the notion that one of these characteristics could plausibly be "the population is less intelligent on average than other populations around the world, controlling for all other factors such as health care, education, happiness, etc.", that's what we call racist. it's why attempting to "study" such things are rightly frowned upon.
Science starts with questions and hypothesis, not with conclusions. I personally don’t think it’s significantly related to any of the factors you’ve listed (or with genetics either), but thanks to perspectives like the one you’re presenting here, the only possible outcome is that the world will permanently have a large low-IQ underclass (which as other have mentioned ITT, perhaps that is something a lot of people want).
