Established: IQ is a well-understood, race/language independent measure of some (large) part of mental functioning, that is strongly positively correlated with almost all positive values in live (perhaps within normal, 2 sigma ranges -- 70-130). Most folks who say otherwise don't have any idea what they're talking about
One of the critiques often leveled by the HBD folks against democracy is that IQ is important, and (for instance), African countries tend towards IQ's about 2standard deviations below (70) American Average, and African Americans tend towards about half-way between the African Average and the American (oddly).
When this discussion happens, I always look at the Flynn effect. On every test we've got...and every way we know how to measure things, IQ has been going up for at least half a century, and probably a lot longer than that. Some folks have suggested that average IQ in 1920 would have been around 70 on today's scales. While clearly the effect would not have been continuous...it is fairly solidly established. I'd always heard a half-standard-deviation in IQ per generation, but the numbers seem all over the place.
My personal opinion is that the Flynn effect probably was solid for about a century, maybe 1890 to 1990, as parasites and disease started being eliminated, and nutrition got better, and stopped after that. Which would put the average IQ in 1789 somewhere near 70. If we take established science, we can see that the average African now is about as smart as the average American when the constitution was written. I don't think that constitutes a group of people who aren't smart enough to govern themselves. I think there's a heckuva lot of crappy institutions that prevent them from governing themselves, but I have to say...it's the system to blame (and in Africa, the system is clan-based kleptocracy left by the failure of the states that the Europeans should never have created), not the people.