The people who take the survey know that their IQ contribution is going to be watered down by the 1000 other people taking the survey.
I have thought of that. But a person who wants to lie about his IQ would think this way: If I lie and other LWers do not, it is true that my impact on the average calculated IQ will be negligible, but at least it will not be negative; but if I lie and most other LWers also lie, then the collective upward bias will lead to a very positive result which would portray me in a good light when I associate myself with other LWers. So there is really no incentive to not lie.
(I’m not saying that they definitely lied; I’m merely pointing out that this is something to think about.)
How likely is it, do you think, that this group of rationality-loving people has reasoned that claiming to have joined a group that anybody can join is a good way to brag about their awesomeness?
Fair point; but very often the kind of clubs you join does indicate something about your personality and interests, regardless of whether you are actually an active/contributing member or not. Saying “I read LessWrong” or “I joined LessWrong” certainly signals to me that you are more intelligent than someone who joined, say, Justin Bieber’s fan club, or the Twilight fan-fiction club. And if there are numbers showing that LW readers tend to have IQs in the gifted range, naturally I would think that X is probably quite intelligent just by virtue of the fact that X reads LW.
One last point is that LWers might not be deliberately lying: Perhaps they were merely victim to the Dunning-Kruger effect when self-reporting IQs. I am not sure if there are any studies showing that intelligent people are generally less likely to fall prey to the Dunning-Kruger effect.
Last but not least, I would again like to suggest that future surveys include questions asking people how much time they spent on average preparing for exams such as the SAT and the ACT—as I pointed out previously, scores on such exams can be very significantly improved just by studying hard, whereas tests like iqtest.dk actually measure your native intelligence.
Clarity cannot be over-valued; plausibility, however, can be under-valued.