The assumption that people reading HN are above average intelligence is probably accurate.
It’s a cognitive blind spot to assume communities you’re a part of have a higher mean intelligence than the general population :)
Simply the writing quality, grammar and sentence structure of the comments on HN would easily be enough to prove that HN readers (well, commenters) are above average.
I spend enough time glazing over headlines, stories and comments from other sites out of casual interest and I'll bet any amount of money this is true.
I like HN, but can’t see it as a social space like most forums are. (And yes I like HN for that reason too!)