Social media goes further, allowing us to construct a reality defined by distinct visions of the world: We pick who we "friend" and who we follow, creating virtual communities of the likeminded. Before social media, you lived out your days in the real world of neighborhoods and workplaces that in many cases had some diversity of views, and also uncertainty about views, which led people to conceal and restrain them out of a concern for manners.
But now we can voluntarily join together with hundreds or thousands of people scattered across the country and the world who share common views about justice. That leads to the intensification of these views, as they get reinforced through combat with those on the outside of the group (who affirm contrary positions on justice) and amplified by the frenzied encouragement of those in the inside.
Science has shown that tribalism is hard-wired. Experiments and evidence dating back generations, in psychology, sociology and anthropology, have established firmly that human opinions and emotions, loyalties and affiliations, religions and customs, and even perceptions are shaped by our need to belong to a group — and by our proclivity to hate rival groups. Experimental subjects will spontaneously form in-group loyalties and out-group antipathies when assigned to teams randomly. Subjects will deny the evidence of their own eyes to agree with those around them, even if the discrepancy is blatant. There need be no trigger for tribalism, no cause or conflict. If we do not already have a tribe and a reason to be loyal to it, we will create a tribe and invent a rationale.
Not only that, but anger is also the quickest emotion to go viral. It strikes me that in the last couple decades, with the rise of the web, we've been able to observe a real-time experiment in which purely ideological tribes can develop, unimpeded by physical limitations. Only a decade ago, in fact, it was still common to read all sorts of gushing panegyrics to social media and how it was going to change the world for the better by allowing people everywhere to "connect," with the blithe assumption that, having connected, people would naturally share happiness and camaraderie for the most part. But it turns out that much like it must have been on the African savanna millions of years ago, once a tribal group has the basic needs of food and shelter met, the next step is consolidating their territory and engaging in violent skirmishes with outsiders. Centuries of civilization and domestication haven't changed that. Only a common extraterrestrial enemy can save human nature from itself.