 2013; Patton 2000). However, the observation of virtually exclusive male involvement in organized violence across human societies and ecologies as well as among chimpanzees suggests deeper evolutionary roots that extend earlier than the emergence of large and small human societies (Micheletti, Ruxton, and Gardner 2018). While evidence of evolutionary influences on the emergence and presence of human warfare is not without criticism, a cross-disciplinary examination of war is merited.The causes of the emergence of warfare are extremely complex and remain a matter of debate by evolutionary anthropologists, psychologists, sociologists, political scientists, historians, and philosophers. In this paper I suggest that evolutionary and life history theory can contribute to the understanding of, but not fully explain, the origins of warfare and violence and the reasons behind the predominant role of males. The following factors are suggested to amplify the likelihood of war as well as the central role of human males:• Age-specific increases in risk tolerance in younger men that are common in many human populations independent of ecology or lifestyle• Accumulation of social, economic, and political power that facilitates the role of older men as war planners• Greater intrinsic mortality that increases risk tolerance in older men• Paternity uncertainty that mitigates the cost of war for combatants and war planners• Technology and coalitional behavior in males that amplify these factors and promote war as a viable strategic group behaviorThese suggestions are not meant to replace other explanations based on social, cultural, economic, or political influences. Because of the necessity of a deep time perspective and changes in populations, evolutionary and life history theories often lack the precision to predict specific events or individual behavior. Instead these theories are meant to augment our understanding of war and the predominant role of males by leveraging the ability to generate testable hypotheses of behavioral change due to natural selection as well as the utility of comparative research on other species.Gender differences in behaviors are not uncommon in humans even when taking into consideration the broad spectrum of gender identities, social behaviors, and biology-independent cultural malleability across a myriad of societies. In many communities, for example, the sexual division of some labor is common although there is significant variability (Bliege Bird and Codding 2015). In communities that rely on foraging for wild food products, men tend to perform most of the hunting while women take on the task of procuring other resources. While human males are unique among great apes and mammals in general in that they can provide significant amounts of childcare (Gray and Anderson 2010), women tend to be the predominant caretakers of children. The causal factors for this allocation of activities are multifactorial, involving physiological, reproductive, sociocultural, economic, and political influences. Similarly, the sexual division of engagement in warfare is multifactorial.The preponderance of males in the planning and waging of war is another example of binary gender-based differences in societal behavior that can be rooted to some degree in evolutionary processes. Various hypotheses have highlighted male-specific reasons for the evolution of warfare. For example, some have offered that psychological differences between males and females drive a “warrior mentality” in men (Van Vugt 2009). Others have invoked coalitional behavior that promotes male reproductive success (van der Dennen 1995). The possibility that humans and chimpanzees share similar evolutionary roots that result in male-based organized conflict has also received significant attention (Wrangham and Peterson 1996). However, it is important to be aware not only of the utility of evolutionary theory but also its limitations.BackgroundEvolutionary Theory: Opportunities and LimitationsSince its formalization in 1859, the theory of evolution by natural selection has been deployed to explain the emergence and natural history of all organisms, including Homo sapiens. Evolution by natural selection provides a mechanism by which nonrandom changes in biological traits can occur in response to environmental influences. In addition to physical characteristics, behaviors can be examined through an evolutionary lens since they are often influenced by the physiology of neurological, endocrine, and genetic function. Most traits evolved as complex and malleable phenotypes that are under varying degrees of influence from genes, physiology, environment, and social variables.The link between evolutionary theory and war was not extensively highlighted by Darwin himself. Instead, he placed a greater emphasis on the interaction between biology and environment as well as the complex nature of humans themselves (Crook 1983). Similarly, it is vital to continue to recognize that the requisite conditions of evolution by natural selection must be taken into consideration when invoking an approach based on evolutionary biology.Natural selection, the primary mechanism of biological evolution, is the nonrandom elimination of individuals or genes