With this embrace of subjectivity came the diminution of objective truth: the celebration of opinion over knowledge, feelings over facts — a developm… - Michiko Kakutani

In addition to amplifying polarization, the report concluded, social media tends to undermine trust in institutions and makes it more difficult to have the sorts of fact-based debates and discussions that are essential to democracy.

[T]hese were grievences exacerbated by changing demographics and changing social mores that had some members of the white working class feel increasingly marginalized; by growing income inequalities accelerated by the financial crisis of 2008; and by forces like globalization and technology that were stealing manufacturing jobs and injecting daily life with a new uncertainty and .

Assorted theories have been advanced to explain confirmation bias — why people rush to embrace information that supports their beliefs while rejecting information that disputes them: that first impressions are difficult to dislodge, that there’s a primitive instinct to defend one’s turf, that people tend to have emotional rather than intellectual responses to being challenged and are loath to carefully examine evidence.
Group dynamics only exaggerate these tendencies, the author and legal scholar Cass Sunstein observed in his book Going to Extremes: insularity often means limited information input (and usually information that reinforces preexisting views) and a desire for peer approval; and if the group’s leader “does not encourage dissent and is inclined to an identifiable conclusion, it is highly likely that the group as a whole will move toward that conclusion.”
Once the group has been psychologically walled off, Sunstein wrote, “the information and views of those outside the group can be discredited, and hence nothing will disturb the process of polarization as group members continue to talk.” In fact, groups of like-minded people can become breeding grounds for extreme movements. “Terrorists are made, not born,” Sunstein observed, “and terrorist networks often operate in just this way. As a result, they can move otherwise ordinary people to violent acts.