there is this theory called "hybrid vigor"
It states that different races have strengths. If you mix these races together, the offspring will have the strengths of both races.
if blacks are good at running, and the Chinese are good at math, maybe you create offspring that's good at both.
But, this is rare. You will likley just create a kid that can't run or do math.
Race mixing usually creates weaker offspring.
A positive example might be different ethnicities of the same race that mix.
You need a natural or artificial selection to remove the people who are not superior at whatever category of thing you are trying to improve in order for "race mixing" to result in superior people this is also true for mixing two different people of the "same race"
By remove I do not mean kill but just not have them reproduce such as by not giving them welfare to reproduce for example
