A long time ago, on a day like any other, Vladimir stood, sweaty and exhausted, dirt and grime caked to his leathery hands and face. Still, he admired the golden palace on the hill. He himself had nothing worth having. Nobody noticed him. Though he did not despair. It was all he had ever known.
Suddenly, a miracle! In an instant, God appeared before him. God looked down on the poor, piteous man and said, “Vladimir, I will grant you one wish. Anything you desire shall be yours.” Vladimir couldn’t believe it! Finally he too would know riches and luxury. He turned to speak his wish to God, but before he could God raised a hand and added, “Anything I grant to you will be given to your neighbor twice over.” This provision stopped Vladimir dead in his tracks. He stood and thought deeply in silence for awhile. After some time, he turned to God and said, “Ok God, take out one of my eyes.”
Whoa! Ok, fast forward to 1970. Henri Tajfel is developing his minimal group paradigm aimed at determining the minimal conditions for the development of intergroup bias. In experiments which take place the following year, participants are randomly assigned to different groups. These groups are made arbitrary in every way possible in the hope that this will strip away any sense of group-identity. Then, the participants make a series of resource distribution decisions. What Tajfel observes shocks even himself. Not only is the minimal condition required for in-group favoritism simply being categorized into a group, but the tendency to maximize in-group gain occurs even when it means sacrificing overall gain for all groups. The enduring eastern European fable known as Vladimir’s choice will later be chosen to illustrate these findings.
Now let’s jump to 1999. Tajfel & Turner’s now nearly 30 year old Social Identity Theory (SIT) has evolved. At its core, it seeks to explain the influence group-membership and group-status have on individual behavior. However, in this year, a new theory will be born. This new theory, the brain child of two American psychology professors, claims that most forms of group conflict and oppression (e.g. racism, sexism, nationalism, classism, ethnocentrism, and so on) can be regarded as different manifestations of the same basic predisposition observed across the entire primate order; the formation of hierarchically organized social systems.
Present day. Welcome to the 21st century. Unfortunately, not much has changed. The world still seems to teeter precariously on a delicate balance between cooperation and potential annihilation. Wait, I take that back. You can unlock your car with your cell phone now. So that’s cool. Anyway, this offshoot of SIT, its “child” if you will, is called Social-Dominance Theory (SDT). It was formally synthesized in Jim Sidanius and Felicia Pratto’s ’99 book, Social Dominance: An Intergroup Theory of Social Hierarchy and Oppression. In the interest of time, we will not dive any further into SIT or SDT. Instead, in an attempt to shed light on their potential biological roots, and because we like brains, we will concentrate our remaining efforts on the neural correlates of this propensity which predates mankind; social hierarchy.
The ubiquity of social hierarchy
From our family structure to the foundation of our governments, hierarchy, defined as a system of arranging or classifying things according to relative importance or inclusiveness, permeates and defines almost every system of organization on our planet. It is so infused in our daily lives that it almost seems trivial. In fact, nearly every human society that has ever existed, at least as far as we know, has been organized hierarchically. However, social hierarchy is not unique to primates. Not even close in fact. Some form of hierarchical social organization has been widely observed across numerous mammal species and even in some species of birds making evident its evolutionary utility.
It is generally accepted that in a world of limited resources, while it offers clear benefits against predation, cohabitation places stress on the individual organism. Animals that live together are simply more likely to come into competition with one another. It is thought that social hierarchy helps to alleviate this stress by favoring a system in which conflict is minimized. Every member knows its place, and subordinate members yield to those above them. However, everyone naturally does not benefit equally. A corollary of this “order“ is that higher ranking members generally have access to a larger share of those resources, and lower ranking members generally do a larger share of the work.
Anti-egalitarian as it may be, the ubiquitous nature of social hierarchy raises interesting implications across the board. At the biological level, what are the neuronal mechanisms driving this potentially prejudiced impulse, and how similar are they across species? The consistent manifestation of social hierarchy in considerably disparate neural systems would seem to underscore its centrality to proper social function. If hierarchical organization is indeed fundamental to an effective society, how then could it compel us into the shadows of oppression and racism — unless, they too are innate? Intriguing as this final question may, it is well beyond the scope of this essay. Instead, we will restrict our focus to neurobiology studies in humans, non-human primates and rodents, however, the interested reader is encouraged to take full advantage of the vast, outstanding literature.
