Thanks for the official statement ;-)

Interestingly, I originally believed HN is pure-upvote based, then I learned it from other HN comments that says karma affects ranking and I believed it.

So I'd say it's just an unsubstantiated rumor/misinformation getting circulated in the comment section from time to time, combined with the impression of HN having a "magic algorithm", so many believed it without any fact-checking. Also, the quick-moving nature of HN comments somehow created a confirmation bias that makes the idea appeared to be true.

I would have believed you, since I feel often the same names are highly upvoted. Though this can have natural causes. Like them writing better comments. Or just popularity based on name recognition. Such feedback loops naturally existing without being explicitly implemented by HN sounds highly plausible.

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Slashdot karma affects ranking? Good to know.