Verified by 
Though the United States has always been polarized, in recent decades polarization has increased dramatically. Why?
There are many moving parts in a full story of modern polarization, and the point of this series is not to challenge the standard empirical accounts of it.
Rather, the point is to challenge their normative interpretations—to argue that our polarized politics arise from reasonable people who care about the truth but face ambiguous evidence. Given that, it’ll suffice to have a simple empirical story on the table; subtle or contested details will be reserved for the Technical Appendix.
The story, in outline, is that a variety of societal changes have led to increased social and informational sorting.
Both the southern realignment and the civil rights movement started the process of making Democrats the party of consistent progressives and Republicans the party of consistent conservatives. In turn, this increase in ideological consistency may have combined with the fading influence of religion to make political party the new key to many people’s identity.
Meanwhile, an increasing urban-rural divide has made it so that the political views of people's friends have become more predictable than ever. Combined with a precipitous fall in civic engagement, this has led to a decrease in cross-party social pollination and fewer friendships across party lines.
At the same time, an increasingly fragmented and political media landscape, along with the rise of web personalization has allowed people greater freedom in choosing their sources of information and opinion.
In short, decades ago, our social circles were ideologically diverse, our news sources constantly confronted us with differing opinions, and our political beliefs were not terribly consistent or central to our identities. Today, all that has changed. As a result, the old mechanisms that always drive polarization—including persuasion, the group polarization effect, confirmation bias, biased assimilation of evidence, and motivated reasoning—all point in increasingly the same direction. That means Democrats are being pulled further to the left and Republicans further to the right.
In my case: The more time I spent talking with Democrats, the more persuaded I was of their arguments, and the more opinionated we all became. The more informational choices I had, the more inclined I was to listen to and trust liberal sources. The more confident I became, the more inclined I was to interpret ambiguous stories about Republican ideas and politicians in negative ways. The more I came to identify with being a Democrat, the more motivated I was to see new evidence as confirming my Democratic beliefs. And the stronger each of these processes became, the more it reinforced the others.
Likewise, of course, for Americans everywhere.
Our question: What does this story—of old forces driving polarization, recently kicked into overdrive—mean for the rationality of politics?
It means that if we understand the old mechanisms, we’ll understand both why we’ve always been polarized, and the route through which polarization has increased.
And it means that if we come to see these old mechanisms as rational, then we’ll be able to see our polarized politics—including our political opponents—as the result of individuals doing the best they can with the information they have.
That’s where we’re heading. We’re going to take an in-depth look at the mechanisms of persuasion, group-polarization, confirmation bias, biased assimilation, and motivated reasoning. We’ll see how each of these is driven by rational attention to ambiguous evidence—and that as societal changes have made our political evidence systematically more ambiguous, they have all been kicked into overdrive.
But before we can dive into those details, we need to establish some background. What do I mean when I say that these mechanisms are to be expected from “rational people who care about the truth” and yet face “ambiguous” evidence? And how could it be that rational people can be predictably, persistently, and profoundly polarized?
Those are the questions I’ll take up next week.
