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  • Writer's pictureArnold Schroder

The Biology of the Left-Right Divide

Updated: May 28, 2023

Abstract art and oppression

In his short story “The Approach to Al-Mu'tasim”, Borges describes a protagonist who participates in a Hindu-Muslim riot, killing someone, only to reflect that he is not certain that god exists, but he is capable of killing someone for conceiving of god differently than he does. This capacity for self-reflection about the nature of one's beliefs justifies his role as protagonist—it is extremely uncommon for us to consider, rather than what we believe and how it differs from the (obviously insane) beliefs of others, why it is that such a radical diversity of perceptions exist in the first place.

In his book Conscilience: The Unity of Knowledge, Edward O. Wilson contrasts the chaotic heterogeneity of findings in social science with the relative cohesion of findings in physical science (with the greatest testament to our confusion being the very fact that we need to arbitrarily segregate the social and physical into non-continuous domains in the first place). He states that the greater attachment we have to the outcome of an inquiry, the less able we are to conduct it accurately. Right-left politics would seem like a particularly strong justification of this statement. There seems to be an inverse proportionality between the strength of the emotional responses this form of division engenders and the presence of clear, consensus definitions of the very terms left-right, or understandings of the nature of the division to which they refer. This is as true in academia as anywhere else (What Is Politics? 2020).

In fact, nothing is so characteristic of a political disagreement as a lack of shared vocabulary, and for as much as “arguing over semantics” is a common way to say a discussion is trivial, a moment's scrutiny over political disputes would seem to indicate that they are almost always arguments over what words mean. While I know of no research that has done so, it seems fairly clear we could predict a great deal about one's politics from how one defines various terms. For right-wingers, for instance, left almost invariably has associations with monolithic, oppressive, centralized authority—with, for instance, Stalin. For leftists, likewise, right-wing politics are conceived of as the politics of coercion and despotism—the politics, for instance, of Hitler. (Interestingly, this points to a common set of values somewhere below the surface; we will see in a later chapter how narratives for freedom and against alienation are ultimately consistent across lines of political division, even if the conditions that produce freedom and alienation are conceived of differently).

We can extend our argument beyond the definitions of individual terms to their immediate associations, to differences in the frames words invoke, which George Lakoff (1994) has demonstrated is so characteristic of left-right political division. For instance, a debate about the so-called criminal justice system in the US is for some a debate about the fact that it is an apparatus of mass coercion without historical precedent, while for others it is a debate about the degree of safety one has from violent acts committed by strangers.

For years, I began my days with the sequence of Fox News, CNN, and Democracy Now! The most conspicuous frame shift, as one progresses along this spectrum from right to liberal to left, is the declining salience of random acts of interpersonal violence. Democracy Now! features plenty of violence—it is actually far more focused on human suffering than Fox—but it is all of the chronic, systemic variety. CNN is truly intermediate—a particularly notorious instance of, say, a blonde lady going missing on vacation will receive coverage, but they do not report on rapes and murders and muggings with anything like the obsessive emphasis of Fox.

In other words, it would seem clear that at least some of what political division is about is fear, and how much it informs one's worldview—and maybe not just fear of anything frightening, but of a small range of stereotypical threat stimuli with strong evolutionary salience. For instance, the litany of warnings from scientists that we are rapidly approaching, or have perhaps already crossed, thresholds beyond which the planet becomes uninhabitable (e.g. Ripple et al. 2017) would never be featured in right-wing media, and rarely in liberal media, as something to take seriously and be frightened of.

If we ask why people find these evolutionarily ancient forms of fear more or less motivating, we will find that it is a product of their objective external conditions, their cultural conditions (i.e. the narratives to which they are exposed about objective conditions), and their innate temperaments. Moreover, these factors can form feedback loops: if one is born with a greater disposition to orient to stereotypical threat stimuli, and one watches Fox News, it will only amplify the salience of such stimuli, and if this process goes on for awhile, it creates a society in which there is legitimately more to be afraid of.

That left-right politics must be about deep temperamental differences is clear from the incredible durability of this form of division across radically different contexts. To select a few items at random from a potentially very long list, the early church had debates about the nature of christianity which have a conspicuous left-right characteristic (Pagels 1995), as did conflicts over the subjugation and extermination of indigenous people among early colonialist settlers in the US (Gwynne 2011), as did conflicts over the cult of Dionysus, and the social upheaval it implied, in ancient Greece (Otto 1965). The terminology itself is commonly asserted to be derived from the 1791 French legislative assembly, although it “appears to be much older” (Hibbing 2014), but somehow it is still clearly a useful taxonomy for classifying political conflicts, from legislative disputes to street fights, anywhere from Hungary to Canada to Chile.

Between 1981 and 2008, researchers conducting a massive inquiry called the World Values Survey asked nearly 300,000 people in 97 countries if they could place themselves on a ten-point left-right political spectrum. About 20% of respondents didn't answer or said they didn't know (this occurred more in more authoritarian countries, giving the impression at least some of the reticence was based on fear of political repression), but the fact that ~80% of respondents could identify their politics in terms of left-right indicates that these terms have meaning in a wide range of conditions. The distribution of responses to this cross-national survey of political orientation from a normal distribution, or a bell curve (Tuschman 2013). This is noteworthy as none of the variables which are considered salient to political behavior in most media discourse—such as income, ethnicity, and gender—form normal distributions. However, a large number of heritable traits, like height and IQ and blood pressure, do.

There are two ways to try to make sense of this. One is to examine the traits that correlate with political difference and try to find overarching themes, the core narratives and perceptual biases which political orientation manifests. This is the approach of political psychology. The other is to ask, humans being a species of animal whose behavior has conspicuous resemblances to that of other animals, whether we can see similar differences in other species. Such differences have been extensively described in those studying the biology of aggression, tolerance, social cognition, behavioral plasticity, and domestication, which are regulated by common developmental mechanisms across a wide range of taxa. These results, however, have not been extensively compared with the findings of political psychologists. Our purpose is to examine these two somewhat disjunct areas of scientific inquiry to describe the biology underlying the psychological trait variation which produces political difference: the biology of the left-right divide.

We will start with the work of the political psychologists, and see how they have produced a fascinating, complex landscape of results that seem to require integration into a unitary theoretical framework. Because of the aforementioned lack of clear, consensus definitions of our most basic political terminology, those who study left-right political division have found themselves in the position of first needing to define it. A major meta-analysis called “Conservativism as socially motivated cognition”, assessing correlations between psychological traits and political orientation in 88 studies from 12 countries, concludes that the meaning of left-right political division is twofold, as described in the previous chapter: variation in perceptions of hierarchy vs. equality, and variation in perceptions of tradition vs. change (Jost et al. 2003).

This paper also identified nine psychological corollaries of political perception, although it must be acknowledged that some of these seem to overlap or even be a rephrasing of one another. Those that positively correlated with right-wing political perception were fear of death; perceptions of system instability; fear of threat and loss; needs for order, structure, and closure; and dogmatism-intolerance of ambiguity. Those that positively correlated with left-wing political perception were openness to new experience; uncertainty tolerance; integrative complexity (a measure of the complexity of written and spoken language); and self-esteem.

