This is a phenomenal question. My gut tells me it has something to do with sequestered/topic-specific forums or boards. That linked with a karma system and ideologies have an environment that is almost cancerous.

High-karma posts and topics help to deter or teach new comers. They also serve to create a narrative and local extrema that begins to differ from more broadly held views.

Next, consider that a sequestered board also limits (even eliminates) any empathy a user may have had. No one thinks differently from them, no one has a differing opinion.

HN is in stark contrast to these other systems. The entire site may be self-contained, but there is no place for users to hide (in PMs, in sub-boards, etc.). If we want to be here (which I think we do) we have to deal with each other, listen to differing opinions, have real conversations. There is a karma system, and I'm not quite sure how it fits in...

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