due to environmental pressures (Darwin 1859; Dobzhansky 1977). The units of selection can be genes, individuals, or, in very specific cases, groups (Lewontin 1970; Maynard Smith 1998). Natural selection can only act on physical and/or genetic traits under the following conditions. First, the trait must exhibit variation. That is, there should be some range of differences on which selection can act. Second, the trait must be genetically heritable. The importance of genes was amplified with the discovery of “kin selection” and the insight that genes could be passed nonrandomly through genetically related individuals (Hamilton 1964). Third, differential reproductive success must be evident between variants. The importance of this third condition cannot be overstated since reproductive biology, behavior, and demography have proven to be vital to understanding contemporary human evolutionary biology (Bribiescas 2006b; Ellison 2001; Wood 1994). If a trait, including those relevant to males and warfare, is not subject to these conditions, then it is unlikely that an evolutionary perspective would be useful in understanding its emergence and history. Recognizing and being aware of these constraints is central for valid testing of hypotheses attempting to deploy evolutionary theory toward understanding warfare and the role of males.Natural selection is not the only mechanism for evolutionary change that can inform male-biased warfare. In contrast to nonrandom change (natural selection), random shifts in gene frequencies can occur as the result of catastrophic events or other influences that affect organisms in a stochastic manner. War is an example of a large-scale catastrophic event that can potentially alter genetic variation in a nonrandom fashion. Small populations experiencing such events or colonizing a new environment are especially susceptible to random changes in genes that result in the fixation of novel traits. This source of evolutionary change is known as genetic drift. The role of genetic drift in the evolution of war is unclear, although indiscriminate conflict can potentially be the source of random mortality that could influence gene frequencies, especially in males (Bigelow 1972). For example, the War of the Triple Alliance between Paraguay, Brazil, and Argentina (1864–1870) reportedly resulted in the death of 60% to 69% of the Paraguayan population, mostly men according to demographic sex ratios (Whigham and Potthast 1999; see also Blinn Reber 1988).Lifetime male mortality due to warfare in forager populations also varies widely, ranging from 5% to 30%. Among agricultural populations, mortality can be as high as 60% among the Waorani (Auca) of the Ecuadorian Amazon (Gat 1999:574–575; Robarchek and Robarchek 1992; Yost 1981; see also Glowacki and Wrangham 2013). It is reasonable to assume that given the extensive involvement of males in war, genetic drift and changes in sex ratios are worthy of consideration as a potential contributing factor to human evolution.Warfare is challenging to engage from an evolutionary perspective as it involves multiple individuals, and the evidence for natural selection with groups as a unit of selection is still very much open for debate (Zefferman and Mathew 2015). Dawson (1999) explores the potential role of group selection in warfare, concluding that the rates of extinction among human groups are too slow to make group selection a viable explanation. We are therefore left with individuals and genes as the most likely units of selection within the context of the evolution of male-biased warfare.Defining MasculinitySince combatants in warfare are largely male, it is important to define what it means to be male in a manner that makes evolutionary theory useful and informative. Masculinity has been defined in several ways by anthropologists. For example, Gutmann (1997) identified four ways in which anthropologists define and use the concept of masculinity as well as notions of male identify, manhood, manliness, and men’s roles.The first concept of masculinity holds that it is, by definition, anything that men think and do. The second is that masculinity is anything men think and do to be men. The third is that some men are inherently or by ascription considered “more manly” than other men. The final manner of approaching masculinity emphasizes the general and central importance of male-female relations, so that masculinity is considered anything that women are not. (Gutmann 1997:386)For other perspectives, see also Connell (2005), Gilmore (1990), Mead (1949), and Mosse (1996). These definitions of masculinity, while useful and important, do not sufficiently engage aspects of maleness that can be examined through an evolutionary lens. If masculinity is an aspect of being a man, what is a man? What is male? To deploy evolutionary and life history theory with effectiveness and validity, an expanded definition of masculinity and men is necessary, one that not only encompasses the important sociocultural perspectives of maleness but also (1) the influences of social and ecological environment, biology, and, most importantly, their interactions; (2) an awareness of the nonbinary physical and behavioral expression of maleness; and (3) an awareness