As I said, a number of these seem to be different ways to conceptualize what we might call, if we were biologists, behavioral plasticity—for instance, the need for order, structure, and closure seems like in some ways it might be simply considered the logical inverse of openness, and openness and uncertainty tolerance seem clearly related. If we make the fairly reasonable conclusion that aversion to the unknown might be born out of fear, variables like fear of death (the strongest correlation they found), may ultimately reflect the same underlying explanations: again, it seems like a whole lot of political difference reduces to differences in fearfulness.

But many questions remain. Why is hierarchy the natural orientation of the fearful? This seems far from a logical inevitability. While some (e.g. Jost et al. 2003; 2007) have posited that support for hierarchy is a logical expression of discomfort with uncertainty and general risk aversion because traditional arrangements have been hierarchical, historical right-wing phenomena like Nazism seem to have embraced an awful lot of risk and upheaval, and contemporary right-wing politics is likewise characterized by what looks an awful lot, from some perspectives, like a wanton love of danger.

We could perhaps say that hierarchies are actually pretty intrinsically risky: even setting aside the very fundamental matter of the right's disregard of the ecological crisis, concentrating wealth in power in the hands of a few and making everyone else desperate and hungry and full of rage is hardly the only plausible expression of an overarching concern with stability and safety. This is pretty intuitively obvious, but the fact that inequality and instability are statistically related is also well studied (e.g. Turchin 2010).

So perhaps the most fundamental question that emerges from these characterizations is why perceptions of risk and novelty and perceptions of hierarchy covary in the way they do, but there are plenty of others.

What does the complexity of one's language have to do with one's fearfulness? And how do these themes of hierarchy and fearfulness and tradition relate to the warlike nature of the right? In the American context, one could make the argument that the invasions of, say, Vietnam and Iraq were about imposing hierarchies, but this would be one case of the phenomena mentioned earlier, that the acceptance or rejection of a frame is highly predictive of one's politics. From the perspective of those who supported either of those wars, this frame would be rejected: such people were often responding to a sense of threat, so we can invoke fearfulness, but of a very specific kind. After all, invading other countries could pretty aptly be described as extremely high-risk behavior—setting aside the harm done to the invaded, this behavior has inflicted grievous wounds in the psyches of generations of soldiers, and engendered an enduring hostility toward the US in much of the world. Rather than as a subordinate clause of threat perceptions or support for hierarchy, it seems like perceptions of war are best conceived of on their own terms, in terms of the evolutionarily inherited psychology of intergroup aggression, with its stereotyped threat perceptions and associated behavioral responses.

Other left-right differences which don't really clearly fall into the framework of hierarchy vs. equality and tradition vs. social experimentation could be noted. Another perennial and readily identifiable one, which increases the lack of cohesive, overarching statements we can make about the major themes of political division, concerns sexual behavior: left-right conflict is somehow about whether one supports the boss or the union, but also about dropping bombs all over the world, but also about the number of sexual partners, and the range of pleasures to pursue with them, that should be considered acceptable.

Some would argue divided perceptions of sexuality is a permutation of openness vs. tradition, but left-right political differences can only be described as reflecting variation in traditionalism when the cultural development is actually somewhat new. One of the earliest political psychometrics with significant predictive power, the C-Scale, asks a number of questions which (rather hilariously) reflect its early 1970s provenance: respondents are asked to react positively or negatively to items like “pajama party,” “astrology,” and “computer music” (Wilson 1973). This seems clearly to relate to preference for tradition vs. social experimentation because things like astrology and computer music were actual novelties in the cultural context at the time. But conflicts over sexuality have been raging for millennia along very recurrent lines, and at a certain point (noting the right-wing didn't have trouble adapting to, say, the internet), we must conclude that it is not the novelty of sexual behavior per se, but sexual behavior itself that is the nexus of divided perception.

So it is clear we are not going to get out of this by merely finding some common logical architecture to every aspect of left-right political division, by finding the coded message or the hidden theme. And even if we did, it is not clear how much we would have actually explained. Even if the differences were of a unitary nature, we would still want to know why these differences exist, to try to understand what they ultimately mean. As another major review which does posit a unitary theme, that of negativity bias, phrases it:

Of course, when we move the explanatory locus back a step from survey self-reports to deeper physiological and psychological forces, the issue immediately becomes the source of variations in these physiological and psychological traits. In other words, if negativity bias leads to the adoption of certain personality traits, basic values, moral foundations, and bedrock political principles, what causes variation in negativity bias in the first place?(Hibbing 2014)

Like so many social and ecological trends, scientific publishing is on an exponential curve, and it is truly remarkable to see the radical change in the state of the literature between these two reviews, the 2003 “Conservativism as socially motivated cognition” and the 2014 “Differences in negativity bias underly variation in political ideology”. The former exclusively reflects questionnaire results and analysis of speech and text, while the latter describes results from experimental games, imaging of brain activity and structure, measures of physiology, and cognitive experiments.

Interesting evidence for the theme of negativity bias in political difference comes from research into the change in electrical conductivity detected by sensors on the skin in response to various stimuli, a common measure of general physiological arousal. A small handful of studies have established that self-reported right-wing political orientation correlates with a stronger reaction to threatening stimuli, such as frightening pictures or sudden loud noises (e.g. Oxley et al. 2008). One, however, also examined physiological arousal in response to averse images in general, as opposed to only those that are threatening—in addition to pictures of things like spiders there were also pictures of things like maggots, intended to promote disgust rather than fear—as well as presenting a number of generally positive images, of rabbits and happy children and bowls of fruit. The researchers then divided study participants into everyone left of their sample mean and everyone right of it, and plotted these groups as two separate lines on the same graph, showing physiological arousal in response to either averse or positive imagery. The lines have opposite slopes: responsiveness among those right-of-center is higher for averse imagery and declines with positive imagery; for those left-of-center, the opposite is true (Dodd et al. 2012)

Such results are somewhat chilling—if a faction involved in political conflict ultimately just sort of thinks the world is a bad place, it is hard to imagine how any permutation of their politics could ever result in anything other than nightmares. And if one thinks about it for a moment, statements to the effect that one must simply be realistic about the intrinsic cruelty of the world, and our intrinsic disconnection from others within it, are an awfully common form of right-wing rhetoric—if one reads Ayn Rand, for instance, it is not so much an argument for an optimal form of society as it is a polemic against feelings of connection with the world and others. Or, as the authors of the paper documenting these differential responses to positive and negative stimuli might phrase it, Ayn Rand's work, and right-wing politics in general, could be plausibly described as the political manifestation of avoidance, as opposed to approach, behavior:

… in addition to recognizing the uniqueness of individual categories of response, many researchers also have found useful a 'biphasic' model which holds that emotion is a product of varying activation of two motivational systems: appetitive and defensive. Both of these systems 'are evolutionarily old, shared across mammalian species and have evolved to mediate the behaviours that sustain and protect life'. Gray has described these two core systems as the behavioural inhibition system (BIS) and the behavioural activation system (BAS), with the BIS typically activated by aversive stimuli and the BAS activated by appetitive stimuli … (Dodd et al. 2012)

Brain imaging provides other lines of converging evidence on the themes of political division. Left-leaning participants in experimental games have shown greater activation of the anterior cingulate cortex (ACC), which monitors for conflict between the environment and habitual behavior, thus promoting novel behaviors (Amodio et al, 2007), as well as greater activation of the right amygdala, central to fear and aggression, in response to risk-taking activities in games (Schreiber et al. 2013). Another study found right-wing participants in an experimental game took less risk and explored less novel stimuli, employing more stable, cautious strategies (Shook and Fazio 2009). These brain activity and game strategic differences clearly relate to the theme of tradition vs. change.

