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Perhaps it has something to do with the kind of person who enjoys interacting via little snippets of plain text on a screen.

Merely reading and writing are "odd" activities. Humans aren't born knowing how to do them (unlike, say, walking or talking) and not all humans enjoy them in the same way.

Maybe the underlying figures that are claimed for the general population are just wild-ass guesses, and there is no "overrepresentation" at all on the Internet. The other replies already posted to your post suggest other possibilities too.

Yup, I've found that as well. For example, here's a poll of 4chan's /jp/:

http://i36.tinypic.com/3011z11.png

It matches up surprisingly well with HN, except that (perhaps as expected) the results are 97% I.

Wow, so why are people interested in Japan extremely shy? It doesn't seem like cosplay is inherently an introverted activity.