of the spectrum of male identities that span the human condition now and in the past.To satisfy these conditions, I expand the definition of masculinity to encompass male gender identity variation that includes physical and behavioral traits that emerge from gene-environment interactions and satisfy the conditions of being subject to natural selection. Note that environment includes ecological and social influences.In many organisms, sex is determined genetically with a specific genotype. The resulting organizational sex phenotype is a fluid state that can be influenced by a myriad of factors depending on the species. Contributing factors to phenotypic variation can include temperature, social condition, and ecological variables. Some organisms are facultatively hermaphroditic; that is, they maintain the ability to change their sex phenotype during their lifetime. Indeed, it is quite common for organisms such as many fish to exhibit adaptive variation and plasticity in their sex phenotypes in response to social influences (de Mitcheson and Liu 2008).Given the broad range of sex phenotypes in nature and the evidence that phenotypic sex malleability is adaptive in many species, including vertebrates and many mammals, it is reasonable to hypothesize that human male phenotypes are subject to considerable variability. From a biological perspective, genetic definitions of being a human male are rooted in the sex genotype XY, in contrast to XX, which is female. However, different sex genotypes exist that contribute to variation in what one would consider to be male or female: XYY (extra Y chromosome) and genotypes that contain multiple X and Y chromosomes add to the variation observed in males, and while XO (only one sex chromosome) are observed, OY are not viable. These genotypes are, however, quite rare due to resulting developmental challenges (fig. 1; Visootsak and Graham 2006).Figure 1. Range of phenotypes, genotypes, and sources of variability. Genotypes with at least one Y chromosome are commonly defined as genetically male. Genotypes in gray are rare; OY genotypes are not viable.View Large ImageDownload PowerPointHormonal milieus that can further influence primary sex characteristics demonstrate the potential for phenotypic variation despite binary definitions based on XY and XX genotypes. That is, hormonal influences can and do influence the expression of sex genotypes, resulting in a spectrum of phenotypes. For example, a genetic condition known as 5-alpha reductase deficiency results in the under production of the hormone dihydrotestosterone (DHT) in utero. This results in an individual with an XY sex genotype being born with female external genitalia despite having internal male sex organs (i.e., testes) that are not apparent. With the onset of puberty around the age of 12 years, the testes begin to produce testosterone that transforms the person’s female external genitalia into a male phenotype. The clitoris enlarges into a penis, labia fuse, and secondary sexual characteristics such as beard growth emerge. Similarly, XY individuals with a condition that renders their androgen (testosterone) receptors nonfunctional manifest an external genital and secondary female phenotype. While both of these conditions are very rare, they do provide evidence of the malleability of phenotypes in spite of the common binary nature of sex chromosomes (Bale, Howard, and Wright 1992).This is not to say that the range of gender identities associated with masculinity are all due to hormonal or biological variation. Investigations attempting to link biological influences to variation in gender identity have met with compelling evidence of the role of genetics and hormones; however, many questions remain unanswered (Roselli 2018). As with many other complex traits, masculinity is heavily influenced by sociocultural factors. While this may induce frustration in some scientists, this is an opportunity for quantitative and qualitative research partnerships across disciplines.WarfareVon Clausewitz and Graham (1873) famously defined war as “the continuation of politics by other means.” Warfare is characterized by organized violence over physical, social, political, and economic resources as well as the decimation of other groups perceived as being a threat or inferior. In humans warfare is also unique compared with other organisms through the development and deployment of high efficiency weapons, that is, tools that optimize the killing or wounding of rivals with minimal effort or risk to one’s own group.Evidence of warfare and other forms of group violence in the archaeological record is widespread, especially in historical and prehistorical nation-states throughout the world (Estabrook 2014; Martin and Harrod 2015). The impact of organized combat and killing is seen in the skeletal remains and material culture of combatants as well as in written records (Kass 2007). Forager populations also engage in organized combat against rival groups as well as colonizers, primarily in the form of short, intense raids on neighboring communities (Gat 1999; Glowacki 2016), although there is variation in how violence is conducted between groups and ongoing debate as to whether these forager conflicts reflect