While controversial in everyday conversation, intelligence has been measured around the world, and in many countries 80 or even below is the average. If you see the Wikipedia article “Heritability of IQ” you will find that it is mostly inherited, like for example height which can be stunted in childhood, but not easily increased.
Many people live good lives like this, although the modern world must present many challenges to these unfortunate many.
And I don’t even understand[0] why you feel the need to bring some racist theorizing from the 70s into it? Yes, I would believe there are many people with severe mental retardation that are, nonetheless, happy. That doesn’t mean, however, that I won’t try to avoid head trauma.
0: I’m kidding… I know exactly why you keep bringing up supposed IQ differences between ethnicities in your comments. But, unfortunately, HN considers it totally ok to say “look at Africa and how stupid but happy they all are” as long as you make it sound sciency, while calling out such racism is considered ill-tempered.