I don’t have all the answers to the questions above, but I may have some. Here they are for you to judge and come up with your own. Grab some warm coffee or fresh whiskey and sit comfortably. You'll be here for 10 minutes.
For as long as there is reason, there will be confrontation, there will be discourse. I used to think that disagreement was the spoken representation of clashes between people's internal nuances. You were born and grew up a certain way and I did another way. You believe X and I believe Y. We'll agree on many things, but also disagree on many others.
All that is true. What I had failed to notice though, was that disagreeing is an anticipatory act. It's a mechanism to manage change, and just like our very human nature, disagreement is predictive and it's social. While differences in opinions come from our past experiences, the spoken or acted representation of our opinions is our attempt to preserve our world view into the future. If the world was to end tomorrow, I think we would all find ourselves not disagreeing much. Let's explain with an example.
Liberals, Conservatives and Superman
Liberalism and conservatism are two ideologies represented in most modern socio-political-economic structures [throughout this essay I'll use 'liberal' and 'conservative' based on the U.S.-based contemporary popular definition, which is not technically correct and represented an entirely different set of people in previous centuries. Here, liberal means progressive and conservative means, well, conservative]. Although their behaviors and beliefs fluctuate from place to place and in time, there are general principles that tend to apply to each. Liberals are progress-oriented. Usually looking for what's next and with a loose regard for what came before. Conservatives support tradition and a preservation of pre-existing order, thus tend to oppose change, especially when it's radical.
Just like there are liberals and conservatives now, there were so 200 years ago. Both groups have evolved. Both are tracking society's thought progress like a needle in a compass. As humanity moves in time, both liberals and conservatives adapt their beliefs and thoughts accordingly. If the collective of humanity's thoughts could be squashed into a ball of clay, and you grabbed it with both hands, stretching it until you had one half of the mass in one hand and another half in the other hand, with a bridge of thinner clay in the middle, that would represent human's collective culture - conservative on the right hand, liberal on the left. But that's an over simplification. Inside the two masses of clay, there are internal masses that represent more and less radical philosophies of liberalism and conservatism.
Imagine that 27 year-old Superman lands on earth for the first time. He doesn't get adopted by any farmer family. Instead he navigates our world by himself. Also, one of his many powers is that when he is flying high up he can discern how humans think based on the clay spectrum we identified previously. He can see a person and immediately identify if he/she is a liberal or conservative. Moreover, this Superman dude grew up in a farm in his native planet (which wasn't destroyed in this alternative universe) where he would herd Super-sheep. His passion is to be the best Super herding dog there ever was.
To practice his herding skills he begins arranging people spatially based on their location in the liberal-conservative spectrum. Liberals on the left. Conservatives on the right. On the far left there are radical liberals. On the far right there are radical conservatives. And in the middle, people who are undecided or neutral.
Superman notices something interesting. As time goes by, the circles change in sizes and also move further up, down, left and right. They are not static! He wonders, "What would happen if I help a leader on the far left establish a communist state?" So, he publicly announces his support for Mrs. Communist. The masses on the far left start to grow in size with people joining from all spheres in the spectrum. After all, if Mrs. Communist has Superman’s support then who are you to disagree? Except, not everyone supports this. Superman notices that some of the spheres of people are shifting to the right slightly AND are merging with other nearby groups. Turns out that those who don't support Mrs. Communist observe the momentum she is gathering and begin to oppose her movement; ever more fervently and ever more organized. These groups agree to temporarily forget trivial differences that marked the boundaries between them and nearby groups. They team up to oppose the newly created left force. People who never thought about communism before are now in disagreement with it because it seeks to create a change to their comfortable lives. Like most people, you too don't like change, or at least are slightly bothered by uncertainty, so you oppose Mrs. Communist. Your neighbor, however, supports this new movement because, as he puts it, his life isn’t that great and he’s looking for change. [Cough] Russia [cough] Cuba [cough] Venezuela.
The dance of the masses goes on. Eventually the result is a new arrangement: not too different from the previous one but not the same either. Humanity emerges an evolved civilization with, hopefully, some lessons learned and an improved, more unified philosophical stance. What is this new order (a.k.a. new equilibrium) though?
Dancing Masses = Individual Psychology + (Social Perception)^2
To understand the new order, we need to understand how we perceive ourselves as individuals in a social world. We are constantly monitoring where we fall in society. Individually we are looking to position ourselves where we are comfortable. I personally want to be a progressive thinker with balanced but slightly edgy views. Where do I position myself? Well, I survey the landscape first to get a sense of the spectrum (meaning, I look around when I’m with fellow humans). I then make a somewhat informed decision. Aaaand I position myself here! Here will do (think of me standing somewhere in the spectrum of circles shown above). If Superman brought a bunch of new people and dumped them into our social group the spectrum would shift based on their views. I would adapt and find my sweet spot again. You observe this even on daily interactions. If you are an 8 on the scale from 1=calm to 10=energetic and are meeting a friend at a coffee shop who you know is a hyperactive 10, then you will probably lower your intensity a little to counteract her exaggerated high levels of energy. Bring the average to an 8 by lowering yourself to a 6 maybe. After all, you want to enjoy your time without causing a frenetic scene. You are pleased being at an 8 atmosphere. So you do what you can to bring the collective behavior thermostat to your comfort level - forcing your friend down has not worked in the past, so you lower your intensity level instead.
