Decline of Violence
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A great deal of evidence indicates that human society has experienced a slow and steady decline in both interpersonal and intergroup violence as cooperation, empathy, and peaceful resolution of conflict have become more dominant.
The emergence of Homo sapiens sapiens was a watershed moment in the natural history of the planet. Highly mobile populations of modern humans quickly replaced all of the other contemporary hominins they encountered, probably through a combination of violent and nonviolent means, and dramatically reshaped every environment they entered. However, human social groups are marked by extensive intraspecific cooperation, empathy, and nonviolent resolution of conflict, at least compared to our closest relatives, the other extant apes. There are several theories that have been developed to explain the decline of violence in the human species that are not mutually exclusive; in fact, synergy among them is likely.
Intelligence, Communication, and Language
Intelligence and sophisticated communication are necessary preconditions for prosocial cooperation. While true referential communication – nouns and adjectives – has been observed in several nonprimate mammal species (and may also exist in birds), complex social communication is a hallmark of primates (Slobodchikoff 2012). The apes have taken this further to include body language and gestural communication that exhibits personal variation and regional dialects (Byrne et al. 2017). A phylogenetic analysis has revealed that, in primates, the size of the vocal repertoire is closely associated with both group size and time spent in close social contact (McComb and Semple 2005). The evolution of intricate sociality and sophisticated communication seems to be linked, perhaps inextricably.
Esther Herrmann and colleagues take this idea further and propose the cultural intelligence hypothesis, which holds that the key difference in intellectual capacities between humans and other animals is not in general intelligence, but in specific cognitive abilities related to our sociality (Herrmann et al. 2007). This includes our social intelligence, the ways in which we communicate, understand, and interact with our fellow humans, but also our meta-social intelligence, that is, the ways in which we form hierarchical subgroups, communities within communities. The activation and ecological shaping of innate abilities is of key concern to this question because primate behavior is not strictly programmed, but rather extensively learned, and socially so.
In 1993, Frans de Waal and Denise Johanowicz performed a pioneering cross-fostering experiment that demonstrated the connection between prosociality and learning. They took juvenile rhesus macaques, which are fairly aggressive and not prone to displays of apparent social reconciliation, and allowed them to be raised with stump-tail macaques for 5 months, a species known for peaceful conflict resolution. The results were dramatic. In addition to acquiring various food preferences and strategies, etc., the cross-fostered rhesus macaques also picked up the behaviors described as “peaceful reconciliation.” These habits persisted long after they were returned to their original troop of conspecifics. This study demonstrates that primate behaviors are not purely genetically encoded but take shape through learning within a social milieu. That the reduced aggressiveness persisted even after they lost other learned habits hints that sociality may involve non-genetic (e.g., hormonal) mechanisms of self-reinforcement. In sum, elaborate communication and social collaboration is a driving force of intelligence across diverse clades of birds and mammals and culminated in apes and hominins.
Competition Drives Cooperation
It has been known for some time that, among primates and possibly other mammals, there is a rough correlation between group size and brain volume and this has led to the “social intelligence” theory of primate intelligence (Reader and Laland 2002). However, Byrne and Whiten (1988) emphasize that social intelligence is not always prosocial and may just as frequently be “Machiavellian” in nature. Importantly, these two notions are not necessarily in tension (Stewart et al. 2016). In fact, prosocial behaviors and manipulative behaviors involve many of the same cognitive abilities related to the interpretation of behaviors and mental states, understanding the past in order to predict the future, etc. Whether we strive to successfully work with or against someone, the necessary interpersonal skills overlap considerably.
Gavrilets (2015) has modeled the role of collaborative and competitive skills in the evolution of the human brain and found that the two are not just overlapping, but reinforcing. Conflict between groups sets up strong selective pressure for the emergence of within-group cooperation leading to an evolutionary arms race of “cooperative competition.” Groups of hominins competing against one another led to groups of humans cooperating with one another.