Not only is the activity of these brain regions correlated with political outlook, but so is their size: the right amygdala, as well as insula, are larger in those with more right-wing politics, whereas the ACC of those with more left-wing politics is larger (Kanai et al. 2012). The amply sized and particularly active amygdalae of right-wingers would seem to be participating in a number of research results which are all variations on measuring differences in fearfulness. Those with more right-wing politics have been found to be more likely to classify ambiguous facial expressions as threatening (Vigil 2010) and are more distracted in cognitive tests by words like 'catastrophe' and pictures of things like sharks and hurricanes (Carraro et al. 2011). One remarkable piece of research affected a leftward shift in study participants' politics simply by asking them to imagine themselves as physically invincible superheroes (Napier et al. 2013). This illustrates, as we examined in the last chapter, that the innate differences between people which are of interest in the creation of sociopolitical realities are differences in thresholds rather than absolute potentials—different stimuli (e.g. different corners of the internet, different objective levels of scarcity and threat) will amplify or diminish our innate potential for fear, and different doses of these stimuli are required to invoke what are ultimately very similar responses.

One of the more idiosyncratic corollaries of political difference—vexatious both because we are bad at knowing how to talk about it in any context and because it has no obvious connection to the other traits that predict political outlook—is IQ, which has a negative relationship with right-wing perceptions (Bouchard et al. 2003; Hodson and Busseri 2012; Stankov 2009). There is evidence, however, that this difference is restricted to some cognitive domains. Those with more left-wing politics score higher on the language portion of the SAT (the SAT is strongly correlated with IQ), but not the math portion, where scores exhibit no ideological skew (Kemmelmeir 2008).

This difference in language use and communication has also been documented outside of testing contexts, in experimental conversations where friendliness and expressivity were measured.

Previous research has found that Openness is associated with more friendliness, more expressivity, less halting speech, and more smiling. Building on prior theorizing and results, we hypothesized that liberals would be more likely than conservatives to exhibit this general pattern of nonverbal behavior across different social interactions … predictions received relatively consistent, albeit modest support. Self-reported liberalism significantly predicted both smiling behavior … and body orientation toward the interaction partner … Liberalism was also marginally associated with greater expressiveness … Conservatism was marginally associated with more halting speech … (Carney et al. 2008)

Considering the aforementioned results concerning the interpretation of facial expression, integrative complexity, and these on the use of language and gesture, we might say that there seems to be a statistical left-right difference not necessarily in general cognitive ability, but in social cognitive ability. This same study that described differences in conversational styles also confirmed one of the intuitions many of us possess, that one can predict someone's politics from their living space—the researchers found that right-wing homes were tidier and had more things like cleaning supplies and postage stamps, whereas the left-wing homes were messier, had more books and music, and more ephemera from foreign countries and items related to travel.

Then there is the abstract art. Preference for abstraction and complexity in paintings have been correlated with left political leanings, preference for representational simplicity with right political leanings (Wilson 1973b).

Then there is the matter of faces. If one were searching for a trait distinction that most radically undermines the implicit premise common to most political dialogues—that political positions are arrived at solely through careful processes of reasoning, and therefore sufficient argumentation should convert political opponents—one could do worse than to note that undergraduates can predict people's party affiliation by looking at their high school yearbook photos (Rule and Ambady 2010). Study participants classified the Republican faces as more masculine.

A list of terms used from 1930-2007, in studies employing a wide range of theoretical structures, to characterize those with left-leaning politics includes items like: eccentric, sensitive, individualistic, life-loving, creative, imaginative, enthusiastic, sensation-seeking, uncontrolled, impulsive, slovenly, complex, and nuanced. Terms used to characterize those with right-leaning politics include: tough, definite, masculine, firm, intolerant, conventional, organized, clean, sterile, anxious, suspicious, withdrawn, cold, and mechanical (Carney et al. 2008). On the one hand, it all feels breathtakingly intuitive, and on the other—just like we all know what dancing or humor is but might struggle to meaningfully define them—it is very difficult to thematically summarize all of these terms and everything else we have discussed.

Which is to say that our search for a unitary theoretical framework for political difference is going to require us to find something that explains a pretty bewilderingly complex, idiosyncratic landscape of traits. We are looking for something that produces differences in perceptions of hierarchy but also of tradition; and strongly biases perceptions in favor of certain types of risk taking and exploratory behavior, and against others; and also other differences which somewhat tentatively or weakly relate to what are described as core themes of left-right division, like perceptions of war and sexual behavior; as well as many differences lacking any apparent nexus of logical connection to anything explicitly political: differences in things like IQ, the salience of positive stimulus, the use of gesture in conversation, cleaning supplies, and the masculinity of faces.

Whatever it is we are looking for, we know it is heritable, as political orientation has been studied using the same paradigm by which the heritability of any trait is studied, involving genetically identical twins raised apart (Alford et al. 2005; Bouchard et al. 2003; Eaves et al. 1999) and that personality variation evident at the age of preschool is predictive of adult political outlook (Block and Block 2006). Whole genome scans have turned up some politically correlated genetic variants that concern, as one might expect, social cognition and fear (Hatemi et al. 2011). However, whole genome scans typically identify genes accounting for only a small proportion of heritable variation in any trait (Chabris et al. 2015), and political orientation is no exception.

Raven youth revolution

Because scientific research is so compartmentalized, sometimes answers to questions in one scientific domain are being produced, unnoticed, in another. I think the biology underlying the trait distinctions that correlate with political perception is one such case. When the relationship between the developmental biology of aggression and political psychology is examined explicitly and in detail, I believe it not only provides a powerful explanatory framework for the strange, complex landscape of trait variations we have just described, but, as good theories often do, it provides answers for additional questions we may not have thought to ask (like why people statistically tend right as they age).

There are varieties of evolutionary reasoning supporting virtually every conceivable worldview, and I want to be clear about what I see as the fundamental distinction between the type of theory we're building here and many others. We are trafficking in evolutionary theory that is grounded in the actual mechanisms whereby traits are constructed—when I am feeling pithy, I sometimes say it is the type of biology that has some biology in it. This isn't just a distinction to be made from haphazard invocations of evolutionary theory to be found in pop culture and political discourse, but also from some of the most prestigious and widely-cited works in biology, that rely solely on mathematical modeling of competition between genes (e.g. Hamilton 1964; Trivers 1971).