Similarly, imagine that someone from ancient times was dumped into our current culture and asked to find his place. He would be an outcast. He would stab people for food, throw rocks at prostitutes, shower once a month and would not give a fuck about our political correctness. Eventually he would learn though, for we might be animals, but we are social animals for _____ (chose a God of your liking) sake! He would gauge what's acceptable and what's not. Most importantly, he would make some mistakes when testing or jumping over the "what's acceptable" boundaries, and in doing so he would stretch those boundaries (picture an elastic membrane surrounding our area of acceptable behaviors). Others would observe, and they will learn that a new boundary has been created: maybe you CAN walk without a shirt on the streets after all (assuming he wasn't punished for that) but you definitely can't eat your neighbor's pet (Caveman dude was in fact put in jail for this action). #freecavemandude.
The same is true as a collective. Being that we are all monitoring our thoughts and behaviors in relation to those of others, collectively humanity is trying to achieve homeostasis. As a whole we perpetually aim to find humanity's place by figuring out what's acceptable, making some mistakes and correcting to then find balance based on newly defined boundaries. To illustrate with a real life example, think that at one point women educating themselves was outside of that boundary. Contemporary liberals pushed the elastic membrane (to contemporary conservatives' dismay) so that it now encompasses women's right to an education. Today, opposing women to go to college is deemed discriminatory and outside the norm (and rightly so). If tomorrow a trend emerges where people fight for protection of both animals AND plants feelings (there are people currently promoting this), you bet a group will emerge to counteract this trend fearing that in 15 years we won't be able to eat neither animals NOR plants and we will instead drink water and swallow nutrient pills. In the end, plants' advocated don't win, but during the process both groups compromise and agree that animals are out of reach at least, thus making the world vegetarian. At that point what used to be acceptable no longer is. We have moved the needle. The end result is a function of some initial movement to one side, followed by disagreement turned into resistance pulling the other way. Where did we end up? That depends on how persuasive, rich, powerful each side was. In this way, disagreement is the first step that is part of our mechanism to achieve the order we would like to enjoy, based on the dance between the eagerness of some to evolve and the efforts of others to keep things the same.
Going back to the liberals and conservatives conversation, both are required in a balanced society. To over-simplify it, liberals are the wheels that drive us forward, and conservatives are the breaks that keep us grounded and prevent humanity's vehicle from flying off a cliff. They are both trying to achieve a greater good, in general, but have different approaches on how to get there, and different ideologies on what "greater good" is.
Keeping all this in mind, it's easier to comprehend (not necessarily support though) drastic behaviors from other groups: they are reacting to what they believe are drastic behaviors from my group. I personally identify myself as more in tune with liberal trends. However, I caution both sides to be more moderate in their public discourse and actions (not in your thoughts though, keep those), as they will be met with resistance proportional to the strength of their drive forward. Give me one view you have and I'll find someone who thinks it's too radical of a view . Whether that opinion is right or wrong, if your goal is for them to share your view, then their interpretation is what matters. For instance, as understanding as I hope I can be, if the plants-have-feelings movement starts to gain traction, they would turn me into a pseudo-conservative fighting back because their proposed change it's a big leap from where my views currently stand: plants are food. But if they came up and said we can slowly and progressively reduce the number of plants we eat by doing X, Y and Z, then I might start listening.
It Takes 2 to Play
At last, this all brings us to analyze the tendency to form not three, not four, not one, but two groups. My theory is explained as follows: There is a natural disagreement, because of the reasons we just looked at, and others. There are personal and group interests in having the world be just like we imagine it to be. Moral, monetary, status and many others. But your view can't be proven necessarily true if there are other views out there, right? If someone is challenging my opinion, then it's hard to argue that my view is 100% correct. So, to prove mine to be “necessarily true” I have to to prove the other view is wrong. But there are many different views to any given topic. So which view do we attack? Pick one randomly, sometimes it's the one you disagree with the most, sometimes is the one with the largest support, sometimes is the most controversial one. Pick one. You bring up arguments against it, and whoever supports that view defends it. At that point you realize you might need backup, so you bring someone else into the conversation. The opponent (let's call him Contrarian) does the same. Now you're both playing a numbers game. Whoever gets the most people on their side wins. After all, views are a human product, so the more humans voicing support for my view the more real my view is. Hey, look at that woman over there! We disagree with her on this and that, but definitely not as much as we both disagree with Contrarian, so let's team up. After all, the world I envision is closer to what she envisions than the one Contrarian envisions. What magic ratio do you reach when both sides have the most number of people they can get? 50-50. Meaning, two groups. The tendency to form two groups is therefore the optimal outcome to the dance of the disagreeing masses. Disclaimer: this is a thought experiment more than an actual formal theory.
Whether consciously or not, we are all trying to protect our views while also protecting the clay of disagreeing masses from over-stretching. As long as keeping it together in some form of harmonic disagreement is better than the chaos of tearing it completely. Who is to say how much of a stretch is too much? Maybe Caveman dude knows.
Our disagreement is not only shaped by our nature and nurture, but by our anticipation of what the future will bring. Individually, we are constantly trying to find views and actions that we are comfortable with and that fall inside social norms. When the norm changes we might support or oppose, but eventually we adapt. Collectively, the new norm is a function of where each individual was, plus how much we moved in which direction. The magnitude and direction of movement is shaped by what is happening at the moment, but from the lens of where that will take us. To support or oppose the direction we focus the most on, we gather people that share our views. Others do the same and eventually we converge to the simplification of 2 groups. These two groups would do each other a favor by understanding that their views appear to be radical from the other side's perspective, and should moderate their message to achieve a common ground first.