warfare compared with violence between individuals (Fry and Soderberg 2013).The introduction of new weapons such as firearms changed the role of physical strength in warfare. Modern weapons such as bombs, missiles, aircraft, and submarines altered the nature and killing efficiency of warfare, facilitating greater involvement of women in conflicts. While women in combat are limited within the US military, many other nations openly incorporate women into combat roles (Carreiras 2006). Other more low-intensity conflicts commonly involve women. This has been the case in guerrilla wars in which a smaller force uses raiding tactics to engage a larger and more technologically advanced opposition (Reif 1986).War also often necessitates the social, economic, and political support of the population and leadership. The run-up and maintenance of war is often accompanied by nationalistic fervor, leadership planning, and actual fighting. Those who experience war firsthand can be divided into two groups, combatants and noncombatants. While mortality and morbidity of noncombatants includes males and females of all age groups, combatants, including those who engage in direct combat and those who plan war, are mostly males, although there is some variation depending on the type of conflict (GBD Mortality 2015 and Causes of Death Collaborators 2016). Combatants can also be subdivided into the attackers and those that are attacked. The predominant role of males in direct combat and war planning is a generalization that appears to be common across human societies. Older men tend to engage in war planning, especially in nation-state societies. Data on the role of older men in forager societies is sparse, although ethnographic records suggest that the influence of older men on community matters can vary depending on the degree of acculturation and economic integration with the dominant (colonial) society. For example, among the Ache of Paraguay,social changes have accompanied these slow economic changes. The political structure has shifted such that very young men now wield the greatest influence in the community. Since the late 1980s there is also a pattern of increasing political dominance by Ache who lived for some time with Paraguayans as children. These Ache, called berupuare or “those who come from the Paraguayans,” are generally more educated, politically astute, and aggressive than the Ache who never lived with Paraguayans. They are also more likely to form alliances with missionaries, Indian rights workers, government officials, and others in order to leverage their way into power at the current reservations. Older men still lead most forest treks and are listened to respectfully by the young leaders in community discussions, but they very rarely participate in formal gatherings. Instead, the older people wield their influence behind the scenes through grumbling and complaining if they are unhappy about decisions reached by younger leaders. Three elected chiefs represent the community legally and settle all disputes both within the community and between most community members and outsiders. (Hill and Hurtado 1996:74–75)It is also tragically common for child soldiers to be conscripted, often involuntarily, and to be directly involved in combat (Song and de Jong 2015). Aside from these exceptions, not only are combatants predominantly male but they also tend to be young adults. The nonrandom nature of combatants raises the question of whether there are evolutionary influences on warfare and the preponderance of male involvement.Are men more prone to engage in warfare compared with women?Evidence of the evolutionary influence on the preponderance of conflict and violence in males compared with females in many organisms, including humans, includes some forms of sexual dimorphism, that is, differences in phenotypes between males and females that are likely the result of selection for traits that embolden and equip males for physical conflict or intimidation. Many organisms have evolved physical characteristics such as antlers, horns, and other physical traits for combat. In humans and many other mammals, sexual dimorphism is evident in body size and composition. Men on average have about 15% more body mass compared with women (Wells 2012), as well as greater skeletal muscle mass, especially in the arms, chest, and back. Sexually dimorphic muscle tissue is under the influence and management of anabolic hormones, especially testosterone, emerging mostly at sexual maturation (Garcia-Mayor et al. 1997; Tanner 1978).An indication of the influence of natural selection on human sexual dimorphism is evident in the metabolic cost of sexually dimorphic muscle. Roughly speaking, the metabolic cost of excess anabolic tissue compared with females is comparable to the metabolic cost of reproduction in women when totaled over a lifespan. This is indicative of sexually dimorphic tissue in men being reflective of reproductive effort and selection for physical strength despite the associated metabolic costs (Bribiescas 1996, 2001; Ellison and Bribiescas 2018). For an expanded perspective on the evolution of sexual dimorphism, see Dunsworth (2020).The deployment of sexually dimorphic body mass is still evident today in