The type of evolutionary theory with which we are concerned is happening across many different scientific disciplines, but a particularly good illustration of it, which we will make extensive use of, is evolutionary developmental biology. Except for reproductive cells, with half the usual number of chromosomes, each cell in our bodies has an identical sequence of nuclear DNA. Developmental biology is concerned with how those cells self-organize into highly differentiated tissues and functions—how genetically identical cells can comprise teeth and heart and skin—by expressing different genes, to different degrees. This science begins with the fascinating process of embryogenesis, wherein varying gene expression profiles act as a coordinate system, telling the rapidly proliferating cells where they are and thus what type behavior to engage in, and proceeds through all stages of life—development ultimately stops at death (Gilbert 2006).

Evolutionary developmental biology examines how changes in this process of development—of gene expression in particular, and trait construction more generally—can produce new traits and ultimately new species.

The first shots in the Evo Devo revolution revealed that despite their great differences in appearance and physiology, all complex animals—flies and flycatchers, dinosaurs and trilobites, butterflies and zebras and humans—share a common “tool kit” of “master” genes that govern the formation and patterning of their bodies and body parts … diversity is not so much a matter of the complement of genes in an animal's tool kit, but, in the words of Eric Clapton, “it's the way that you use it.” The development of form depends upon the turning on and off of genes at different times and places in the course of development … (Carroll 2005)

Particularly crucial to understand, in terms of distinguishing it from other forms of evolutionary theory, is that changes in the developmental process reflect the experiences of an individual throughout a lifetime, meaning it is concerned with evolutionary change that is sometimes quite rapid and directly adaptive, rather than being the result of very slow-moving, entirely random, fortuitous mutations of DNA.

The origin of a new direction of adaptive evolution starts with a population of variably responsive, developmentally plastic organisms. That is, before the advent of a novel trait, there is a population of individuals that are already variable, and differentially responsive, or capable of producing phenotypic variants under the influence of new inputs from the genome and the environment. Variability in responsiveness is due partly to genetic variation and partly to variations in the developmental plasticity of phenotype structure, physiology, and behavior that arise during development and may be influenced by environmental factors, including maternal effects that reflect genetic and environmental variation present in previous generations. (West-Eberhard 2005)

Evolutionary developmental biology, and the myriad other disciplines that examine traits in ways that are not agnostic on their construction (e.g. psychology, neuroscience) make use of the fact that any trait of interest—like adventurousness or musical ability—is polygenetic, the result of many genes, and that some genes are pleiotropic, affecting many traits (Chabris et al. 2015). This massively expands the range of potential behaviors and more generally phenotypes we might expect to see. Indeed, these facts help us account for why there are millions of species, instead of just a few who have settled on highly constrained, mathematically optimal strategies for existence—if one really considers the implications of the old forms of evolutionary theory, it is difficult to see how they account for the sheer diversity of life.

Because it has been so extensively debated and factors so significantly into the implicit or explicit political dimensions of evolutionary theory, let's use empathy, and its behavioral corollary of altruism, as an example. If traits were constructed by single genes, and we could reliably assume that alternative alleles to any given gene would emerge to engage in evolutionary combat with them (as some varieties of evolutionary theory so often seem to imply), this would profoundly constrain empathy.

But if we talk about how traits are constructed, we could easily see ways in which profound empathy might develop, promoting behaviors which have no clear underpinnings in the strict logic of survival and reproduction—we could see, in other words, how someone like John Brown might exist, or why dolphins might sometimes save scuba divers from sharks. A particularly plausible hypothesis might be that the experience of empathy is a byproduct of the exquisite neural architecture that gives rise to social intelligence generally, to the ability to know what someone else is thinking (theory of mind). One could easily posit that the development of this ability, to comprehend others, has been the target of evolutionary pressure, with altruistic behavior a corollary.

We might, upon a moment's reflection, even begin to suspect that this is exactly what we mean when we say someone is able to understand someone else's mind, that on some level it is simply the sharing of experience, and while this might be great for outsmarting one's enemies in battle and navigating the politics of one's group, it also means that, if the evolved capacity for intersubjective experience lacks any particular filters, when we see pain we will feel pain, and that some of us will risk everything to free someone else from a cage.

A final note on individual variation within species is required, as this will ultimately be what we are examining in the biology of the left-right divide. Where a trait like openness, which so predicts political orientation, is concerned, where the biggest differences are between individuals within a culture rather than between cultures (Tuschman 2013), and where in any given culture individuals exhibit a normal distribution of the trait, I think we can assume that to a large degree this simply reflects continuous variation in the process of trait construction, rather than some evolutionary pressure or another to produce a hypothetical optimal trait distribution. So if, for instance, certain cells differentiate into the adrenal medulla during embryogenesis, and how many of these cells do so during a crucial developmental stage is a determinant of fearfulness (and its inverse, openness) later in life, and we see a normal distribution of openness, this is probably because the proliferation of these cells is subject to random, normally distributed variation, and not necessarily because evolution is “trying” to create a range of personalities. Again, this is crucial to understand because it vastly expands the range of potential differences we might see between individuals of the same species, as opposed to if there were some strictly adaptive reason for these differences.

With this (mercilessly summarized) evolutionary theoretical framework in place, let's begin by noting that one of the distinctions we can see in other species with the most intuitive similarity to the left-right divide is between bonobos and chimpanzees. Chimpanzees are a group-living species which, while also sometimes exhibiting great empathy, have a demeanor that has often been described as warlike. The intergroup hostilities of the species are frequent, sometimes fatal, and occasionally manifest in an extreme form, with the wholesale extermination of neighboring groups and subsequent inhabitation of their territory (Goodall 1986; Mitani et al. 2010).

Within groups, there is a strict male dominance hierarchy, which determines access to food and mating rights, and which is established by fights between males (Goodall 1986), which are often brutal and sometimes (although less frequently than in aggressive encounters between groups) result in death (de Waal 2007). As is the case with so many other species (Lorenz 1954), the hierarchical social order is characterized less frequently by outright aggression than by the exchange of formal cues which are derived from the language of aggression—by behaviors which are stereotyped representations of dominant and submissive cues exchanged in actual conflicts, but which do not involve a real fight. These formal cues give dominance hierarchies an institutional character which appears directly homologous with those found within human societies—the social order is constantly being expressed, rather than only becoming apparent when there is an overt conflict, as is evident from this description of the ritualized dominance behavior of the alpha male of the Arnhem Zoo chimpanzee colony:

In his heyday he would charge straight at a dozen apes, his hair on end, and scatter them in all directions. None of the apes dared remain seated when Yeroen approached, stamping his feet rhythmically … the air would be filled with the sound of screaming and barking as the apes fled in panic … Then, as suddenly as the din had begun, peace would return. Yeroen would seat himself, and the other apes would hasten to pay their respects to him. Like a king he accepted this mass homage as his due, obviously regarding some of his subjects as unworthy even of a glance. After these “formalities” everyone sat down quietly again … (De Waal 2007)

Because I know at least some of my intended audience has been exposed to it, I have to briefly address what I can only think to call the banana feeding thing. This is the claim that this representation of chimpanzee society is completely wrong, as aberrant as describing the dynamics in a maximum security prison as representative of humanity in general, and that it is the consequence of Jane Goodall feeding the chimps she studied bananas, which apparently drove them so insane that they abandoned their previously benevolent ways and took up murder, subjugation, and rape.