Cultural Group Selection
Although cooperation is a successful strategy for survival, flourishing, and biological fitness, it can nevertheless be purely self-directed in nature. Human cooperation goes beyond this into true prosociality in which individuals act for the benefit of others without regard for the immediate benefit to oneself. The extensive prosociality of humans, not to mention human altruism, has been a conundrum for evolutionary science since Darwin first proposed natural selection as the primary mechanism of adaptation. The problem of deception, cheating, and free-loading often overtakes cooperation in attempts to model the evolution of cooperation in animal species. Leticia Avilés (2002) has proposed that freeloaders can theoretically be kept rare by frequency-dependent group-level selection. This selection process may similarly function to restrain the proliferation narcissistic sociopaths (Lents and Trivers 2018). Christopher Boehm (2009) and others have argued that collective action to enforce rules and punish cheaters has resulted in the evolution of an egalitarian social structure among human societies dating back to at least the Paleolithic.
Nevertheless, however much between-group competition drives within-group cooperation, the emergence of collaboration, collective action, and altruism is difficult to model without the invocation of group-level selective forces. Although first fully articulated by the political and economic theorist Friedrich Hayek (Henrich 2004), a phenomenon called Cultural Group Selection (CGS) has its root in The Descent of Man (Darwin 1871). Darwin noted that the biological differences between groups of humans are negligible compared to the differences in cultural practices and therefore latter are far more determinant of success when the groups clash. Because this phenomenon does not appear true in other species, human may have evolved in a unique selection environment, rendering obsolete selection models based on data from other animals. Not surprisingly, CGS has met with strong criticism (West et al. 2011).
Recently, Richerson et al. (2016) have mounted a vigorous defense of CGS as a key selective force in the emergence of human cooperation. This defense is predicated on the assertion that cultural traits, i.e., memes, can evolve in a Darwinian fashion (Dawkins 1976). To function in this way, CGS requires that a species meet the following criteria: (1) possesses high capacity for social learning, (2) exhibits conformist behavioral tendencies, (3) gains strong selective advantage from cooperation, (4) engages effective punishment of social deviants, (5) reputation (prestige) leads to reproductive success, (6) groups marked by symbolic, non-biologic differences, and (7) has established “institutions” through which collective action can bring group benefits. Interestingly, CGS relies on the actions of gene-level elements to facilitate group-level selection, a point underscored by Richard Dawkins’ coinage of the term cultural meme within a larger defense of the gene-centric view of natural selection (Dawkins 1976). It is worth noting that most scholars of human cooperation agree with many of the individual tenants of CGS, if not the entirety of the theory (Singh et al. 2016; and for a comprehensive discussion of some the counter-arguments to CGS, see West et al. 2011).
Self-Domestication: Survival of the Friendliest
An even more specific theoretical framework for the emergence of highly cooperative and nonviolent social structures in humans is the notion of self-domestication. This theory holds that the behavioral changes that occurred in animals as they were domesticated, such as increased docility, are similar to the “civilization” of human beings over the last 200–500 ka. Although this possibility was obliquely mentioned by Darwin (1871), it was first fully articulated by Franz Boas (1901). In the time since then, the possibility has been raised that animal domestication may have been less something that humans intentionally did to other species, and more something that those animals did to themselves by pursuing the unique ecological niche of mutualistic cohabitation with humans. The very specific selective pressure of that niche resulted in what could fairly be called self-domestication (Zeder 2006, 2012).
Some of the features of domestication that cut across many species are reduced external ears, snouts, teeth, and brain size; curly tails, floppy ears, and skin depigmentation; as well as the all-important behavioral changes of increased docility, feminization, and neotany. That such specific features appeared in such divergently domesticated species argues for a common mechanism and has been called domestication syndrome (Wilkins et al. 2014). The search for this common cause has implicated deficits in neural crest cells during embryonic development and a host of candidate genes (Sánchez-Villagra et al. 2016).
Central to the theory of self-domestication is the notion that reduced aggression brought gains in biological fitness. Despite the nuance brought by the theory of cultural group selection, fitness, inclusive or otherwise, must be the output for adaptive natural selection. Whether mammals cuddling for warmth or hominins working together to hunt megafauna, personal benefit is key to the decline of aggression. Inspired by the theory of endosymbiosis that made her famous, Lynn Margulis remarked that “Life did not take over the globe by combat, but by networking” (Margulis and Sagan 2000). When applied to the emergence of modern human behavior, this idea is frequently expressed as “survival of the friendliest,” a term that, tellingly, grew out of research into intergroup contact theory (Pettigrew 1998). In the past 10 years, substantial progress has been made in fields as diverse as developmental biology, comparative behavior, neuroscience, paleoanthropology, cognitive psychology, and of course evolutionary psychology, toward supporting the notion that reduced aggression, enhanced cooperation, and in-group prosocial behaviors were naturally selected features in the emergence of behavioral modernity (Hare 2017).