many cultures with physical contact sports as well as ritual combat. While women also engage in physical sports and ritual combat, the segregation of men and women in these activities is often based on the overall differences in strength (Walker and Hill 2003). Evolutionary changes in the morphology of the male shoulder have facilitated the ability to throw objects that may have been leveraged during early conflicts (Roach et al. 2013).It is therefore reasonable to hypothesize that human males have been selected to engage in activities that require specific needs of strength compared with women. Whether this is still the case in contemporary societies remains a matter of debate. However, this does not automatically mean that these strength activities and associated anabolic hormones are always linked to violence or the desire to inflict harm or death (Trumble et al. 2013), nor does this provide definitive evidence that sexual dimorphism coevolved with warfare. Sexual dimorphism is evident in many species without warfare but who exhibit significant male-male competition. Sexual dimorphism is suggestive of selection for conflict but not necessarily for group aggression or war.Comparative approaches to warfareEarly suggestions that warfare was conducted by our hominid ancestors were made by Raymond Dart (1953) and Robert Ardrey (1961). These assertions were based on assumptions derived from the fossil record and modern characteristics of humans. It was not until the 1970s that evidence of warfare emerged from other great apes, particularly chimpanzees (Pan troglodytes). Intergroup conflict and violence in chimpanzees is well documented and has been shown to be one of the distinguishing social features compared with their close evolutionary relative, the bonobo (Pan paniscus). The most notable example of warfare in chimpanzees is the decimation of the Kahama group observed by Jane Goodall and her research team at the Gombe field site in Tanzania. Between 1974 and 1978, consistent group conflict existed between the Kasakela and Kahama groups, resulting in numerous male deaths and a few females. The Kasakela group eventually defeated the Kahama individuals and took over their territory (Goodall 1986).It has been argued that our species shares the proclivity toward warfare due to evolutionary similarities with chimpanzees (Wrangham and Peterson 1996). As with humans, male chimpanzees are much more likely than females to engage in attacking males from rival groups, although females have been observed to accompany males on what have been defined as patrols. There is considerable demographic variability in chimpanzees such as community size, sex ratios, encroachment by humans, ecological differences, and habitat quality that are likely to affect intergroup conflict. However, a consistent characteristic of chimpanzee intergroup conflict is the predominant role of males (Mitani 2006). Wrangham (2018) has also suggested that human group conflict and aggression is more proactive, while chimpanzees are more reactive. While human influences have been suggested to contribute to chimpanzee group conflict (Ferguson 2011), evidence supports the influence of species-specific selection for violence that can be interpreted as war (Wilson et al. 2014; Wrangham and Peterson 1996).The conclusion that chimpanzee warfare is an innate trait resulting from natural selection has not gone unquestioned. Ferguson (2011) has argued that food competition linked to human impact is the more salient cause of coalitional aggression and violence in chimpanzees. Power (1991) has also suggested that food provisioning is a major contributor to organized violence in chimpanzee communities. However, this view is challenged by long-term field data that document significant organizational aggression, violence, and territorial expansion in a large community of wild chimpanzees that have never been provisioned (Mitani, Watts, and Amsler 2010), but Mitani, Watts, and Amsler (2010) caution that inferences from chimpanzee research into the evolution of warfare in humans is “moot” because of the heterogeneity of human warfare compared with chimpanzees.Evolutionary, life history, and comparative perspectives hold the promise of providing insights into the evolution of warfare. However, incorporating these perspectives with discussions of masculinity and male-specific life history traits has largely gone unexplored. A specific focus on male life history traits and how they might contribute to the evolution of warfare merits discussion and holds promise for new directions and a greater understanding.Do men who engage in warfare have higher reproductive success?If warfare by males is influenced by natural selection, there should be nonrandom variation in fitness or traits that affect reproductive success between males who engage in warfare and those that do not. Reproductive success in mammalian males is generally constrained by access to mates and mating opportunities compared with females, whose fitness is more limited by energetic resources devoted to pregnancy and lactation (Bribiescas 2001; Ellison 2003; Hirshfield and Tinkle 1975). Females can also benefit from greater access to
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