I have absolutely no idea where this claim originates, but it seems to have been popularized by the 2010 book Sex at Dawn, which has a curious habit of repeatedly landing on interpretations of texts that I would describe generously as not particularly literal and ungenerously as disingenuous. It is true that Goodall fed some chimps some bananas, and it is true there was an increase in aggression as a result—she is the one who says so, and she also describes how she terminated this practice accordingly. The banana feeding phase of the research described in her classic Chimpanzees of the Gombe: Patterns of Behavior was a brief interval in a years-long research project, and it was only within one group. It is hard to imagine it was still determining the social dynamics a decade later (or exactly what this would say about the ostensibly peaceful animals if this were the case), or that it inculcated conflict with another group, that was never fed bananas. Fatal intergroup conflict, including extermination of one chimpanzee group by another, has been observed in other portions of the animal's range, where banana feeding never occurred (Mitanni et al. 2010; Wrangham and Glowacki 2012), and chimpanzees reliably organize into dominance hierarchies both in captivity (De Waal 2007) and everywhere they are observed in the wild (Hare et al. 2012; Muller and Mitani 2005).

With this (admittedly somewhat digressive; but I cannot count how many times banana feeding has been presented, in my subcultural milieu, as a counterargument to a basic characterization of chimpanzee behavior) qualification out of the way, let us contrast these tendencies with those of the bonobo.

… even though, like chimpanzees, bonobos are territorial and can have hostile intergroup interactions in which males and females display at members of neighboring groups … These interactions infrequently result in physical contact aggression … In addition, intergroup interactions can sometimes be highly social events with members of both groups sitting in close proximity, playing, and in some cases, even copulating. (Hare et al. 2012).

Within groups, bonobo behavior also radically diverges from that of chimpanzees. Their hierarchies are much less pronounced, and the aggression used to establish and maintain those hierarchies is much less severe. Females have greater power than males, with the power they exercise distributed throughout broader coalitions (i.e. in a more egalitarian manner) than among chimpanzee males, who dominate their societies as individual despots (although their individual despotism is often maintained by a small coalition of males). Bonobos “do not express dominance through formal signals and have very low-intensity displays relative to those of chimpanzees” (Hare et al. 2012). And of course, bonobos are famous for their more frequent and more adventurous sexual behavior.

In this very brief summary, we have clearly encountered a number of distinctions between these two animals that conspicuously resemble the left-right political differences we just described: differences in intergroup aggression, hierarchy, openness (in intergroup encounters), and sexual behavior. It is somewhat commonplace to say that humans clearly have tendencies of both the bonobo and the chimpanzee (e.g. De Waal 2022), so maybe this is true not just of phases of history or different types of societies, but of the differences between individuals to be found in any one place and time.

What about that vexatious difference in cognitive ability we previously examined, which some evidence suggests is mostly a distinction in language, and perhaps other forms of social cognition? Can we find parallels in this domain? We can. Chimpanzees have greater cognitive ability than bonobos in some experimental tasks involving tools and causality, whereas bonobos have greater cognitive ability in tasks related to theory of mind, requiring the comprehension of intentions underlying communicative gestures and gazes (Hermann et al. 2010).

Bonobos are also able to cooperate, and thus solve cooperative problems, in contexts where chimpanzees cannot. Chimpanzees are constrained by their tendency to aggressive monopolization of food, by a politics that does not allow them to share the rewards of a cooperative effort (Hare et al. 2007). Bonobos voluntarily share food with one another (Hare and Kwetuenda 2009), and are thus not constrained in this manner: aggression and hierarchy can function as an inhibition to the development of a theory of mind.

… certain social emotions (elicited during interactions with another animate being) that are normally adaptive in noncooperative interactions, such as direct competition over food and mates, potentially limit an individual’s or species’ behavioral flexibility in approaching novel social problems. Increased behavioral flexibility can result if selection acts on these social emotions so that they no longer constrain cooperative interactions. Finally, cognitive evolution can result if the cognitive ability responsible for the revealed flexibility then itself becomes the target of selection. (Hare et al. 2007)

What about the fact that Republican faces are apparently more differentiated by sex than Democratic faces—does this state of affairs also prevail among our two closest relatives? It does. Chimpanzee males are more pronouncedly different from females in their teeth and faces (more sexually dimorphic, in the nomenclature) than are bonobo males (Hare et al. 2012).

Where do these differences come from, and what can they concretely tell us about the differences between humans? Let us traverse three descriptive levels, from evolutionary pressures to traits to the process of trait construction. These levels can be distinguished by scale, starting with ecosystems and populations, progressing to individual animals, and finally to the activity of cells, genes, and molecules.

It is interesting to note that in the wild chimpanzee females do not form coalitions which determine group power dynamics, but in captivity they do. Female coalitions in captivity have been observed to decisively intervene in male power contests, establishing or maintaining the dominance of a favored male (De Waal 2007). Arguably, a primary reason that captive females exhibit more political agency is because of the lack of threat from neighboring groups. Chimpanzee dominance hierarchies are a tradeoff: subjugation within a group is the cost of protection from outright annihilation by those outside the group (note how this is the same logic whereby humans were historically subjugated by feudal overlords, and are currently subjugated by modern police forces).

It is hypothesized that bonobos were isolated from a chimp-like ancestor, south of the Congo river, roughly one million years ago, in a more abundant habitat. This abundance, it is further hypothesized, diminished the ecological pressures for territoriality and thus intergroup aggression. Consequently, at some point female coalitions formed, like those observed in captive chimps, but rather than simply determining which males were dominant, these coalitions directly asserted power (Hare et al. 2012).

So bonobo behavioral biology is the result of a feminist revolution that happened a million years ago, and we will return to this theme in earnest later—that not only does biology pervade politics, but that politics pervades biology, and that the course of evolution has been and will continue to be determined by what we can only call political struggle. But what is actually different between bonobos and chimpanzees, that produces these distinctions of behavior? Let us focus on two realms: brain structure and the persistence of juvenile trait into adulthood.

