It could very well be, of course, that the people taking IQ tests are a biased sample.
A serious IQ test should not have this kind of bias—I think they pay randomly selected people to participate. (Of course a few people will refuse anyway, but you can make an estimate how big error that makes.)
A serious IQ test should not have this kind of bias—I think they pay randomly selected people to participate. (Of course a few people will refuse anyway, but you can make an estimate how big error that makes.)