How, and why, do our brains create ’Us versus Them’ fault lines?
Reetika Sud, NIMHANS, Bengaluru

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In an increasingly polarized world, we are separated by an ever expanding list of divides— from religion, castes, sub-castes, to political beliefs, sports teams loyalties, you name it. While the specific divisive reasons may differ across the globe, throughout history, humans have displayed preferential behavior towards in-group members (the “us”) over out-group members (“thems”). This bias towards those we deem similar to us is not something unique to humans.
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Rats use scents from chemicals called pheromones to tell relatives from enemies, their equivalent of “us” from “them”. Fish have a similar system. In humans though, no such chemical cues exist and we must rely on our brain’s system of sorting — who counts as “us” and who are “them”? This is why we ask others— where are you from? Where did you grow up? Beyond the impressions [stereotypes] from surnames, even if not done consciously, people ask these questions as it can help classify others into us or them.

While this can seem like harmless, a “getting to know you” exercise, a cursory glance just through modern history can provide glaring examples of how easy it is to game this system (exploit it). Ideologues have manipulated masses, creating us-them divides with horrendous consequences. During the Rwandan genocide, Tutsis (the victims of the genocidal campaign) were referred to by the Hutu (the majoritarian group), as cockroaches. The Nazis presented Jews as vermin. All that it requires is for leaders with mass appeal to invoke feelings of such disgust in their followers towards “them” that they barely qualify as humans.

Biology of the human brain behind the “us-them” divide
Why do we people behave this way? Let’s try to answer this through the lens of neuroscience, and what it tells us about our brains. Reference to “them” activates a part of the brain called the insular cortex. In many animals including humans, this part does what’s called gustatory disgust. If you unknowingly bite into a rotten fruit, this part of the brain activates, causing you to spit out the fruit. The same with the gag reflex, etc - that’s all thanks to the insular cortex.

In humans though, it does something extra it doesn’t do in other animals. In addition to physical disgust, it also activates from emotional disgust. Invoke “thems”, and the brain’s disgust response is hijacked by the popular narrative.

At this point, we must talk about another part of the brain called amygdala. This almond shaped part is situated deep inside the brain. Part of the emotional circuitry, this part does a lot of things though it is most famous for fear and aggression. As a response from insular activation, neurons in amygdala fire up, and they can’t tell whether the insular activation is from physical or moral disgust.

So, what does the amygdala do? Mention “them”, and you’ve activated fear response, a measure of aggression towards “them”, grounded in the fear. In humans, multiple neuro-imaging studies have documented increased firing from the amygdala, in a fraction of a second, in response to those deemed as out-group members.

Because a lot of this circuitry is similar in humans and other animals, we do learn a lot from experimental studies in animals. In fact, recent studies in rats showed young rats helped both in-group and out-group distressed rats, whereas adults lent a helping hand preferentially towards in-group members. The rats that didn’t extend help also showed increased amygdala activity.

*This suggests that preference or bias towards those similar to us is not innate but learned. Amygdala activation is a reflection of culturally learned associations.*

So far in the story, “you” the individual, have little role to play. You can’t help the insular cortex or the amygdala from getting fired up, but luckily the amygdala also communicates with the thinking and decision-making part of the brain, the prefrontal cortex, the part that sits right behind the forehead. On the grand scale of evolution, this is the most recent part of the human brain. The figure shows the amygdala and various parts of the frontal (FC) and prefrontal (PFC) cortex.

Research into racial bias shows that individuals who better understand historical/cultural biases and who themselves are internally motivated to not follow along the same path, are also the ones who demonstrate helping behavior towards out-group members. This is the domain of the cortex. This is the cortex communicating back to the amygdala, “I hear you, but this is not a dangerous person”.

Why should we care about what seems like a confused bit of brain biology? Does this matter in real life? Once we start to differentiate ourselves from “them”, we also start to see meaningful differences in values and beliefs; so this becomes immensely critical. Because the one story, based either on one personal experience, or pieced together by your brain from various sources, becomes the only story. “They” are reduced to their difference from “us”.

Hardwired response?
During the Vietnam war, a US Army helicopter pilot Hugh Thompson, while flying his helicopter during a survey mission, noticed American soldiers executing civilians in the Vietnamese village of My Lai. What he witnessed prompted Thompson to land his aircraft between the soldiers (his colleagues in a sense), and the fleeing villagers. He threatened to open fire at them unless they stopped the killing.

In World War I, British and German soldiers entrenched along the western front entered into an impromptu ceasefire around Christmas of 1914. It began as singing of carols from their trenches, and shouting out seasons’ greetings to the other side. Soldiers soon emerged from their trenches to wish each other, exchange tokens, sharing food and drinks, and playing soccer. They also helped the other side bury their dead. Eventually, the men had to be ordered back by Generals on either side to retreat to their respective trenches and resume fighting.

These are just some but nonetheless remarkable examples of how fluid the “us- them” divide lines really are.

Because your brain does it, does that mean it is inescapable destiny? Brain activity is highly malleable. Decades of research has shown that people process information about in-group (us) and out-group (them) differently, but the software in our brains is constantly getting updated and influences us on who counts as “us” and who as “them”.

Science tells us there are no humans that are so different to us. Thus, when we hear societal or cultural narratives presented to us as “facts of life” — “they are bad people”, whoever the “they” are, and find ourselves getting sucked into it, somewhere behind it is the misappropriated bit of brain biology.

Being aware of our own biology can make us better informed especially when faced with narratives that arouse rather than inform.