Concerning brain structure differences, one study found that “bonobos had higher local gray matter volume estimates in several areas of particular relevance to social cognition, and these were among the largest differences overall. These areas included the right anterior insula, right dorsal amygdala, right hypothalamus and right dorsomedial prefrontal cortex” (Rilling et al. 2011). In addition to social cognition, these differences have a bearing on empathy, tolerance, play, sexuality, and aggression. The pathway linking the amygdala and the ventromedial prefrontal cortex (VMFPC) is larger in bonobos than in chimpanzees, a distinction also noted in humans between non-psychopaths and psychopaths, who are characterized by a lack of empathy. Psychopathy scores are higher among those in positions of power than in the population at large (Babiak 2010; Boddy 2011), and psychopathy is one of three traits in the so-called dark triad, which are found at higher rates among those with right-wing politics (Duspara and Greitemeyer 2017). Interestingly, the functional connectivity between the amygdala and the VMPFC also declines proportionally to the severity of abuse humans experience as children (Herringa et al. 2013).

The fact that the brain regions described as varying by political orientation in humans also vary between bonobos and chimpanzees is intriguing, but there are caveats born out of the complexities at work in determining the relationship between brain structure and behavior. Bonobos, with their egalitarian politics, have a larger right amygdalae and right anterior insula than chimpanzees (Rilling et al. 2011), whereas in humans, right-wingers also have a larger right amygdalae and right anterior insula (Kanai et al. 2012). It is true that the amygdala is central to the production of fear and aggression, but it also plays a significant role in empathy and many other aspects of behavior and perception (Rilling et al. 2011), and experimental removal of amygdalae has resulted in both hyper and hypo-aggression (Wickens 2005). Brain regions serve roles of inhibiting as well as promoting behavior. For our purposes, it is probably sufficient to say that similar behavioral and perceptual differences, associated with variation in the same neural architecture, appears to exist between bonobos and chimpanzees on the one hand, and those with left and right politics on the other.

In terms of juvenile trait duration, it is not only true that bonobo faces are more similar between the sexes, but that they are juvenile, throughout the lifespan, with respect to chimpanzees. And this developmental delay extends to the realms of behavior and cognition. Bonobos are dependent on their mothers for longer, and some of their most important divergences appear to be the result of the retention of juvenile traits through the entire lifespan. While chimpanzees and bonobos both become less socially tolerant with age, this process is much slower in bonobos, such that adults display levels of social intolerance and aggression characteristic of juvenile chimpanzees. Their propensity for play throughout adulthood likewise resembles juvenile chimpanzees. (Wobber et al. 2010a; 2010b).

We have discussed evolutionary pressures and of traits, and can now examine the mechanisms of trait construction. The prolongation of development, such that juvenile (or in some cases, larval or embryonic) features persist, is sometimes called neoteny, or paedomorphosis, or simply developmental delay. All domestic animals are characterized by a set of correlated traits, many of which are neotenous, called the domestication syndrome (although not all the traits of the syndrome are present in every domestic species). These traits include:

… increased docility and tameness, coat color changes, reductions in tooth size, changes in craniofacial morphology, alterations in ear and tail form (e.g. floppy ears), more frequent and nonseasonal estrous cycles, alterations in adrenocorticotropic hormone levels, changed concentrations of several neurotransmitters, prolongations in juvenile behavior, and reductions in both total brain size and of particular brain regions. (Wilkins et al. 2014)

Some wild animals also exhibit this syndrome, and so are described by scientists as self-domesticated. Bonobos are one species to which this term is applied, as are humans (Hare et al. 2012; Hare and Woods 2020). (It should be noted, however, that domestication is usually defined partially by an inability to survive outside of a highly managed environment, which obviously doesn't apply to either species, although it does apply to civilized humans—it seems more precise to say that there is a diminished aggression syndrome than a domestication syndrome.)

The domestication syndrome reflects delayed development of aggression, such that adult animals retained a juvenile aggressive behavioral repertoire, with the remaining traits believed to be byproducts, shaped by the same developmental process (Hare and Woods 2020; Trut et al. 2004). Developmental delay is theorized to result from the diminished activity of neural crest cells. The developing embryos of complex animals differentiate into three so-called germ layers, which serve as the primordial foundations for further refinements of complex anatomy—the outer layer forms skin, brain, and nervous system; the middle layer generates organs like the heart and kidneys, and the inner layer produces, for instance, the stomach and the lungs. The neural crest, sitting atop the developing neural tube, has sometimes been called the fourth germ layer. Its cells migrate all over the developing embryo, differentiating to become, among many other things, neurons, pigment cells called melanocytes, and the adrenal glands, which produces substances like cortisol and epinephrine, crucial in determining stress responses (Gilbert 2006).

Changes to the adrenal glands and the sympathetic nervous system, resulting from diminished neural crest cell proliferation, are thought to underly the reduced aggression of animals under the domestication syndrome with respect to their wild counterparts.

… we trace the mechanistic basis of tameness to reduced size and function of the adrenal glands, which play a central role in the physiology of both fear and stress responses. Adrenal hypofunction and reduced stress hormone levels are well documented in domesticated species and have been induced experimentally by selection for tameness during experimental domestication of foxes and rats … (Wilkins et al. 2014)

The syndrome of traits affected by neural crest cells is a good illustration of the general principle that we cannot reason our way to the evolutionary underpinnings of any given trait, no matter how much math we use, without understanding the mechanism of its construction, because multiple traits are governed by unitary developmental mechanisms (Sih et al. 2004). For instance, the condition called piebald in domestic animals, where patches of fur are depigmented, is the result of the diminished presence of melanin-producing cells which are derived from the neural crest (Trut et al. 2004). One could easily imagine agonizing theoretical convolutions attempting to explain why this trait repeatedly evolved in domestic animals (and, for that matter, why bonobos have depigmented noses and ravens who retain juvenile behavior throughout their lifespan have depigmented mouths). Efforts have, in fact, been made to explain the domestication syndrome in terms of selective pressures for each trait, requiring each to be strictly adaptive. But here we see many traits, from reduced tooth size to floppy ears to patchy coats, resulting from a common selective pressure and developmental process.

This unitary developmental mechanism is polygenic. In any given species, the domestication syndrome is the result of “multiple genetic changes of moderate, quantitative effect” (Wilkins et al. 2014), and while species under the domestication syndrome do share significant overlap in genes thought to play a role in producing it, this overlap is far from complete (Theofanopoulou et al. 2017). In other words, different genetic changes in different species result in the common outcome of the domestication syndrome.

The fact that these correlated trait variations emerge repeatedly, in species after species, and do so from different genetic pathways, is intriguing. It indicates there are aspects of the evolutionary process that we are only beginning to tentatively understand. This is particularly the case because of the extraordinary speed with which the domestication syndrome has been produced under experimental conditions. Researchers have experimentally bred foxes (Trut 1999; Trut et al. 2004) and mice (Gariépy et al. 2001) for tameness. In both cases, while diminished aggression was the sole criterion used to select individuals for the breeding population, developmental delay in other traits was observed, and these changes only required a few generations.

