Big lies prevent big goods
The fact that the distributions of abilities in all tribal and racial groups varies considerably does nothing to help us choose behavior. It is not as if the lower will agree to the demands of the higher or vice versa. However we can trade with each other and in this way cooperate. Yes it is probably true that the lower are more burdensome than the upper can compensate for. So there is a maximum difference in populations that can cooperate. Because at some point the difference is large enough that we can no longer find mutual beneficial exchanges. The only moral objective is to reduce the rates of reproduction of the lower end of the spectrum until the remainder of the tribe nation or race can cooperate with the higher end of the spectrum. The twentieth century has been an exercise in crippling the able in order to attempt to advance those who are unable. Whereas it would be preferable to create specific developmental programs that suit abilities rather than create falsehoods.