What causes polarization? Three core mechanisms: (1) *Informational sorting:* people increasingly [get their information](https://www.journalism.org/2020/01/24/u-s-media-polarization-and-the-2020-election-a-nation-divided/) from sources that align ideologically with them. (2) *Social sorting:* people increasingly [talk, work, and live](https://www.nytimes.com/interactive/2021/03/17/upshot/partisan-segregation-maps.html) with people who align with them ideologically. (3) *Enclave Deliberation:* when people talk with like-minded others, their opinions tend to become [more extreme in the direction of their initial inclination.](https://www.amazon.com/Going-Extremes-Minds-Unite-Divide/dp/0199754128) And each of (1)–(3) is individually rational. (1) **Informational Sorting**. Suppose some political news breaks—a new police shooting; a change in the crisis at the border, what have you. First question: Where do you go to find out about it: the New York Times, or Fox News? Second question: Who do you trust to more accurately and objectively report on the developing story: the New York Times or Fox News? No doubt your answers to the two questions will be the same. People's tendency to exhibit [selective exposure](https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100452931)—a type of confirmation bias influencing what information they seek out—is driven by *selective trust*. And [selective trust is rational](https://aeon.co/essays/why-its-as-hard-to-escape-an-echo-chamber-as-it-is-to-flee-a-cult): if you have liberal opinions down the line, you *should* think the NYT is more reliable than Fox (you'd be incoherent if you didn't). If you have conservative opinions down the line, vice versa. (2) **Social Sorting**. What determines where you live and work? Many things, but part of it is having a culture and values that you identify with—if everyone in your workplace had alien values and a different way of life than you did, then (if you have the option) you'll be likely to change workplaces! This is old news: [*homophily*](https://en.wikipedia.org/wiki/Homophily)—the tendency of people to be similar to their friends and acquaintances—has always been with us. The trouble is: *partisan ideology is increasingly becoming a way of life*. (See e.g. [Mason 2016](https://press.uchicago.edu/ucp/books/book/chicago/U/bo27527354.html), [Klein 2020](https://en.wikipedia.org/wiki/Why_We%27re_Polarized), and [Talisse 2019](https://global.oup.com/academic/product/overdoing-democracy-9780190924195?cc=us&lang=en&).) Quick quiz: Jill carries a Whole Foods tote bag in her backpack and bikes instead of driving; Bill has an open-carry permit and drives a pickup. Can you guess their politics? And can you guess who you're more likely to become friends with? Yes and yes, of course. And you are not irrational for that. (3) **Enclave Deliberation**. Consider some controversial opinion you have—say, that it's a good idea to Defund the Police. Suppose you're about to attend a workshop on this idea, full of people who think it has merit. What do you think will happen to your opinion—and why? You'll probably get more confident that defunding the police is a good idea. And the reason is simple: you're about to hear a bunch of arguments and considerations in its favor, from people who you generally agree with and therefore trust. So *of course* it's reasonable to get more confident in your opinion; [that's what arguments are for.](https://deepblue.lib.umich.edu/bitstream/handle/2027.42/22888/0000452.pdf?sequence=1) (Vice versa, of course, if you have the opposite opinion and were instead attending a Def**e**nd the Police workshop.) So there we have it. Each of (1)–(3) is the result of individually rational behavior; put together, they are what lead to massive polarization. The problem of polarization has less to do with people being unreasonable, and more to do with the fact that society has changed in a way that leads *reasonable* people to polarize.

Ok sure- 'has causes' doesn't say anything about magnitudes so I can go with 'agree'... (rational causes could account for 0.0001% :))