Thirty years ago the developmental psychologist Howard Gardner published his most famous work, Frames of Mind: The Theory of Multiple Intelligences. Inside, he laid out a theory so obvious in hindsight it's amazing it took humanity so long to put it all together. In brief, Gardner argued that "intelligence" is not a single quality, but a series of sub-divisions of skill within a person which may or may not be correlated . Thus, simply describing a person as "intelligent" betrays a bias within the speaker towards certain forms of intelligence that are subjectively viewed as more important.
This should come as no shock to anyone who has taken an IQ test, which tend to focus on one's skills with linguistics, mathematics, and spatial relations. Even there, cracks can appear; I do pretty well with the first, exceptionally well with the second, and utterly abysmally with the third. Gardner argues that there are many other forms of intelligence (six more in the original book, though last I checked this had grown into something more like eleven more) that are no less real. Musical intelligence, bodily intelligence, inter- and intrapersonal; there's a whole range of skills born within the mind that for centuries were viewed as either too hard to or not important enough to try and measure (or both),
I mention all this by way of a reminder that what it means to be called "intelligent" is in large part a social construct, just as it is to be called "beautiful". An IQ test measures (imperfectly) those strands of intelligence society has judged to be most valuable. Not only does it prize linguistic and logical intelligence over interpersonal or artistic intelligence, but it prizes it's own conception of linguistic and logical intelligence over all others. That's how western nations can create intelligence tests, pass them over to Africans, and then act surprised if they don't do quite so well (see Bell Curve, The).
As between societies, so within. The rich and the powerful determine what the rich and the powerful consider most important to being rich and powerful, and then are amazed to find those qualities lacking in those not hoovering up seven-figure salaries. Doubtless certain forms of intelligence are very useful in making your way to the top. But in this country at least we may as well call those strands the "succeeding in a neoliberal capitalist marketplace" strands. We can't draw any conclusions beyond that. Perhaps we could call this strand "low cunning" for short, or even just "luck", as if the mechanisms by which you enter the world with high aptitudes for given intelligence strands weren't entirely down to luck in the first place.
There is no idea in the world more toxic to Boris Johnson's classical approach to Conservatism (the Jacobins would doubtless have recognised much in his comments, except of course they probably weren't "intelligent" enough to speak English) than the possibility that the systems by which some "cornflakes reach the top" are anything but the purest expressions of meritocracy, and moreover, that the merits the meritocracy reward are anything other than universal qualities worthy of equally universal respect. The existential terror that the rich and powerful might have to share their riches and power with others less deserving - as defined by the rich and powerful themselves - can only be allayed by convincing themselves the world is better off with a strict social heirarchy in which those on the lower rungs accept they are objectively unsuited for a larger slice of the pie.
(Yes, I have been reading Corey Robin's Reactionary Mind recently. Why do you ask? Jack Graham deserves some credit here, too, though all mistakes and bouts of idiocy remain forever my own.)
So is the game rigged, and those that rig the game comforted by hearing their own voices, louder than those they sit above, as they explain to people that it's really their own fault they haven't risen higher. After all, if they were smarter they could have rigged their own game, and reached the final level, where the only rule is to lie to each other over the champagne flutes.
 In fact, as I understand it Gardner was of the opinion that the correlations were very weak (I've only read bits and pieces of the original text, so I may have misunderstood, however). I'm less convinced by this; I think it depends very much on the pair of intelligences under consideration and the population one is observing. The key point though is that these correlations can be very weak, which is why trying to divide the population into "intelligent" and "not intelligent" is so ridiculous an endeavour.