Verified by 
We all know that polarization is on the rise in the United States. But to know why, we need to know what’s normal.
Polarization has always been with us. A set of basic psychological and sociological mechanisms explain why human societies have always been characterized by local conformity and global diversity: There tends to be agreement within small social circles, but disagreement between them. As a result, when people go off on different life trajectories, it’s always been normal for their attitudes to drift apart.
This drift is guided by a set of mechanisms that drive people to have stronger (and often more conflicting) attitudes over time—especially when they are put in different social and informational environments.
To illustrate them, let's take a look at me and my old friend Becca. When we went our separate ways, I went to a liberal university in a city, while she went to a conservative college in the country. Unsurprisingly, our opinions polarized—I became more liberal, she became more conservative. Why?
Mechanism #1: Most obviously, and most simply, people are persuaded by arguments. That means that if two people enter different environments in which they’ll tend to encounter arguments for different positions, their opinions will predictable diverge.
For example, since I was headed to a liberal university, I could expect to hear arguments in favor of progressive taxation and the existence of oppressive sexism; since Becca was headed to a conservative college, she could expect to hear arguments in favor of the value of capitalism and the importance of being a good woman. As a result of these opposing arguments, we might have been able to predict that we’d come to disagree. (But that apparent obviousness is also a bit misleading—as we’ll see, the rationality of this predictable disagreement hinges on the subtleties of ambiguous evidence.)
Mechanism #2: When groups of like-minded individuals share and discuss their opinions, they tend to become both more homogenous and more extreme in those opinions. This is known as the group polarization effect, and is one of the most robust findings in social psychology.
For example, when I started talking about gun rights in groups of mostly-liberal university students (the majority of whom had never held a gun), we all grew more confident that interpretations of the second amendment have been bastardized, leading to the rise in mass shootings. Meanwhile, when Becca did the same in groups of mostly-conservative friends (many of whom owned guns), she grew more confident that the right to bear arms was central to American identity, and that since we’ve always had guns, the rise in mass shootings has other causes.
These first two mechanisms—persuasion and group polarization—are what gets polarization started: simply by putting me and Becca in different social environments, they started to pull our opinions apart.
Now let's jump to spring of 2011, when our opinions had already been pulled slightly further apart by our new environments. Once this happened, a new set of mechanisms kicked in: Our opposing beliefs started to ratchet up on their own.
Much of this phenomenon goes under the rather disunited label of "confirmation bias,” people’s tendency to gather and interpret evidence in a way that confirms their prior or favored beliefs. This tendency can be helpfully divided into three different mechanisms.
Mechanism #3: Selective exposure—when given a choice, people tend to prefer to see new information that they expect to confirm their prior beliefs, rather than information they expect to disconfirm them.
For example, in the spring of 2011, I began to consistently check the New York Times and the Washington Post to get my news. Becca, meanwhile, became more consistent in checking Fox News and the National Review.
Mechanism #4: Biased assimilation of evidence—when confronted with conflicting or messy evidence, people tend to interpret it in a way that favors their prior beliefs.
For example, in 2012 when Republicans stonewalled Obama’s proposals to use government funds to boost the economy, I took this to show that Republicans tend to put party over country—but Becca took it to show that Democrats tend to resort to government over-reach.
Mechanism #5: Motivated reasoning, a.k.a. identity-protective cognition—people tend to gather and make use of evidence in a way that confirms the things that they want to believe, especially when the belief is tied to their identity.
For example, when Tara Reade accused Biden of sexual assault, I must admit that I was inclined to be a bit skeptical—to spend a bit longer looking at articles that questioned her credibility than at those that supported it. Yet when Christine Blasey Ford made an accusation against Brett Kavanaugh, I made no such skeptical effort. Becca likely reacted in exactly the opposite way.
These mechanisms should sound familiar—we all know that people tend to be persuaded by arguments, that their motivations tend to affect the way they reason, and so on.
This is as it should be. Polarization is a familiar fact of life—it’s always been with us. Thus any adequate explanations of it will be built upon other familiar facts of life; an explanation built solely upon new or surprising findings would be missing the bigger picture.
Nevertheless, there is something new in our polarized politics—and there is something surprising lying behind these familiar mechanisms.
What’s new is that a series of factors have come together to align these various mechanisms—and kick them into overdrive. As a result, now when people like me and Becca go off on different life trajectories, their opinions diverge in predictable and consistent directions, and do so faster and farther than before.
What’s surprising is that all of them—including howlers like biased assimilation and motivated reasoning—are to be expected from rational people who care about the truth but face systematically ambiguous evidence.
That, at least, is what I’ll argue in the posts to come.
