What I think is never said about IQ is how it is the result of a series of small decisions that you make during the test, and therefore at least as much about emotion and personality as about intellectual capacity. When you do an IQ test some questions will come easily, but the harder ones are slightly stressful. You have to decide how much stress you want to endure in order to get one more point on your total.
It seems to me that the personality traits that might make you decide to accept the mild discomfort in return for the point include aggressiveness, competitiveness, self-confidence, craving for status/reward, and a belief in the value (to you) of getting a high score. That latter one in particular I think is very important. A person from a culture which doesn't have IQ tests, or a person in a social role which wouldn't be enhanced by a high 'score', has much less motive to put themselves through the stress.
I've also said before that I think acting dumb is significator of social submissiveness, and in a sophisticated modern society, not trying too hard at IQ tests is a part of that social dance. In other parts of the culture, refusing to try hard at tests of this kind can be an act of social defiance, and enhancing self-esteem (paradoxically). So it's complex.
(Note in all this I am talking about IQ not intelligence, which is a whole other subject).
Also, I think the best comment on the thread is this one from 'badjim' who is responding to the people who argue that white men are naturally more intelligent than other humans.
It’s been doctrinal belief for thousands of years that women and other men, typically dark, are less than human, and supporting evidence has always been ready to hand, and reliably eventually discredited. We’ve always been told that members of X can’t do Y, where X always includes women and blacks, and Y could be any desirable activity from medicine to mathematics, and it always turns out to be false. And now, as then, we’re told “This time it’s different. Now there’s solid proof.”