In the case of the famous farm fox experiment, begun by the Russian geneticist Dmitri Belyaev in the 1960s, it required only three generations of selective breeding for animals to be born that did not exhibit the species-typical fear response to humans, and another three generations for animals to be born which the researchers called the domestication elite, whose behavior is “similar to that of the domestic dog: they do not escape humans but actively seek contact with them. Seeing a human even at a distance, they whine, yelp, and wag their tail anticipating a contact; during the contact, they try to lick the experimenter’s face and hands.” (Trut et al. 2004)

This is not at all what we would expect if the only mechanism of evolution was the very slow process of DNA mutation, with new alleles occurring entirely at random and then propagating if they fortuitously happen to engender a beneficial trait. The timespan of six generations is far too short to account for such radical changes, a problem that emerges repeatedly in the evolutionary record:

In an observation that has evolved into the modern theory of punctuated equilibrium, Darwin inferred from the fossil record that evolution frequently occurs in rapid bursts. It is now well established that significant morphological evolution generally occurs on short time scales … the rapid pace of morphological evolution … [is] not easily reconciled with the rates at which point mutations occur or with the sensitivity of complex organisms and their proteins to random point mutations. (Fondon and Garner 2004)

Moreover, it is not as if a number of other, similarly dramatic trait variations emerged in this time frame. Domestication experiments appear to be cases of the environment causing genetic change, rather than genetic change occurring at random, and subsequently subject to natural selection. This is one major sense in which contemporary evolutionary theory is showing us a world with vastly more possibilities than older forms of evolutionary theory implied: organisms have sufficient plasticity of form and behavior to produce novel traits over very short timeframes, including a single lifetime, and these adaptations are subsequently accommodated by genetic change. “I consider genes followers, not leaders, in adaptive evolution. A very large body of evidence shows that phenotypic novelty is largely reorganizational rather than a product of innovative genes …” (West Eberhard 2005).

Mechanisms for rapid evolutionary change continue to be elucidated, such as variation in tandem repeat length of gene sequences, which occurs at up to 100,000 times more often than point mutations (Fondon and Garner 2004). The researchers involved in the farm fox experiments have long maintained that the neurophysiological corollaries of a diminished stress response are heritable: “some of the initial changes may not be DNA-sequence alterations but epigenetic changes. In particular … hormonal states in the mother, associated with the less stressful conditions of domesticity, are involved in generating the DS.” (Wilkins et al. 2014)

I want to rephrase this, to convey the deep significance of this evolutionary understanding to the possible future journeys of the human species: the subjective experience of fear, or the lack thereof, can have a direct and rapid impact on the biological apparatus that governs the fear response itself. If making people less afraid is both the fundamental requirement and fundamental mandate of any meaningful or useful social transformation—the means and the end of revolution—this fact is clearly salient. Or, to phrase it more generally and perhaps more dramatically: if our sense of sociopolitical possibility is constrained by human nature, it is worth noting that not only does human nature encompass a wide range of potentials we don't necessarily see expressed, but the very apparatus that generates these potentials has the capacity to change in response to the conditions we create for ourselves.

In addition to telling us about mechanisms of evolution, Belyaev's farm fox experiments are a window into one aspect of the domestication syndrome that is particularly useful in understanding the left-right divide in humans: the diminished fear response of the experimental animals was expressed as heightened exploratory activity. Many species are subject to a critical period of development that precedes the establishment of an adult repertoire of fairly stable behavior, wherein a great deal of behavioral plasticity and exploration occurs (Lorenz 1950; Scott 1962); this is, after all, what we commonly understand to characterize youth in our own species. The experimentally domesticated farm foxes exhibited a prolongation of this critical period.

The sensitive period of such adaptation (or primary socialization) is known to begin in the early postnatal development by functional maturation of sensory systems and locomotion. Owing to that, the animals can perceive the environment and respond to it. The development of the fearful response is regarded as a factor that substantially complicates (if not altogether blocks) the processes of their exploration of the social environment and adaptation to it, which is true for all animals regardless of the level of their social organization. (Trut et al. 2004)

Just as humans statistically tend to become more right-wing with age (Fieldhouse et al. 2019; Wilson 1973), so does xenophilia and low aggression give way to xenophobia and high aggression in the developmental trajectory of many other species. In dogs, the fearful response is evident at four to six months of age, whereas in wolves it occurs at 1.5 months—we can see how this difference in the timing of developmental stages corresponds to an absolute difference in the trait throughout the lifespan, adult dogs being less prone to fear and aggression than adult wolves. The case of the farm foxes is similar. Like wolves, the control foxes in the experiment began to exhibit the fear response at 45 days, and—crucially for our purposes—the onset of the fear response was “characterized by a reduction in exploratory activity in an novel environment” (Trut et al. 2004), whereas the domesticated foxes did not exhibit the fear response even at the age of three months.

Nonhuman animals are sometimes assessed using the Big Five personality inventory (Gosling and John 1999; Nettle 2006), and while I know of no research that has done so, but it seems obvious that if one were to rank the personalities of these foxes, those subject to selection against aggression would score much higher on the dimension of openness, which so predicts a disinclination to hierarchy and war (i.e. aggression) in humans. Here we have come to the heart of the mysterious covariation, in political psychology, of perceptions of hierarchy vs. equality on the one hand and tradition vs. novelty on the other. We are not alone in this.

Hare et al. (2012) describe four categories of trait variations comprising the domestication syndrome, each one of which, while the precise terminology may vary somewhat between disciplines, has a clear connection to traits that predict political perception. First, they describe “physiological changes include those related to aggression … as well as others with no clear relation to aggression, such as more frequent reproductive cycles.” Political difference is indeed sometimes described explicitly in terms of an aggressive differential, for instance in the work of Robert Altemeyer, whose Right Wing Authoritarianism scale is one of the most widely-cited and predictive political psychometrics (Altemeyer 1998; Dean and Altemeyer 2020). Moreover, while the specific terminology of aggression is often missing from texts of political psychology, the term hierarchy is central, and aggression is precisely how hierarchies are established and maintained. While humans with more egalitarian outlooks don't have more reproductive cycles than authoritarians, they certainly have a more permissive attitude toward frequent sexual encounters, a ceaseless source of right-wing anxiety and repulsion.

Second, they note “reduced aggression and increased tolerance, and also increased pro-social behaviors, particularly play, non-conceptive sexual behavior and grooming.” Again we find that the language describing differences in humans and nonhumans alike does indeed overlap, as variation in social tolerance is frequently used to characterize left-right difference. Non-conceptive sexual behavior is specifically cited as troubling by those with more authoritarian politics, and thus perceptions of contraception and abortion are highly politically predictive. And these other descriptions, of generally prosocial behavior and increased play, would seem to converge elegantly with descriptions of egalitarians as being more concerned with stimulation than security (Piurko et al. 2011), more open (Jost et al. 2003), expressive (Carney et al. 2008), and “skilled in social techniques” (Block and Block 2006).

Third, they describe “anatomical changes such as reduction in cranial capacity, shortening of the face, reduction in tooth size, depigmentation of parts of the body, floppy ears and reductions in sexual dimorphism of crania”, and while this would seem the most likely category of trait variation to lack any corollary in the literature on human political difference, the aforementioned research differentiating Republican and Democrat faces by their masculinity (Rule and Ambady 2005) even provides us a connection to sexual dimorphism.