Aggression and Testosterone
The elephant in the room with the survival of the friendliest paradigm is the reality that human aggression and violence almost always aligns well with adaptive benefits (Buss and Shackelford 1997; Shackelford and Weekes-Shackelford 2012). However, the realization that aggression is actually comprised of two very different behavioral modes, proactive aggression and reactive aggression (Poulin and Boivin 2000), may allow a breakthrough of this apparent paradox. Wrangham (2018) has recently argued that these two modes of aggression may have been differentially shaped by natural selection over the last 200–500 ka. While proactive aggression (sometimes called predatory attack) refers to the willful, planned attack focused on obtaining or achieving a goal with clear survival benefits, reactive aggression (sometimes called affective violence) is a response to an immediately threatening stimulus and is directed only at escaping or neutralizing the threat. Under this model, the recent natural history of our species has seen a decline of reactive aggression, not proactive aggression. This can be expressed as gains in self-control, peaceful conflict resolution, etc. Intriguingly, calculated proactive aggression may have even contributed to the decline of reactive aggression through capital punishment. As Boehm (2000) has argued, the majority of the victims of capital punishment in traditional societies are those who fail to control their reactive aggression.
At the same time, there may also have been a turn away from direct physical violence toward subtler forms of social manipulation collectively referred to as indirect aggression (Ingram 2014). While it remains to be examined if the switch to indirect forms of aggression is more of a cause or an effect of the decline of violence, the phenomena of reputation, status, gossip, and coercion enact some of the same social functions in humans as aggression does in other species. In summary, it seems plausible, then, that a combination of domestication syndrome and cultural group selection acted to reduce reactive aggression and promote self-control in humans, while cognitive advances allowed more sophisticated forms of proactive aggression, which can either promote or restrain violence, depending on context.
While a great deal of solid theoretical work has been done on the decline of violence, most is focused on the ultimate adaptive gains, with comparatively little progress on the proximate mechanisms (the exception being recent work on the genetics of domestication syndrome in humans, see Theofanopoulou 2017 and references therein). However, new insight into the mechanism of cooperative prosociality has emerged from the study of behavioral endocrinology. Specifically, average levels of circulating testosterone have been linked to aggression in several mammal species including the African apes (Book et al. 2001). There is evidence that circulating testosterone has steadily deceased in modern humans over the last 100 ka, and possibly extending back 1–2 Ma (Cieri et al. 2014). This evidence comes from the study of emerging skull features that have been linked to lower testosterone including the reduced prominence of the brow and jaw, smaller teeth and eye sockets, and rounding of the face. While it is not currently possible to measure testosterone in fossilized remains, the evidence for “craniofacial feminization” in our recent ancestry is compelling.
The careful analysis of Wardecker and colleagues (2015) adds important nuance to the role of testosterone and estrogens. For example, “in species where social dominance is integral to mating success, testosterone is very tightly linked to mating effort; in species where social cooperation is essential for mating success, testosterone is more loosely linked to mating effort.” (Wardecker et al. 2015). In addition, while testosterone is linked to dominance toward sexual rivals in human males, it is also associated with affiliation and cooperation with males that are not sexual rivals. Testosterone also seems to play a different role in short- versus long-term mating, although, as noted by Wardecker et al., the causal arrow could also point the other direction. Hormones don’t just influence social behaviors, they are influenced by social behavior.
While a great deal of work remains to parse out the role of the respective ultimate and proximate causal mechanisms, the social and cultural milieu of the last 200 ka of human history has been marked by increasing empathy, prosociality, altruism, and in-group cooperation. While violence has always been a part of our culture, it has been in steady decline as our species transitioned to so-called behavioral modernity, a transition that continues to the present (Pinker 2011). Understanding the social, cultural, genetic, and hormonal roots of this decline offers great promise for continued progress on the pacification and long-term success of our species.
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