The asymmetric ascription of social value
Advances in neuroimaging and neurorecording techniques over the past few decades have made it possible to systematically explore the neural basis of of evermore complex animal behaviors. In humans and non-human primates, increasing evidence from several independent studies identify activations in portions of the prefrontal cortex, amygdala, hippocampus and striatum in the performance of tasks involving the formation of social hierarchy and the perception of both social hierarchy and dominance. Prefrontal activations are believed to represent, among other things, the elevated engagement of perceptual-attentional resources potentially reflecting the greater saliency of higher-ranking individuals.
According to a 2014 review by Wang et al., prefrontal regions are believed to act as a central regulator with upstream areas conveying information about social status and downstream brain regions executing the behaviors associated with the expression of dominance. While potentially an oversimplification, at the very least this view suggests a useful schematic for the neural processing of social hierarchy. IMAGE
Timothy Behrens et al. divide the brain networks associated with social behavior into two distinct groups; those mediating social preferences, and those associated with evaluating the intentions of others. Strikingly, they suggest that the brain regions important for mediating social preferences are the same as those for reward-based learning. According to this view, we learn the hierarchical rank of other group members through experience in the same way that we learn the relationship between stimuli and rewards in reinforcement learning!
Further evidence for this shared neural encoding comes from a more recent study by Munuera et al. In their paradigm, Rhesus monkeys viewed images of the faces of other members of their group as well as images predicting different reward amounts. They observed activity in the same neuronal ensembles of the amygdala for both the processing of hierarchical rank and nonsocial reward encoding suggesting that, at least in primates, members of a group are seen as having a subjective value with higher-ranking individuals being “worth more” than those beneath them.
Knowing what we know about the mechanisms behind reward-based learning, one would also expect the asymmetric ascription of social value which defines social hierarchy to be associated with the neuromodulator Dopamine (DA). Evidence that this is indeed the case comes from Yamaguchi et al. who showed that acute administration of a D2 antagonist, a drug which blocks the activity of D2 DA receptors, tends to attenuate the social dominance (SD) traits in both Macaques and mice. Interestingly, in both species, this manipulation differentially affected the stability of the established hierarchy contingent upon the rank of the animal involved. Specifically, the hierarchical social structure was generally destabilized by drug administration (decreased SD expression) in higher-ranking animals and stabilized by drug administration (decreased SD expression) in lower-ranking animals. In other words, the stability of the hierarchical social structure was shown to be directly dependent on the rank-appropriate expression of social dominance behavior.
In further support of the role of DA in social hierarchy, a very recent study by Stagkourakis et al. employed optogenetic techniques to investigate the importance of DA-neurons to the expression of aggression between male mice. Optogenetics is a very cool and fairly new technique which allows for extremely precise temporal control of neuronal activity by way of pulses of light. Using this technique, they showed that light-induced activation of specific DA-neurons triggers attack-behavior and silencing these same neurons interrupts attacks. Critically, the manipulation of inter-male aggression had both immediate and long-lasting effects on the hierarchical social structure of these mice highlighting once again its plasticity and the essential role of behavior in its maintenance.
A cooperative game
Alright, let’s recap. In our discussion of social hierarchy, we began with the social-perspective by very briefly reviewing classical theories and examining its prevalence. We then shifted to the neuro-perspective by having a look at the overall organization of neural circuits in the perception and expression of social hierarchy, highlighting the shared neural encoding of hierarchical rank and reward-based learning and pointing out the importance of the neuromodulatory activity of DA in the maintenance of the hierarchical social structure of several species. Essentially, we’ve worked to connect behavior at the level of the individual organism and society as a whole to brain processes at both a systems and cellular level.
Besides the fact that this is fun and thought-provoking, we’ve done this in order to make a few very important points. (1) Social hierarchy is fluid and largely established and upheld by individual behavior, although the influence of institutions (e.g. governments, religions) is certainly not to be ignored. (2) Social hierarchy’s intrinsic connection to oppression and discrimination is potentially rooted in its neural mechanisms making them a highly-relevant target for potential social-reform, and (3) Social hierarchy is ubiquitous and potentially harmful to the individual organism. The study of its neural-basis has very real implications for social stress-related mental and physical health problems. Social status strongly predicts well-being, morbidity, reproductive-success, and even survival especially in primates, and the relationship between stress and social status continues to receive significant attention.
This bring us full circle, back to our poor peasant, representative every-day man, Vladimir. He understood his position in the hierarchy. He acknowledged and perhaps even subconsciously consented to being “worth less” than those above him. For better or worse, he did his part to maintain the order certainly established long before he took his first breath. And when his defining moment arrived, he decided that gain is relative, and the only way for one to climb is for others to fall. But we know his tale as “Vladimir’s choice” precisely because there was an alternative — another path in which everyone wins. Choose wisely.