Sad, but more or less what I expected.

Sometimes people ask me what is so special about the rationalist community. Like, aren't there smart and rational people all around the world? Are we the only ones who use their brains properly? And it feels extremely arrogant to say "yes"...

...and then I read e.g. the Hacker News, and it makes my point better than I could. Actually, I still can't articulate the point properly, but my best attempt is that there is a difference between rational individuals and rational groups. The rationalist community is a group that is open to newcomers (we don't require money or credentials, don't reject by race or gender or whatever) and still succeeds to keep relatively high epistemic norms. I am sure there are many rational people on Hacker News, and yet the epistemic level of their discussion is quite low. There may be individuals out there much smarter than everyone in the rationalist community combined... but they probably can't have a group discussion of the same quality.

I think I'd place Hacker News somewhere about in the middle between Mensa and Less Wrong. Better than most, but not good enough once you have experienced a higher quality. The special thing about rationalist community is precisely that ingredient that is missing at Hacker News. People actually caring to figure out the truth, and keeping each other accountable. A place where obvious bullshit gets downvoted, dubious general statements get challenged or receive requests for specific examples.

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Applying the usual rationalist nipicking to the comment you complained about...

they often confidently and aggressively spew a set of beliefs

If they do it often, it shouldn't be a problem to provide three good examples.

One of a few issues I have with groups like these [...] This is common everywhere, but I feel especially pronounced in communities like this.

Wait a moment, do you even have specific data about the community that is being discussed, or is this all mere reasoning by assumed similarity to other groups?

The smartest people I have ever known have been profoundly unsure of their beliefs

I really doubt that there are e.g. smart mathematicians who are profoundly unsure about whether 2+2=4.

BTW, what do you mean by "profoundly unsure"? Assigning less than 100% probability? Less than 50%? It makes a huge difference whether their probabilities are really low, or whether they just really solemnly say that they are lower than 100%. Which one is it?

...is the kind of question that a typical Hacker News reader usually doesn't ask. (Instead they probably go like "uh, this feels like deep wisdom, and criticizes a group we are currently criticizing... upvoted".)

This was not supposed to mean that the rationalist community is perfect, it's just that other online places seemingly don't even try. (Then again maybe the smartest people don't waste time debating online.)