In the late '70s, a couple of sociobioloists, Wilson and Freeman, found that Hopi Indian children developed faster cognitively and motorically, had better eyesight and in general outdid their white counterparts with respect to the advancement and culmination of their mental and physical abilities. It was hypothesized, in typical liberal environmentalist theorizing, that the custom of Hopi mothers of carrying their infant children strapped to their backs somehow caused those children to develop so much faster.
Immediately, it became fashionable and prevalent for upper class, wine and cheese, Whole Foods type, white liberal mothers to carry their babies on newly designed and mass produced infant back carriers that were suddenly manufactured and made available for the occasion of this new fad. But it was all just a fad and had no basis in fact. That the speculation that back carrying caused quicker, stronger children's developement was unfounded soon became clear as children of iKhung! "African Bushmen" children and children of many other technologically primitive cultures whose mothers did not have the infant back carrying custom showed developmental rates similar to those of the Hopi Indians.
It was, in fact, found that children from any non Western culture developed significantly faster and better than those of white Americans or white Europeans and within Western civilization, children of non dominant or less dominant, poorer groups - as those on the Native American reservations (and something vaguely similar was discovered among African American children) - had similar positive disparities with white, middle and upper class children.
The only logical conclusion that can be drawn from this is that the dominant groups of society and civilization, the rich and the white, grow up and live in relatively safer, more protected environments significantly more sheltered from the forces of Natural Selection, than do the less dominant groups, the poorer and darker skinned. IOW, the less dominant groups, much more than the dominant, live in environments where the stupid, feeble and inept will not survive. So then, in just a few generations, the less dominant groups outstrip the more dominant ones in evolutionary advancement.
Thus it is at least as logical to conclude or presume that the rich, higher status people are less intelligent and less mentally fit than the poor and lower status as it is to presume the converse, i.e., that the rich and dominant are naturally superior to the poor and less donminant.