Fourth and finally, they note that “cognitive changes are suggested by evidence of differences in problem-solving abilities where domesticates and non-domesticates have been compared.” Because of the brain size reductions associated with the domestication syndrome, one might think any associated changes in problem solving ability would be of the declining variety, but this is not the case. Some non-social cognitive enhancements have been observed, such as the increased ability of guinea pigs to navigate mazes with respect to their wild counterparts (Lewejohann et al. 2010), but the most intriguing changes are in social cognition.

Domestic foxes, like dogs, can understand forms of human communication that require following a gaze or a pointed finger, whereas wild foxes and wolves cannot (Hare et al. 2005), and bonobos, as previously described, excel at social cognition compared to chimpanzees (Hare et al. 2007; Hermmann et al. 2010). This difference in social cognition would seem to have connections to everything from egalitarian's heightened performance on the language portions of the SAT (Kemmelmeir 2008), tendency to interpret facial expressions as less threatening (Vigil 2010), greater use of gesture in conversation (Carney et al. 2008), and general emphasis on empathy and harm avoidance (Lakoff 1994; Piurko et al. 2011). In human children, non-aggressive, observant temperament is positively correlated with the development of theory of mind (LaBounty et al. 2016; Wellman et al. 2011).

That diminished aggression would increase social comprehension is perhaps a logical inevitability. However, the developmental delay that underlies diminished aggression is widely theorized to be the fundamental driver of humanity's unique cognitive abilities in general (Hare 2011; Hare and Woods 2020; Shea 1989). Our cognitive and behavioral flexibility is thought to reflect lifelong prolongation of the developmentally plastic phase that is temporary in other animals (Lorenz 1950), and a great deal of evidence (which we will examine in a later chapter) converges in favor of this notion. For instance, primate neocortical volume is predicted by group size (Dunbar 1992); infant humans have greater social cognitive abilities than chimpanzees but similar capacities for reasoning about the physical world (Hermmann et al. 2007); and human brains exhibit significant transcriptional neoteny, gene expression profiles resembling those of juvenile chimpanzees, particularly in genes involved in brain growth and maturation (Somel et al. 2009).

This helps us see how we can characterize developmental delay in a flattering or unflattering manner, depending both on one's values and on how it is framed. One can obviously say that neoteny ultimately describes less development, in the sense that stages of the lifespan characteristic of more accelerated development are not reached. However, one can obviously also say that neoteny ultimately describes more development, in the simple sense that more of it happens—calling yourself a guitarist after a few hours of practice is not quite as credible as calling yourself a guitarist after years of practice.

While many of the nonhuman trait distinctions we have discussed have been between different species, or at least domestic and wild versions of species, we are clearly describing variation that occurs within species as well. After all, for domestication to occur, there must be a range of variation in aggression with which to begin the process of selection. In the case of the farm foxes, about 10% of the initial population showed sufficiently diminished aggression to be recruited for experimental breeding (Trut et al. 2004). Some chimpanzee males show little to no interest in the dominance hierarchy, and this lack of interest is not for lack of aggressive capacity (Goodall 1986; de Waal 2022). Famously, a troop of baboons whose aggressive males all died from eating contaminated garbage simply adopted a more egalitarian culture—the remaining males, prone to more affiliative behavior as well as diminished aggression, eschewed much of the dominance behaviors otherwise described as species-typical (Sapolsky and Share 2004).

Clearly, innate variation in a set of traits correlated with the left-right divide exists in other species. But can we observe any other species with an actual divide, where variation in these traits is so significant that their entire societies have a bimodal character, rather than individual personality variation being subsumed under a unitary social structure? I believe one of the closest parallels is to be found in ravens.

One of the most fundamental questions evolution has to answer is when and to what extent to institutionalize behavior, to cease exploring and say that the current paradigm is sufficient and should be preserved. Any answer to this question implies tradeoffs and risks. To eat an unfamiliar plant is to risk dying from being poisoned, and to not eat an unfamiliar plant is to risk dying from starvation. So it is interesting that at least a few of the more cognitively and behaviorally sophisticated species seem to simply be divided on the subject. Orcas, for instance, have resident populations and transient populations (Neiwert 2016). Both are extremely behaviorally sophisticated, but one of these populations interacts with a far wider range of environments and thus exhibits more behavioral plasticity.

Ravens, too, have territorial and transient individuals. The transient stage is a juvenile stage, ending when a raven finds a mate and begins to defend a territory, at which point its social interactions are restricted to those with mate and offspring (Heinrich 1991). Some ravens, however, simply never settle down. In the juvenile, transient stage, the mouths of ravens are pink, turning black with the onset of territoriality—those who remain vagrants throughout adulthood retain the pink mouth coloration (Heinrich 2011). Interestingly, bonobos also have pink lips throughout adulthood in some cases, mirroring the depigmented coats of domestic animals, and it seems entirely plausible melanocyte deficiency from diminished neural crest activity is the cause of this phenomenon in ravens. Birds exhibit some of the trait variations associated with the domestication syndrome as well as mammals (Hare et al. 2012).

What is most interesting for our purposes is that, unlike orcas, ravens are in chronic conflict along the dividing line of xenophilic egalitarianism vs. xenophobic hierarchy. Transients sleep in communal roosts, which are sometimes quite large and have extremely fluid membership. These roosts provide opportunities for interaction and exuberantly social behaviors, such as “social soaring”, and function as information exchange centers, allowing ravens to follow those who have knowledge of major food bonanzas, like the carcasses of large animals, to them. This is beneficial for the birds that are followed as well as those who do the following, because a large group of transient ravens is required to successfully defend against the aggression of a raven family in whose territory they are feeding. Territorial adults are more aggressive, so defense requires a disproportionate number of allies (Heinrich 1991; Marzluff et al. 1995). Additionally, when vagrant ravens discover the nests of mated, territorial ravens, they destroy them (Heinrich 2011).

Thus, ravens present a case where individuals who are socially tolerant and constantly sampling novel environments routinely conflict with individuals who are socially intolerant and behaviorally rigid. This division is more closely associated with age in ravens than in humans, but in both cases there are exceptions to the paradigm of becoming more authoritarian and less adventurous with age, and it is worth noting that in humans the association between social revolution and disaffected youth is very deep.

If it is the case with other behaviorally complex species like ravens, it is no surprise that humans have not settled on an answer to the question of when and to what extent to institutionalize behavior into a stable regime—that we, too, exhibit not just a continuous range of variation on the subject, but a range so wide that there is a bimodal (at the very least) character to our basic outlook on life. Likewise if not only ravens, but also chimpanzees and baboons and foxes, can exhibit significant variation in aggression and social tolerance, it isn't terribly surprising that humans should do so as well. And if all of this correlated trait variation, and what we know of its mechanisms, points to a unitary developmental biology in other species, it would seem extremely likely this same biology is at work producing the variation exhibited by humans.


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